Space-time monitoring of tropical forest changes using observations from multiple satellites
Hamunyela, Eliakim - \ 2017
Wageningen University. Promotor(en): M. Herold, co-promotor(en): J.P. Verbesselt. - Wageningen : Wageningen University - ISBN 9789463436403 - 188
tropical forests - monitoring - satellites - deforestation - ecological disturbance - tropische bossen - monitoring - satellieten - ontbossing - ecologische verstoring
Forests provide essential goods and services to humanity, but human-induced forest disturbances have been on ongoing at alarming rates, undermining the capacity for forests to continue providing essential goods and services. In recent years, the understanding of the short-term and long-term impacts of deforesting and degrading forest ecosystems has improved, and global efforts to reduce forest loss are ongoing. However, in many parts of the globe, significant forest areas continue to be lost. To fully protect forest ecosystems efficiently, timely, reliable and location-specific information on new forest disturbances is needed. Frequent and large-area forest mapping and monitoring using satellite observations can provide timely and cost-effective information about new forest disturbances. However, there are still key weaknesses associated with existing forest monitoring systems. For example, the capacity for forest monitoring systems to detect new disturbances accurately and timely is often limited by persistent cloud cover and strong seasonal dynamics. Persistent cloud can be addressed by using observations from multiple satellite sensors, but satellite sensors often have inter-sensor differences which make integration of observations from multiple sensors challenging. Seasonality can be accounted for using a seasonal model, but image time series are often acquired at irregular intervals, making it difficult to properly account for seasonality. Furthermore, with existing forest monitoring systems, detecting subtle, low-magnitude disturbances remains challenging, and timely detection of forest disturbances is often accompanied by many false detections. The overall objective of this thesis is to improve forest change monitoring by addressing the key challenges which hinders accurate and timely detection of forest disturbances from satellite data. In the next paragraphs, I summarise how this thesis tackled some of the key challenges which hamper effective monitoring of forest disturbances using satellite observations.
Chapter 2 addresses the challenge of seasonality by developing a spatial normalisation approach that allows us to account for seasonality in irregular image time series when monitoring forest disturbances. In this chapter, I showed that reducing seasonality in image time series using spatial normalisation leads to timely detection of forest disturbances when compared to a seasonal model approach. With spatial normalisation, near real-time forest monitoring in dry forests, which has been challenging for many years, is now possible. Applying spatial normalisation in areas where evergreen and deciduous forests co-exist is however challenging. Therefore, further research is needed to improve the spatial normalisation approach to ensure that it is applicable to areas with a combination of different forest types. In particular, a spatial normalisation approach which is forest type-specifics is desirable. In this chapter, forest disturbances were detected by analysing single pixel-time series. Spatial information was only used to reduce seasonality.
Taking in account the fact that forest disturbances are spatio-temporal events, I investigated whether there is an added-value of combining both spatial and temporal information when monitoring forest disturbances from satellite image time series. To do this, I first developed a space-time change detection method that detects forest disturbances as extreme events in satellite data cubes (Chapter 3). I showed that, by combining spatial and temporal information, forest disturbances can still be detected reliably even with limited historical observations. Therefore, unlike approaches which detect forest disturbances by analysing single pixel- time series, the space-time approach does not require huge amount of historical images to be pre-processed when monitoring forest disturbances. I then evaluated the added-value of using space-time features when confirming forest disturbances (Chapter 4). I showed that using a set of space-time features to confirm forest disturbances enhance forest monitoring significantly by reducing false detections without compromising temporal accuracy. With space-time features, the discrimination of forest disturbances from false detections is no longer based on temporal information only, hence providing opportunity to also detect low-magnitude disturbances with high confidence. Based on the analysis for conditional variable importance, I showed that features which are computed using both spatial and temporal information were the most important predictors of forest disturbances, thus enforcing the view that forest disturbances should be treated as spatio-temporal in order to improve forest change monitoring.
In Chapter 2 – 4, forest disturbances where detected from medium resolution Landsat time series. Yet, recent studies showed that small-scale forest disturbances are often omitted when using Landsat time series. In Chapter 5, I investigated whether detection of small-scale forest disturbances can be improved by using the 10m resolution time series from recently launched Sentinel-2 sensor. I also investigated whether the spatial normalisation approach developed in Chapter 2 can be used to reduce inter-sensor differences in multi-sensor optical time series. I showed that the 10m resolution Sentinel-2 time series improves the detection of small-scale forest disturbances when compared to 30m resolution. However, the 10m resolution does not supersede the importance of frequent satellite observations when monitoring forest disturbances. I also showed that spatial normalisation approach developed in Chapter 2 can reduce inter-sensor differences in multi-sensor optical time series significantly to generate temporally consistent time series suitable for forest change detection. Spatial normalisation does not completely remove inter-sensor differences, but the differences are significantly reduced.
Monitoring of forest disturbances is increasingly done using a combination of Synthetic Aperture Radar (SAR) and optical time series. Therefore, Chapter 6 investigated whether the spatial normalisation approach developed in Chapter 2 can also reduce seasonal variations in SAR time series to facilitate the integration of SAR-optical time series for forest monitoring in dry tropical forests. This Chapter demonstrated that seasonal variations in SAR time series can also be reduced through spatial normalisation. As a result, observations from SAR and optical time series were combined to improve near real-time forest change detection in dry tropical forest. In Chapter 7, it is demonstrated that spatial normalisation has potential to also reduce inter-sensor differences in SAR-optical time series, resulting into temporally consistent SAR-optical time series.
In conclusion, this thesis developed a space-time forest monitoring framework that addresses some key challenges affecting satellite-based forest monitoring. In particular, new methods that allow for timely and accurate detection of forest disturbances using observations from multiple satellites were developed. Overall, the methods developed in this research contribute to our capacity to accurately and timely detect forest disturbances in both dry and humid forests.
Super-performance in a palm species
Jansen, Merel - \ 2016
Wageningen University. Promotor(en): Niels Anten; Pieter Zuidema, co-promotor(en): Frans Bongers; M. Martínez-Ramos. - Wageningen : Wageningen University - ISBN 9789462579996 - 193
chamaedorea elegans - understorey - tropical forests - spatial variation - leaves - growth - population ecology - defoliation - genetic variation - chamaedorea elegans - onderlaag - tropische bossen - ruimtelijke variatie - bladeren - groei - populatie-ecologie - ontbladering - genetische variatie
The world is changing rapidly due to anthropogenic disturbance. Effects include: global warming, massive pollution, a changed global nitrogen cycle, high rates of land-use change, and exotic species spread. This has a tremendous impact on both natural and agricultural systems. To understand these impacts, good understanding of ecological systems and underlying drivers is necessary. Ecological systems can be studied at different levels of aggregation. Different levels of aggregation influence each other and are also influenced by external drivers like the environment. The population level is of particular interest, because many important ecological processes occur at the population level, like evolution, extinction, and invasion. Ecologists are increasingly recognizing that population processes are strongly influenced by one level of aggregation lower, the individual level. Individual heterogeneity (i.e. differences between individuals in performance), determines many population processes including population growth rate. However, the exact relations between individual heterogeneity, the external drivers of it, and the population level are not always well understood. Furthermore, methods to analyze these relations are not always available.
Individual heterogeneity occurs at different temporal scales, ranging from short- to long-term performance differences between individuals, where short- and long-term refer to the expected lifespan of the species in question. Short-term differences between individuals are relatively easily identifiable and are common in almost all species. But long-term differences are much harder to determine especially for long-lived organisms. Long-term differences between individuals in reproduction have been identified for several animal species, and in growth for several tree species, but less is known about the existence of such differences in other life forms (e.g. palms, lianas or clonal plants). Quantifying the extent to which individuals differ is essential for understanding the influence of individual heterogeneity on population processes. Super-performing individuals (i.e. individuals that persistently grow faster and reproduce more than others), probably contribute more to the growth of the population and therefore to future generations. Future populations will, therefore, have the genetic characteristics of the super-performers. Which characteristics this will be, depends on the genetic and environmental drivers of super-performance. Full understanding of the influence of individual heterogeneity on population processes, therefore, requires knowledge of the underlying causes of individual heterogeneity.
For many species, it is known that spatial variation in environmental conditions can cause short-term performance differences between individuals, but it is often not clear if the same environmental factors that cause short-term performance differences are also the environmental factors that cause long-term performance differences. Furthermore, genetic variation is known to cause performance differences, but to what extent is not well studied in natural long-lived plant populations. Within-population genetic variation can be maintained in habitats that are characterized by strong temporal or spatial heterogeneity in environmental conditions if the performance of a genotype relative to others depends on the environment it experiences.
Super-performing individuals possibly play an important role in the resistance and resilience of populations to disturbance (i.e. maintaining and recovering population growth rate under stress), because super-performers potentially contribute more to the recovery of the population. However, this depends on the relative tolerance to disturbance of super-performers compared to under-performers. A positive relation between performance and tolerance would make super-performers more important, while a negative relation would make them less important. Many types of disturbances entail leaf loss and tolerance to leaf loss is associated with performance being larger than what one would assume based on the amount of leaf area loss. Tolerance can be achieved by compensating for leaf loss in terms of growth rate, which entails either allocating more new assimilates to leaves, allocating new assimilates more efficiently to leaf area (i.e. by increasing specific leaf area), or growing faster with existing leaf area (i.e. by increasing net assimilation rate). Genetic variation in tolerance and compensatory responses would allow populations to adapt to changes in disturbance events that entail leaf loss.
Individual heterogeneity could also have implications for management. Plant and animal populations are managed at many different levels ranging from harvest from natural populations to modern agricultural practices. When harvesting from natural populations, it might be beneficial to spare the individuals that are most important for future production. Individuals could be spared, either because they contribute most to population growth, because they are tolerant to harvesting (which is relevant when only part of a plant is harvested), or when they start producing less or lower quality product. The productivity of natural populations could also be increased by actively promoting those environmental conditions and genotypes that allow for high productivity, which is the basis of agriculture and common practice in forest management. To determine how this can best be done, knowledge of the causes of individual heterogeneity is necessary.
The general aim of this thesis is to identify and quantify the mechanisms that determine individual heterogeneity and to determine how this heterogeneity, in turn, affects population level processes. This aim was divided into four main questions that I addressed: (1) To what extent do individuals differ in performance? (2) What causes individual heterogeneity in performance? (3) What are the demographic consequences of individual heterogeneity? (4) Can individual differences be used to improve the management of populations? To answer these questions, we used the tropical forest understorey palm Chamaedorea elegans as a study system, of which the leaves are an important non-timber forest product that is being used in the floral industry worldwide. We collected demographic data, measured spatial variation in environmental conditions, and applied a defoliation treatment to simulate leaf harvesting, in a natural population in Chiapas, Mexico. Furthermore, we grew seedlings from different mothers from our study population in the greenhouses of Wageningen University, where we also applied a defoliation treatment.
In Chapter 2 we quantified the extent to which individuals differ in long-term growth rate, and analyzed the importance of fast growers for population growth. We reconstructed growth histories from internodes and showed that growth differences between individuals are very large and persistent in our study population. This led to large variation in life growth trajectories, with individuals of the same age varying strongly in size. This shows that not only in canopy trees but also in species in the light limited understorey growth differences can be very large. Past growth rate was found to be a very good predictor of current performance (i.e. growth and reproduction). Using an Integral Projection Model (i.e. a type of demographic model) that was based on size and past growth rate, we showed that fast-growing individuals are much more important for population growth than others: the 50% fastest growing individuals contributed almost two times as much to population growth as the 50% slowest growing individuals.
In Chapter 3 we analyzed the extent to which observed long-term growth differences can be caused by environmental heterogeneity. Short-term variation in performance was mainly driven by light availability, while soil variables and leaf damage had smaller effects, and spatial heterogeneity in light availability and soil pH were autocorrelated over time. Using individual-based simulation models, we analyzed the extent to which spatial environmental heterogeneity could explain observed long-term variation in growth, and showed that this could largely be explained if the temporal persistence of light availability and soil pH was taken into account. We also estimated long-term inter-individual variation in reproduction to be very large. We further analyzed the importance of temporal persistence in environmental variation for long-term performance differences, by analyzing the whole range of values of environmental persistence, and the strength of the effect of the environmental heterogeneity on short-term performance. We showed that long-term performance differences become large when either the strength of the effect of the environmental factor on short-term performance is large, or when the spatial variation in the environmental factor is persistent over time. This shows that an environmental factor that in a short-term study might have been dismissed as unimportant for long-term performance variation, might, in reality, contribute strongly.
In Chapter 4 we tested for genetic variation in growth potential, tolerance to leaf loss, compensatory growth responses, and if growth potential and tolerance were genetically correlated in our study population. We quantified compensatory responses with an iterative growth model that takes into account the timing of leaf loss. Genetic variation in growth potential was large, and plants compensated strongly for leaf loss, but genetic variation in tolerance and compensatory growth responses was very limited. Growth performances in defoliated and undefoliated conditions were positively genetically correlated (i.e. the same genotypes perform relatively well compared to others, both with and without the stress of leaf loss). The high genetic variation in growth potential and the positive correlation between treatments suggests that the existence of super-performing individuals in our study population likely has (at least in part) a genetic basis. These super-performing individuals, that grow fast even under the stress of leaf loss, possibly contribute disproportionately to population resistance and resilience to disturbance. The low genetic variation in tolerance and compensatory responses, however, suggests that populations might have limited ability to adapt to changes in disturbance regimes that entail increases in leaf loss. Furthermore, the high genetic variation in growth potential could potentially be used in management practices like enrichment planting.
In Chapter 5 we explore the potential of using individual heterogeneity to design smarter harvest schemes, by sparing individuals that contribute most to future productivity. We tested if fast and slow growers, and small and large individuals, responded differently to leaf loss in terms of vital rates, but found only very limited evidence for this. Using Integral Projection Models that were based on stem length and past growth rate, we simulated leaf harvest over a period of 20 years, in several scenarios of sparing individuals, which we compared to “Business as usual” (i.e. no individuals being spared, BAU). Sparing individuals that are most important for population growth, was beneficial for population size (and could, therefore, reduce extinction risk), increased annual leaf harvest at the end of the simulation period, but cumulated leaf harvest over 20 years was much lower compared to BAU. Sparing individuals that produced leaves of non-commercial size (i.e. <25cm), therefore allowing them to recover, also resulted in a lower total leaf harvest over 20 years. However, a much higher harvest (a three-fold increase) was found when only leaves of commercial size were considered. These results show that it is possible to increase yield quality and sustainability (in terms of population size) of harvesting practices, by making use of individual heterogeneity. The analytical and modeling methods that we present are applicable to any natural system from which either whole individuals, or parts of individuals, are harvested, and provide an extra tool that could be considered by managers and harvest practitioners to optimize harvest practices.
In conclusion, in this thesis, I showed that in a long-lived understorey palm growth differences are very large and persistent (Chapter 2) and that it is likely that long-term differences in reproduction are also very large (Chapter 3). I also showed that spatial heterogeneity in environmental conditions can to a large extent explain these differences and that when evaluating the environmental drivers of individual heterogeneity, it is important to take the persistence of spatial variation into account (Chapter 3). Individual heterogeneity also is partly genetically determined. I showed that genetic variation in growth potential to be large (Chapter 4), and that fast growers keep on growing fast under the stress of leaf loss (Chapters 4,5). Therefore it is likely that genetic variation contributes to long-term differences between individuals. Genetic variation for tolerance and compensatory responses was estimated to be low (Chapter 4), suggesting that the adaptive potential of our study population to changes in disturbance events that entail leaf loss might be low. I also showed that super-performing individuals are much more important for the growth of the population (Chapter 2) and that individuals that are important for future production could be used to improve the management of natural populations (Chapter 5).
This study provides improved insight into the extent of individual heterogeneity in a long-lived plant species and its environmental and genetic drivers, and clearly shows the importance of individual heterogeneity and its drivers for population processes and management practices. It also presents methods on how persistent performance differences between individuals can be incorporated into demographic tools, how these can be used to analyze individual contributions to population dynamics, to extrapolate short-term to long–term environmental effects, and to analyze smart harvesting scenarios that take differences between individuals into account. These results indicate that individual heterogeneity, underlying environmental and genetic drivers, and population processes are all related. Therefore, when evaluating the effect of environmental change on population processes, and in the design of management schemes, it is important to keep these relations in mind. The methodological tools that we presented provide a means of doing this.
Multidimensional remote sensing based mapping of tropical forests and their dynamics
Dutrieux, L.P. - \ 2016
Wageningen University. Promotor(en): Martin Herold, co-promotor(en): Lammert Kooistra; Lourens Poorter. - Wageningen : Wageningen UR - ISBN 9789462578906 - 146
tropical forests - remote sensing - mapping - biodiversity - forest structure - monitoring - land use - landsat - tropische bossen - remote sensing - cartografie - biodiversiteit - bosstructuur - monitoring - landgebruik - landsat
Tropical forests concentrate a large part of the terrestrial biodiversity, provide important resources, and deliver many ecosystem services such as climate regulation, carbon sequestration, and hence climate change mitigation. While in the current context of anthropogenic pressure these forests are threatened by deforestation, forest degradation and climate change, they also have shown to be, in certain cases, highly resilient and able to recover from disturbances. Quantitative measures of forest resources and insights into their dynamics and functioning are therefore crucial in this context of climate and land use change. Sensors on-board satellites have been collecting a large variety of data about the surface of the earth in a systematic and objective way, making remote sensing a tool that holds tremendous potential for mapping and monitoring the earth. The main aim of this research is to explore the potential of remote sensing for mapping forest attributes and dynamics. Tropical South America, which contains the largest area of tropical forest on the planet, and is therefore of global significance, is the regional focus of the research. Different methods are developed and assessed to: (i) map forest attributes at national scale, (ii) detect forest cover loss, (iii) quantify land use intensity over shifting cultivation landscapes, and (iv) measure spectral recovery and resilience of regrowing forests.
Remote sensing data are diverse and multidimensional; a constellation of satellite sensors collects data at various spatial, temporal and spectral resolutions, which can be used to inform on different components of forests and their dynamics. To better map and monitor ecological processes, which are inherently multidimensional, this thesis develops methods that combine multiple data sources, and integrate the spatial, temporal and spectral dimensions contained in remote sensing datasets. This is achieved for instance by assembling time-series to fully exploit the temporal signal contained in the data, or by working with multiple spectral channels as a way to better capture subtle ecological features and processes.
After introducing the general objectives of the thesis in Chapter 1, Chapter 2 presents an approach for mapping forest attributes at national scale. In this chapter, 28 coarse resolution remote sensing predictors from diverse sources are used in combination with in-situ data from 220 forest inventory plots to predict nine forest attributes over lowland Bolivia. The attributes include traditional forest inventory variables such as forest structure, floristic properties, and abundance of life forms. Modelling is done using the random forest approach and reasonable prediction potential was found for variables related to floristic properties, while forest attributes relating to structure had a low prediction potential. This methodological development demonstrates the potential of coarse resolution remote sensing for scaling local in-situ ecological measurements to country-wide maps, thus providing information that is highly valuable for biodiversity conservation, resource use planning, and for understanding tropical forest functioning.
Chapter 3 presents an approach to detect forest cover loss from remote sensing time-series. While change detection has been the object of many studies, the novel contribution of the present example concerns the capacity to detect change in environments with strong inter-annual variations, such as seasonally dry tropical forests. By combining Landsat with Moderate Resolution Imaging Spectroradiometer (MODIS) time-series in a change detection framework, the approach provides information at 30 m resolution on forest cover loss, while normalizing for the natural variability of the ecosystem that would otherwise be detected as change. The proposed approach of combining two data streams at different spatial resolutions provides the opportunity to distinguish anthropogenic disturbances from natural change in tropical forests.
Chapter 4 introduces a new method to quantify land use intensity in swidden agriculture systems, using remote sensing time-series. Land use intensity — a parameter known for influencing forest resilience — is retrieved in this case by applying a temporal segmentation algorithm derived from the econometrics field and capable of identifying shifts in land dynamic regimes, to Landsat time-series. These shifts, or breakpoints, are then classified into the different events of the swidden agriculture cycle, which allows to quantify the number of cultivation cycles that has taken place for a given agricultural field. The method enables the production of objective and spatially continuous information on land use intensity for large areas, hence benefiting the study of spatio-temporal patterns of land use and the resulting forest resilience. The results were validated against an independent dataset of reported cultivation frequency and proved to be a reliable indicator of land use intensity.
Chapter 5 further explores the concept of forest resilience. A framework to quantify spectral recovery time of forests that regrow after disturbance is developed, and applied to regrowing forests of the Amazon. Spatial patterns of spectral resilience as well as relations with environmental conditions are explored. Regrowing forests take on average 7.8 years to recover their spectral properties, and large variations in spectral recovery time occur at a local scale. This large local variability suggests that local factors, rather than climate, drive the spectral recovery of tropical forests. While spectral recovery times do not directly correspond to the time required for complete recovery of the biomass and species pool of tropical forests, they provide an indication on the kinetics of the early stages of forest regrowth.
Chapter 6 summarizes the main findings of the thesis and provides additional reflections and prospects for future research. By predicting forest attributes country-wide or retrieving land use history over the 30 years time-span of the Landsat archive, the developed methods provide insights at spatial and temporal scales that are beyond the reach of ground based data collection methods. Remote sensing was therefore able to provide valuable information for better understanding, managing and conserving tropical forest ecosystems, and this was partly achieved by combining multiple sources of data and taking advantage of the available remote sensing dimensions. However, the work presented only explores a small part of the potential of remote sensing, so that future research should intensively focus on further exploiting the multiple dimensions and multi-scale nature of remote sensing data as a way to provide insights on complex multi-scale processes such as interactions between climate change, anthropogenic pressure, and ecological processes. Inspired by recent advances in operational forest monitoring, operationalization of scientific methods to retrieve ecological variables from remote sensing is also discussed. Such transfer of scientific advances to operational platforms that can automatically produce and update ecologically relevant variables globally would largely benefit ecological research, public awareness and the conservation and wise use of natural resources.
Biodiversity and the functioning of tropical forests
Sande, M.T. van der - \ 2016
Wageningen University. Promotor(en): Lourens Poorter, co-promotor(en): Marielos Pena Claros; Eric Arets. - Wageningen : Wageningen University - ISBN 9789462578029 - 282
tropical forests - biodiversity - forest ecology - forest management - climatic change - tropische bossen - biodiversiteit - bosecologie - bosbedrijfsvoering - klimaatverandering
Tropical forests are the most diverse terrestrial ecosystems. Moreover, their capacity for removal of carbon from the atmosphere makes them important for climate change mitigation. Theories predict that species use resources in a different way, and therefore high species diversity would result in more efficient resource use and higher total carbon removal. These theories, however, have yet not been clearly demonstrated for tropical forests. In this thesis, I evaluated how biodiversity of plants and their traits influenced carbon removal. I used data collected in different tropical forest types and at different spatial and temporal scales. I found that biodiversity was important for carbon removal especially at large spatial scales (e.g. the Amazon) where biodiversity varies strongly, and at long temporal scales (e.g. >200 years) where high biodiversity functions as a buffer for changing environmental conditions. In this way biodiversity contributes to long-term stable forests and a safe climate.
Remote sensing of land use and carbon losses following tropical deforestation
Sy, V. de - \ 2016
Wageningen University. Promotor(en): Martin Herold, co-promotor(en): Jan Clevers; L. Verchot. - Wageningen : Wageningen University - ISBN 9789462578036 - 142
remote sensing - tropical forests - land use - carbon - losses - environmental degradation - forest monitoring - remote sensing - tropische bossen - landgebruik - koolstof - verliezen - milieuafbraak - bosmonitoring
The new Paris Agreement, approved by 195 countries under the auspice of the United Nations Framework Convention on Climate Change (UNFCCC), calls for limiting global warming to “well below" 2°Celsius. An important part of the climate agreement relates to reducing emissions from deforestation and forest degradation, and enhancing carbon stocks (REDD+) in non-Annex I (mostly developing) countries. Over the last decades the growing demand for food, fibre and fuel has accelerated the pace of forest loss. In consequence, tropical deforestation and forest degradation are responsible for a large portion of global carbon emissions to the atmosphere, and destroy an important global carbon sink that is critical in future climate change mitigation.
Within the REDD+ framework, participating countries are given incentives to develop national strategies and implementation plans that reduce emissions and enhance sinks from forests and to invest in low carbon development pathways. For REDD+ activities to be effective, accurate and robust methodologies to estimate emissions from deforestation and forest degradation are crucial. Remote sensing is an essential REDD+ observation tool, and in combination with ground measurements it provides an objective, practical and cost-effective solution for developing and maintaining REDD+ monitoring systems. The remote sensing monitoring objective for REDD+ is not only to map deforestation but also to support policy formulation and implementation. Identifying and addressing drivers and activities causing forest carbon change is crucial in this respect. Despite the importance of identifying and addressing drivers, quantitative information on these drivers, and the related carbon emissions, is scarce at the national level.
The main objective of this thesis is to explore the role of remote sensing for monitoring tropical forests for REDD+ in general, and for assessing land use and related carbon emissions linked to drivers of tropical deforestation in particular. To achieve this, this thesis investigates the following research questions:
What is the current role and potential of remote sensing technologies and methodologies for monitoring tropical forests for REDD+ and for assessing drivers of deforestation?
What is the current state of knowledge on drivers of deforestation and degradation in REDD+ countries?
What are land use patterns and related carbon emissions following deforestation, capitalising on available land use and biomass remote sensing data?
The research conducted in this PhD thesis contributes to the understanding of the role of remote sensing in forest monitoring for REDD+ and in the assessment of drivers of deforestation. In addition, this thesis contributes to the improvement of spatial and temporal quantification of land use and related carbon emissions linked to drivers of tropical deforestation. The results and insights described herein are valuable for ongoing REDD+ forest monitoring efforts and capacity development as REDD+ moves closer to becoming an operational mitigation mechanism.
Conservation genetics of the frankincense tree
Bekele, A.A. - \ 2016
Wageningen University. Promotor(en): Frans Bongers, co-promotor(en): Rene Smulders; K. Tesfaye Geletu. - Wageningen : Wageningen University - ISBN 9789462576865 - 158
boswellia - genomes - dna sequencing - tropical forests - genetic diversity - genetic variation - genetics - forest management - plant breeding - boswellia - genomen - dna-sequencing - tropische bossen - genetische diversiteit - genetische variatie - genetica - bosbedrijfsvoering - plantenveredeling
Boswellia papyrifera is an important tree species of the extensive Combretum-Terminalia dry tropical forests and woodlands in Africa. The species produces a frankincense which is internationally traded because of its value as ingredient in cosmetic, detergent, food flavor and perfumes productions, and because of its extensive use as incense during religious and cultural ceremonies in many parts of the world. The forests in which B. papyrifera grows are increasingly overexploited at the expense of the economic benefit and the wealth of ecological services they provide. Populations of B. papyrifera have declined in size and are increasingly fragmented. Regeneration has been blocked for the last 50 years in most areas and adult productive trees are dying. Projections showed a 90% loss of B. papyrifera trees in the coming 50 years and a 50% loss of frankincense production in 15 years time.
This study addressed the conservation genetics of B. papyrifera. Forty six microsatellite (SSR) markers were developed for this species, and these genetic markers were applied to characterize the genetic diversity pattern of 12 B. papyrifera populations in Ethiopia. Next to this, also the generational change in genetic diversity and the within-population genetic structure (FSGS) of two cohort groups (adults and seedlings) were studied in two populations from Western Ethiopia. In these populations seedlings and saplings were found and natural regeneration still takes place, a discovery that is important for the conservation of the species.
Despite the threats the populations are experiencing, ample genetic variation was present in the adult trees of the populations, including the most degraded populations. Low levels of population differentiation and isolation-by-distance patterns were detected. Populations could be grouped into four genetic clusters: the North eastern (NE), Western (W), North western (NW) and Northern (N) part of Ethiopia. The clusters corresponded to environmentally different conditions in terms of temperature, rainfall and soil conditions. We detected a low FSGS and found that individuals are significantly related up to a distance of 60-130 m.
Conservation of the B. papyrifera populations is urgently needed. The regeneration bottlenecks in most existing populations are an urgent prevailing problem that needs to be solved to ensure the continuity of the genetic diversity, species survival and sustainable production of frankincense. Local communities living in and around the forests should be involved in the use and management of the forests. In situ conservation activities will promote gene flow among fragmented populations and scattered remnant trees, so that the existing level of genetic diversity may be preserved. Geographical distance among populations is the main factor to be considered in sampling for ex situ conservation. A minimum of four conservation sites for B. papyrifera is recommended, representing each of the genetic clusters. Based on the findings of FSGS analyses, seed collection for ex situ conservation and plantation programmes should come from trees at least 100 m, but preferably 150 m apart.
Tropical forests in a changing world
Zuidema, P.A. - \ 2015
Wageningen : Wageningen University - ISBN 9789462573765 - 24
tropical forests - forests - forest ecology - climatic change - forest management - tropische bossen - bossen - bosecologie - klimaatverandering - bosbedrijfsvoering
Monitoring tropical forest dynamics using Landsat time series and community-based data
DeVries, B.R. - \ 2015
Wageningen University. Promotor(en): Martin Herold, co-promotor(en): Lammert Kooistra; Jan Verbesselt. - Wageningen : Wageningen University - ISBN 9789462574762 - 161
tropische bossen - bosdynamiek - monitoring - landsat - satellieten - tijdreeksen - remote sensing - tropical forests - forest dynamics - monitoring - landsat - satellites - time series - remote sensing
Tropical forests cover a significant portion of the earth's surface and provide a range of
ecosystem services, but are under increasing threat due to human activities. Deforestation
and forest degradation in the tropics are responsible for a large share of global CO2
emissions. As a result, there has been increased attention and effort invested in the
reduction of emission from deforestation and degradation and the protection of remaining
tropical forests in recent years. Methods for tropical forest monitoring are therefore vital
to track progress on these goals. Two data streams in particular have the potential to
play an important role in forest monitoring systems. First, satellite remote sensing is
recognized as a vital technology in supporting the monitoring of tropical forests, of which
the Landsat family of satellite sensors has emerged as one of the most important. Owing
to its open data policy, a large range of methods using dense Landsat time series have
been developed recently which have the potential to greatly enhance forest monitoring
in the tropics. Second, community-based monitoring is supported in many developing
countries as a way to engage forest communities and lower costs of monitoring activities.
The development of operational monitoring systems will need to consider how these data
streams can be integrated for the effective monitoring of forest dynamics.
This thesis is concerned with the monitoring of tropical forest dynamics using a combi-
nation of dense Landsat time series and community-based monitoring data. The added
value conferred by these data streams in monitoring deforestation, degradation and re-
growth in tropical forests is assessed. This goal is approached from two directions. First,
the application of econometric structural change monitoring methods to Landsat time
series is explored and the efficacy and accuracy of these methods over several tropical
forest sites is tested. Second, the integration of community-based monitoring data with
Landsat time series is explored in an operational setting. Using local expert monitoring
data, the reliability and consistency of these data against very high resolution optical
imagery are assessed. A bottom-up approach to characterize forest change in high the-
matic detail using a priori community-based observations is then developed based on
Chapter 2 presents a robust data-driven approach to detect small-scale forest disturbances
driven by small-holder agriculture in a montane forest in southwestern Ethiopia. The
Breaks For Additive Season and Trend Monitoring (BFAST Monitor) method is applied
to Landsat NDVI time series using sequentially defined one-year monitoring periods. In
addition to time series breakpoints, the median magnitude of residuals (expected versus
observed observations) is used to characterize change. Overall disturbances are mapped
with producer's and user's accuracies of 73%. Using ordinal logistic regression (OLR)
models, the extent to which degradation and deforestation can be separately mapped is
explored. The OLR models fail to distinguish between deforestation and degradation,
however, owing to the subtle and diffuse nature of forest degradation processes.
Chapter 3 expands upon the approach presented in Chapter 2 by tracking post-disturbance
forest regrowth in a lowland tropical forest in southeastern Peru using Landsat Normalized
Difference Moisture Index (NDMI) time series. Disturbance between 1999 and 2013 are
mapped using the same sequential monitoring method as in Chapter 2. Pixels where
disturbances are detected are then monitored for follow-up regrowth using the reverse of
the method employed in Chapter 2. The time of regrowth onset is recorded based on a
comparison to defined stable history period. Disturbances are mapped with 91% accuracy,
while post-disturbance regrowth is mapped with a total accuracy of 61% for disturbances
Chapter 4 and 5 explore the integration of community-based forest monitoring data and
remote sensing data streams. Major advantages conferred by community-based forest dis-
turbance observations include the ability to report on drivers and other thematic details
of forest change and the ability to detect low-level forest degradation before these changes
are visible above the forest canopy. Chapter 5 builds on these findings and presents a
novel bottom-up approach to characterize forest changes using local expert disturbance
reports to calibrate and validate forest change models based on Landsat time series. Using
random forests and a selection of Landsat spectral and temporal metrics, models describ-
ing forest state variables (deforested, degraded or stable) at a given time are produced.
As local expert data are continually acquired, the ability of these models to predict forest
degradation are shown to improve.
Chapter 6 summarizes the main findings of the thesis and provides a future outlook, given
the prospect of increasing availability of satellite and in situ data for tropical forest mon-
itoring. This chapter argues that forest change methods should strive to utilize satellite
time series and ground data to their maximum potential. As “big data" emerges in the
field of earth observation, new data streams need to be accommodated in monitoring
methods. Operational forest monitoring systems that are able to integrate such diverse
data streams can support broader forest monitoring goals such as quantitative monitoring
of forest dynamics. Even with a wealth of time series based forest disturbance methods
developed recently, forest monitoring systems require locally calibrated forest change esti-
mates with higher spatial, temporal and thematic resolution to support a variety of forest
Interactive community-based tropical forest monitoring using emerging technologies
Pratihast, A.K. - \ 2015
Wageningen University. Promotor(en): Martin Herold, co-promotor(en): L. Ribbe; Sytze de Bruin; Valerio Avitabile. - Wageningen : Wageningen University - ISBN 9789462574786 - 164
tropische bossen - bosmonitoring - remote sensing - satellietbeelden - monitoring - technologie - sociale netwerken - geografische informatiesystemen - participatie - tropical forests - forest monitoring - remote sensing - satellite imagery - monitoring - technology - social networks - geographical information systems - participation
Forests cover approximately 30% of the Earth’s land surface and have played an indispensable role in the human development and preserving natural resources. At the moment, more than 300 million people are directly dependent on these forests and their resources. Forests also provide habitats for a wide variety of species and offer several ecological necessities to natural and anthropological systems. In spite of this importance, unprecedented destruction of tropical forest cover has been witnessed over the past four decades. Annually, approximately 2.1x105 hectares of forests are lost, with serious negative consequences on the regulation of the world’s climate cycle, biodiversity and other environmental variables. To mitigate these consequences, the United Nations Framework Convention on Climate Change (UNFCCC) has requested the developing countries to adapt new policy in reducing emissions from deforestation and forest degradation (REDD+). Under this policy, countries have been mandated to engage local communities and indigenous groups as critical stakeholders in the design and implementation of a national forest monitoring system (NFMS) that supports measuring, reporting and verification (MRV) of actions and achievements of REDD+ activities.
Current schemes for tropical monitoring are based on remote sensing and field measurements which typically originate from national forest inventories. Remotely sensed imagery has been considered as the principal data source used to calculate forest area change across large areas, assess rates of deforestation and establish baselines for national forest area change databases. Advancements in medium and high resolution satellites, open data policies, time-series analysis methods and big data processing environments are considered valuable for deforestation monitoring at local to global scales. However, cloud cover, seasonality and the restricted spatial and temporal resolution of remote sensing observations limits their applicability in the tropics. Enhancing the interpretation of remote sensing analysis require substantial ground verification and validation. Accomplishing these tasks through national forest inventory data is expensive, time-consuming and difficult to implement across large spatial scales.
Next to remote sensing, community-based monitoring (CBM) has also demonstrated potential in the collection and interpretation of forest monitoring data. However effective implementation of community-based forest monitoring systems is currently lacking due to two reasons: 1) the role of communities in NFMS is unclear and 2) tools that can support local communities to explore opportunities and facilitate forest monitoring are still scarce. This thesis addresses these two issues by proposing technical solutions (computer and geo-information science) and assessing the capacities and needs of communities in developing countries with a REDD+ implementation and forest monitoring context.
The main goal of this thesis, therefore, is to develop an approach that combines emerging technologies and community-based observations for tropical forest monitoring. To accomplish the main goal, four specific research questions were formulated: 1) What are the potentials to link community-based efforts to national forest monitoring systems? 2) How can information and communication technologies (ICTs) support the automation of community data collection process for monitoring forest carbon stocks and change activities using modern handheld devices? 3) What is the accuracy and compatibility of community collected data compared to other data (e.g., optical remote sensing and expert field measurements) for quantifying forest carbon stocks and changes? and 4) What is a suitable design for an interactive remote sensing and community-based near real-time forest change monitoring system and how can such system be operationalized?
In Chapter 2, scientific literature and 28 readiness preparation proposals from the World Bank Forest Carbon Partnership Facility are reviewed to better define the role and technical conditions for CBM. Based on this review, a conceptual framework was developed under which CBM can contribute as a dedicated and independent stream of measuring and monitoring data to national level forest monitoring efforts. The following chapters are built upon this framework.
Chapter 3 describes a process of designing and implementing an integrated data collection system based on mobile devices that streamlines the community-based forest monitoring data collection, transmission and visualization process. The usability of the system is evaluated in the Tra Bui commune, Quang Nam province, Central Vietnam, where forest carbon and change activities were measured by different means such as local, regional and national experts and high resolution satellite imagery. The results indicate that the local communities were able to provide forest carbon measurements with accuracy comparable to that of expert measurements at lower costs. Furthermore, the results show that communities are more effective in detecting small scale forest degradation caused by subsistence fuelwood collection and selective logging than image analysis using SPOT imagery.
To support the findings of chapter 3, the data acquisition form (mostly activity data related to forest change) for mobile device was further improved in chapter 4. The system was tested by thirty local experts in the UNESCO Kafa Biosphere Reserve, Ethiopia. High resolution satellite imagery and professional measurements were combined to assess the accuracy and complementary use of local datasets in terms of spatial, temporal and thematic accuracy. Results indicate that the local communities were capable of describing processes of change associated with deforestation, forest degradation and reforestation, in terms of their spatial location, extent, timing and causes within ten administrative units. Furthermore, the results demonstrate that communities offer complementary information to remotely sensed data, particularly to signal forest degradation and mapping deforestation over small areas. Based on this complementarity, a framework is proposed for integrating local expert monitoring data with satellite-based monitoring data into a NFMS in support of REDD+ MRV and near real-time forest change monitoring.
Having identified the framework for integrated monitoring systems in chapter 4, chapter 5 describes an interactive web-based forest monitoring system using four levels of geographic information services: 1) the acquisition of continuous data streams from satellite and community-based monitoring using mobile devices, 2) near real-time forest disturbance detection based on satellite time-series, 3) presentation of forest disturbance data through a web-based application and social media and 4) interaction of the satellite-based disturbance alerts with the end-user communities to enhance the collection of ground data. The system was developed using open source technologies and has been implemented together with local experts in UNESCO Kafa Biosphere Reserve, Ethiopia. The results show that the system was able to provide easy access to information on forest change and considerably improve the collection and storage of ground observation by local experts. Social media lead to higher levels of user interaction and noticeably improved communication among stakeholders. Finally, an evaluation of the system confirmed its usability in Ethiopia.
Chapter 6 presents the final conclusions and provides recommendations for further research. The overall conclusion is that the emerging technologies, such as smartphones, Web-GIS and social media, incorporated with user friendly interface improve the interactive participation of local communities in forest monitoring and decrease errors in data collection. The results show that CBM can provide data on forest carbon stocks, forest area changes as well as data that help to understand local drivers of emissions. The thesis also shows, in theory and in practice, how local data can be used to link with medium and high resolution remote sensing satellite images for an operational near real-time forest monitoring system at a local scale. The methods presented in this thesis are applicable to a broader geographic scope. Hence, this thesis emphasizes that policies and incentives should be implemented to empower communities and to create institutional frameworks for community-based forest monitoring in the tropics.
Inauguratie Pieter Zuidema
Zuidema, P.A. - \ 2015
tropische bossen - ontbossing - ecosysteemdiensten - bosfragmentatie - klimaatverandering - bosbouweconomie - bosexploitatie - tropical forests - deforestation - ecosystem services - forest fragmentation - climatic change - forest economics - forest exploitation
Hoogleraar Pieter Zuidema vertelt over zijn onderzoek naar invloed van global change op tropische bossen.
Perspectives for sustainable Prunus africana production and trade. Factsheets
Ingram, V.J. ; Loo, J. van; Dawson, I. ; Vinceti, B. ; Duminil, J. ; Muchugi, A. ; Awono, A. ; Asaah, E. - \ 2015
Wageningen : LEI Wageningen UR (LEI Factsheet 2015-102) - 10
prunus africana - tropische bossen - afrika - duurzaamheid (sustainability) - medicinale planten - internationale handel - overheidsbeleid - prunus africana - tropical forests - africa - sustainability - medicinal plants - international trade - government policy
This brief documents current knowledge about pygeum (Prunus africana). It aims to inform decision makers in governments in producing and consumer countries, international and civil society organisations and researchers, about sustainable (international) trade and governance of the species.
Combining SAR and optical satellite image time series for tropical forest monitoring
Reiche, J. - \ 2015
Wageningen University. Promotor(en): Martin Herold, co-promotor(en): Dirk Hoekman; Jan Verbesselt. - Wageningen : Wageningen University - ISBN 9789462573130 - 151
satellietbeelden - satellieten - satellietkarteringen - tropische bossen - bosmonitoring - tijdreeksen - satellite imagery - satellites - satellite surveys - tropical forests - forest monitoring - time series
Tropical forests are the largest of the global forest biomes and play a crucial role in the global carbon, hydrological and biochemical cycles. Increasing demand for resources rapidly increases the pressure on tropical forests. As a result tropical regions have been undergoing rapid changes in forest cover in recent decades. These changes are the second largest contributor of greenhouse gas emissions in the atmosphere. Spatially and timely consistent detection of tropical deforestation and forest degradation is fundamental to reliably estimate greenhouse gas emissions, and to successfully implement climate mechanisms like reducing emissions from deforestation and forest degradation (REDD+).
To assess historical and future changes in forest cover, satellite remote sensing at medium resolution scale constitutes a powerful tool. Reviewing satellite-based optical and Synthetic Aperture Radar (SAR) efforts for tropical forest monitoring revealed that operationalised optical-based approaches exist, but frequent cloud cover limits their applicability in the tropics. SAR remote sensing has also demonstrated its capability, but the observation frequency of SAR imagery and appropriate time series methods are limited. Research has indicated there is potential for multi-sensor approaches to overcome the limitations of the single-sensors, but so far developments are restricted to mapping approaches. This thesis addressed the need for advancing multi-sensor methods that combine time series imagery from medium resolution SAR and optical satellites to improve tropical forest monitoring. The main scientific contributions include the introduction of three novel SAR-optical approaches, two of them capable of exploiting the full observation density of time series. Furthermore, an approach for multi-model land cover dependent SAR slope correction was proposed.
Chapter 2 introduced an approach for feature level fusing of multi-temporal L-band SAR and optical forest disturbance information. Using Landsat and ALOS PALSAR imagery from 2007 and 2010, we applied the approach to map forest land cover and to detect deforestation and forest degradation of a persistently cloud covered mining region in Central Guyana. By making use of the complementarities of Landsat and ALOS PALSAR, we demonstrated the reduction of Landsat (cloud cover, Landsat 7 scan line corrector error) and PALSAR data gaps (SAR layover and shadow in mountainous area) to a negligible amount.
Chapter 3 described a practical approach for multi-model land cover dependent slope correction of SAR images that can handle a wide range of terrain and topographic conditions. We corrected ALOS PALSAR images of two topographically complex sites in Fiji (study site of Chapter 4 and 5) and Brazil and showed that the remaining slope effects for the multi-model case are marginal for all land cover types. Particularly, this improves the detection of forest degradation and biomass changes. Considering the large change in the L-band backscatter signal caused by the removal of forest, however, remaining slope effects are already sufficiently small after applying a single-model approach already.
Chapter 4 presented a novel multi-sensor time series correlation and fusion (MulTiFuse) approach that was applied to fuse Landsat NDVI and ALOS PALSAR time series. The fused Landsat-PALSAR time series was used in a change detection framework to detect deforestation at a managed forest site in Fiji for the period 01/2008 - 09/2010. We tested the impact of persistent cloud cover in the tropics by increasing the per-pixel missing data percentage of the Landsat time series in a stepwise manner. The results were evaluated against three-monthly reference data that covered the entire study area. For the Landsat-only case, a very strong decrease in spatial and temporal accuracies were observed for increasing Landsat missing data. This highlights the vulnerability of tropical forest monitoring systems that rely only on optical data. In contrast, the results for the fused Landsat-PALSAR case remained high with increasing missing data and were observed to be always above the accuracies for the Landsat- and PALSAR-only cases.
To address the need for near real-time monitoring systems at medium resolution scale, Chapter 5 introduced a Bayesian change detection approach to combine SAR and optical time series for near real-time deforestation detection. We applied the approach in a simulated near real-time scenario using Landsat NDVI and ALSO PALSAR time series already used in Chapter 4. Once a new image of either of the two time series was available, the probability of deforestation was calculated immediately and deforestation events were indicated. These near real-time capabilities are essential to support timely action against illegal forest activities. Spatial and temporal accuracies for the fused Landsat-PALSAR case were consistently higher than those of the Landsat- and PALSAR-only cases, even for increasing Landsat missing data.
With these studies we demonstrated the potential of SAR-optical time series approaches to use the historical and upcoming streams of medium resolution optical and SAR satellite image time series for improving forest monitoring in the tropics.
|Ecology of lianas
Schnitzer, S.A. ; Bongers, F. ; Burnham, R.J. ; Putz, F.E. - \ 2015
Oxford : Wiley-Blackwell - ISBN 9781118392492 - 504
klimplanten - plantenecologie - plantenanatomie - plantenfysiologie - evolutie - tropische bossen - bossen - climbing plants - plant ecology - plant anatomy - plant physiology - evolution - tropical forests - forests
A liana is a long-stemmed, woody vine that is rooted in the soil at ground level and uses trees to climb up to the canopy to get access to well-lit areas of the forest. The main goal of this book is to present the current status of liana ecology in tropical and temperate forests. In essence, it is a forum to summarize and synthesize the most recent research in liana ecology and to address how this research fits into the broader field of ecology.
Long-term trends in tropical tree growth: a pantropical study
Groenendijk, P. - \ 2015
Wageningen University. Promotor(en): Pieter Zuidema; Frans Bongers. - Wageningen : Wageningen University - ISBN 9789462572362 - 244
bosbomen - tropische bossen - bomen - groei - jaarringen - bosecologie - bosbedrijfsvoering - centraal-afrika - forest trees - tropical forests - trees - growth - growth rings - forest ecology - forest management - central africa
Tropical forests cover only 7% of the earth’s land surface, but harbour almost half of the world’s biodiversity. These forests also provide many ecosystem services, such as the storage of carbon and the regulation of local and regional climate, and many goods such as timber and fruits. Furthermore, tropical forests contribute disproportionately to the global carbon cycle, storing an estimated 25% of the carbon stocks on land and accounting for a third of the terrestrial net primary productivity. Therefore, changes in forest cover or in the net uptake or loss of carbon by forests directly influences the global carbon cycle. Tropical forests are under increasing anthropogenic pressure and are undergoing rapid changes due to deforestation, conversion to other land uses and logging. Additionally, there is evidence that pristine and intact tropical forests are undergoing changes due to the effects of climate change. Concerted increases in biomass and tree growth have been found in studies monitoring intact tropical forests, suggesting that they acted as considerable carbon sinks over the past decades. On the other hand, decreasing or fluctuating forest growth and biomass have also been noted. These different changes have been attributed to different climatic drivers: growth increases have been suggested to arise from growth stimulation by increasing atmospheric CO2 concentrations, while growth decreases have been interpreted to reflect the limiting effects of increased temperature on growth. As monitoring plots usually cover only a few decades, it is still unclear whether these changes are pervasive or whether they simply reflect the effect of short-term climatic fluctuations on tree growth. Assessing whether changes have occurred over centennial scales is thus crucial to understanding whether and how tropical forests are reacting to climatic changes.
In this thesis we apply tree-ring analysis on a pantropical study to assess longterm changes in growth of tropical forest trees. Tree-ring analysis was used to measure long-term growth rates of ~1350 trees of different species coming from three sites across the tropics. Trends in growth over the last two centuries were then analysed using an established an a new trend-detection method. Additionally, we applied the long-term growth data from rings to improve the evaluation of forest management practices in Cameroon. All samples were collected and measured within the TROFOCLIM project led by Pieter Zuidema. The project also includes two other PhD theses and sample collection was divided among the three PhD projects and the three sites: in Bolivia (samples collected by Peter van der Sleen), Cameroon (by me) and in Thailand (by Mart Vlam). The main objectives of this thesis were: (1) to assess the potential for using treerings in a wet tropical forest in Central Africa; (2) to project timber yields in the next logging round for four Cameroonian tree species; (3) to evaluate the sensitivity and accuracy of four commonly used methods to detect long-term trends in tree-ring data; and (4) to detect whether growth rates of tropical forest trees have changed over the past ~150 years.
In Chapter 2 of this dissertation, we evaluated whether growth rings are formed annually in the wood of tree species growing under very high levels of precipitation (>4000 mm) in an African tropical forest. For this purpose, we assessed whether ring structures are formed in the wood of the 22 commercially exploited tree species and found that ring structures are indeed formed by more than half of these species (in 14 species), though with varying ring clarity. On four species we proved the annual character of ring formation using radiocarbon bomb-peak dating. That rings are formed under such high levels of precipitation is surprising, as these conditions are considered improper to induce ring formation. These results suggest that the potential of tree-rings analysis is more or less similar across the tropics. Based on our results and that of other studies, we estimate that tree rings can be used to measure tree growth and ages for around a quarter to a third of tropical tree species.
Worldwide, over 400 million hectares of tropical forests are set aside for timber production. Attaining sustainable use of these forests is very important, in the light of the important role of tropical forests in retaining biodiversity and storing carbon. Ensuring that timber species are not overexploited is key to ensure that forest use is sustainable and entails finding a balance between economic gains and the (ecological) sustainability of logging operations. In Chapter 3, we integrated growth data from tree-rings with logging inventory data to forecast whether timber yields can be sustained in the next harvest round for four timber species in Cameroon. Under current logging practices, future logging yields were predicted to reduce moderately to strongly for all species. These yield reductions are worrisome for forest conservation, as loss of economic value may lead to conversion of forests to other land uses. We recommend that such calculations are needed for more species and argue that these simulations should include the effects of logging and eventual silvicultural measures on the growth and survival of trees.
Lifetime tree growth data – as acquired by tree-ring analysis – contains longterm trends in growth that reflect the ontogenetic development of an individual or species, i.e., these data contains an age/size signal in growth. In Chapter 4 we evaluate the sensitivity, accuracy and reliability to detect long-term trends in growth of four methods that are commonly used to disentangle these age/size trends from long-term growth trends. We applied these growth-trend detection methods to measured growth data from tree rings and to simulated growth trajectories on which increasing an decreasing trends were imposed. The results revealed that the choice of method influences results of growth-trend studies. We recommend applying two methods simultaneously when analysing long-term trends – the Regional Curve Standardization and Size Class Isolation – as these methods are complementary and showed the highest reliability to detecting long-term growth changes.
In Chapter 5, we analysed long-term growth trends in tropical forest trees using a pantropical approach applying the two recommended growth-trend detection methods. We showed that growth rates for most of the 13 tropical tree species, from the three sites across the tropics, decreased over the last centuries. These species-level changes may have important demographic consequences and may eventually lead to shifts in the species composition of tropical forests. We found no strong growth changes when analysing trends aggregated per site or across sites: only weak growth reductions were detected for the Thai site and across sites. These findings contrast growth increases that would be expected if tree growth is stimulated by increased ambient CO2. These growth reductions suggest worsening growth conditions for several tropical tree species, and could result from the negative effect of temperature increases on tree growth, or reflect the effect of large-scale disturbances on these forests.
If one image becomes clear from this thesis it is that long-term data are crucial to enhance the management of tropical forests and to quantify changes happening in these forests. Tree-ring analysis provides this long-term perspective for tree growth and is thus a great tool to evaluate changes in the growth of trees, including for tropical species. One of the most important finding of this thesis is that many tropical species show long-term decreases in growth. These results suggest that the commonly assumed growth increases tropical forests, based on measurements over the last couple of decades, may be incorrect. This discrepancy in results could have strong consequences, among others leading to erroneous predictions of the carbon dynamics of tropical forests under future climate change. Combining monitoring plot data (to analyse short-term changes in growth and species composition) with remotely sensed data (to accurately determine forest land cover) and with the long-term growth data from tree rings is probably the best way forward to relate recent findings of short-term changes in tree growth and forest biomass to changes over the past centuries. Such integrative approaches are needed to better quantify and understand the effects of climate change on tropical forests.
Functional ecology of tropical forest recovery
Lohbeck, M.W.M. - \ 2014
Wageningen University. Promotor(en): Frans Bongers; M. Martínez-Ramos, co-promotor(en): Lourens Poorter. - Wageningen : Wageningen University - ISBN 9789462571617 - 223
bosecologie - ecologie - tropische bossen - bossen - plantensuccessie - biodiversiteit - bosbedrijfsvoering - forest ecology - ecology - tropical forests - forests - plant succession - biodiversity - forest management
Electronic abstract of the thesis for the library for the acquisitions department of Wageningen UR library (published as a html file so hyperlinks may be included)
In English, one or 2 pages.
Functional ecology of tropical forest recovery
Currently in the tropics, the area of secondary forest exceeds that of mature forest, and the importance of secondary forest will probably continue to increase in the future. Understanding secondary forests’ potential for maintaining biodiversity and critical ecosystem functions is thereby vital. The aim of this study was to mechanistically link tropical forest succession with the recovery of ecosystem functioning after agricultural field abandonment using a trait-based approach. Such an approach makes use of functional traits; components of an organism’s phenotype that are key to assess ecosystem responses to global change drivers, and are at the same time indicators of how organisms drive changes in ecosystem functioning. Trait-based approaches could therefore provide a mechanistic way to scale up from organisms to ecosystems and thereby contribute towards a more predictive biodiversity and ecosystem functioning science. For this study, I made use of secondary forest data from a wet forest region in Chiapas (main study site), that cover the first 3 decades of succession, complemented with data from a dry forest region in Oaxaca, that cover the first 6 decades of succession. Both are tropical regions in Mexico, characterized by high biodiversity levels and rapid forest loss for agricultural expansion.
In this study I found that functional diversity (the range of different functional traits) increases rapidly and functional composition (the weighted average functional trait value) changes directionally with succession (chapter 2 and 3). These reflect changing habitat filters (changing environmental gradients that underlie succession), and also a gradual shift from habitat filtering towards an increasing effect of competitively driven limiting trait similarity (chapter 4 and 5). Such successional changes in community functional properties suggest strong changes in ecosystem functions, however in situ ecosystem function rates were primarily explained by the total amount of biomass present rather than by biodiversity or functional trait properties of secondary forests (chapter 6). Only the more controlled ex situ decomposition rates were additionally significantly influenced by functional diversity and functional composition. When evaluating the identity of species that drive most of the ecosystem functions I found that different functions were largely driven by the same (dominant) species, implying a limited effect of biodiversity for multifunctionality at a given moment in time. This suggests that biodiversity is mainly important for maintaining multifunctional ecosystems across temporal and spatial scales (chapter 7).
Deforestation is a major threat to natural forests and biodiversity, and I recognize that secondary forests are generally a poor substitute of mature forest. Nevertheless, I show that unassisted recovery through natural succession can be rapid, and contribute considerably to maintenance of biodiversity, functional strategies and ecosystem functions. So while protecting the remaining tracts of mature forests is vital, secondary forests are key components of multifunctional human-modified landscapes where synergies between biodiversity, ecosystem functions and human wellbeing can be optimized.
Costs and benefits of a more sustainable production of tropical timber
Arets, E.J.M.M. ; Veeneklaas, F.R. - \ 2014
Wageningen : Wettelijke Onderzoekstaken Natuur & Milieu (WOt-technical report 10) - 57
houtkap - houtproductie - kosten-batenanalyse - bosbeheer - tropische bossen - duurzaamheid (sustainability) - tropisch hout - ecosysteemdiensten - biobased economy - zuid-amerika - zuidoost-azië - logging - timber production - cost benefit analysis - forest administration - tropical forests - sustainability - tropical timbers - ecosystem services - biobased economy - south america - south east asia
This study is part of the TEEB (The Economics of Ecosystems and Biodiversity) study of trade chains, and assessed the impact of harvesting tropical timber on ecosystem services and the costs and benefits of more sustainable production. The costs of implementation and the benefits from increased ecosystem services levels were assessed for two alternatives to conventional selective logging (CL), sustainable forest management (SFM) and forest plantation. The SFM alternative involves certified forest management implementing reduced impact logging techniques. The forest plantation alternative involves high-yield plantations that have a larger impact on ecosystem services than CL on the actual plantation area, but require only an equivalent of 11-42% of the CL area due to the higher yields per unit of area, and thus allows a larger area of primary forest to be conserved. The majority of Dutch imports of tropical timbers are from South America and South East Asia. We conducted separate analyses for South America and South East Asia to account for regional differences in terms of logging practices, timber yields and the extent and value of ecosystem services
Functional traits, drought performance, and the distribution of tree species in tropical forests of Ghana
Amissah, L. - \ 2014
Wageningen University. Promotor(en): Frits Mohren, co-promotor(en): Lourens Poorter; B. Kyereh. - Wageningen : Wageningen University - ISBN 9789462570726 - 196
tropische bossen - bomen - droogteresistentie - plantengeografie - bosecologie - regen - temperatuur - plantenfysiologie - ghana - tropical forests - trees - drought resistance - phytogeography - forest ecology - rain - temperature - plant physiology - ghana
Tropical forests occur along a rainfall gradient where annual amount, the length and intensity of dry season vary and water availability shapes therefore strongly the distribution of tree species. Annual rainfall in West Africa has declined at a rate of 4% per decade, and climate change models predict a further reduction in rainfall and an increase in frequency and intensity of drought. This will have large consequences for the diversity, composition and distribution of tropical tree species. Understanding the factors that shape tree species distribution will help to understand current forest functioning and to predict the potential impact of climate change on forests.
In this thesis, I used a combination of forest inventory data, greenhouse and field experiments to determine the responses of 10-23 species to drought and shade, and analyse the underlying mechanisms. I addressed 4 questions: (1) What is the relative importance of rainfall and temperature on tree species distribution? (2) How do tree species acclimatise to drought and shade in terms of their physiology, morphology, growth and survival? (3) What morphological and physiological traits determine species drought performance and distribution? (4) How do seedling survival, growth and physiology vary between dry and wet forests, and does drought tolerance and growth determine species distribution along the rainfall gradient?
Forest inventory data showed that the distribution of 95% of 20 species was significantly associated with annual rainfall, 60% with rainfall seasonality, 45% with isothermality and 40% with temperature seasonality. Thus, a reduction in annual rainfall, and an increase in frequency and intensity of drought as predicted by climate change models may affect the distribution of many tree species. A greenhouse experiment indicated that shade facilitated the survival of seedlings subjected to drought, rather than reducing it. This contrasts with the trade-off hypothesis that suggests a stronger impact of drought in shade conditions. Across 23 species, I found a trade-off between drought avoidance (by a deciduous leaf habit during drought) and physiological drought tolerance (by having tough and persistent tissues that allow plants to function during drought) strategies. These strategies were closely associated with species’ shade tolerance and growth rates. A suite of functional traits predicted drought survival and tree species position on the rainfall gradient. Across species, drought survival was enhanced by having less biomass allocation to transpiring leaves, a low leaf area per unit plant mass, and by dense and tough leaf and wood tissues that allow plants to function during drought. The field experiment showed that drought survival (and growth) in the dry forest relative to the wet forest correlated negatively with species position on the rainfall gradient. Hence, species that survive and grow relatively well in dry forests are found at the drier end of the rainfall gradient. This suggests that species sensitivity to low water availability determines the distribution of tree species. The predicted increase in drought frequency and intensity may, therefore, cause a shift in the distribution of tree species in tropical forests.
Environmental and physiological drivers of tree growth : a pan-tropical study of stable isotopes in tree rings
Sleen, J.P. van der - \ 2014
Wageningen University. Promotor(en): Pieter Zuidema; Frans Bongers; Niels Anten. - Wageningen : Wageningen University - ISBN 9789461739544 - 174
bomen - groei - plantenfysiologie - jaarringen - milieueffect - isotopen - tropische bossen - tropen - trees - growth - plant physiology - growth rings - environmental impact - isotopes - tropical forests - tropics
Forests in the wet tropics harbour an incredible biodiversity, provide many ecosystem services and regulate climatic conditions on regional scales. Tropical forests are also a major component of the global carbon cycle, storing 25% of the total terrestrial carbon and accounting for a third of net primary production. This means that changes in forest structure and forest cover in the wet tropics will not only affect biodiversity and ecosystem services, but also have implications for the global carbon cycle and – as a result – may speed up or slow down global warming. Deforestation rates are still high in the tropics and have profoundly affected the extent of forests in many countries. Additionally, there are indications that undisturbed and pristine tropical forests are changing. The most notable changes found by the monitoring of permanent forest plots are an increase of tree growth and forest biomass per unit of surface area over the last decades. If this is indeed the case, it would entail that the world’s tropical forests are potentially absorbing a significant fraction of human caused CO2emissions and as such are mitigating global warming. However, increased tree growth and forest biomass have not been found in all studies. Furthermore, it is unknown whether the observed changes in intact forests are part of a long-term change, or merely reflect decadal scale fluctuations. These uncertainties lead to an ongoing debate on whether tree growth and forest biomass have increased in tropical forests and – if so – to what extent. In addition, there is also a scientific discussion on the factor(s) that could underlie the potential changes in tree growth and forest biomass. Possibly, they are caused by an internal driver, like the lasting effect of large scale disturbances in the past, or by external drivers. Possible external factors affecting tropical forest dynamics are (1) climate change (temperature and precipitation), (2) increased nutrient depositions and (3) increased atmospheric CO2concentration.
In this thesis, I investigated the environmental changes that could have formed the basis for changes in tropical tree growth. I used two relatively new tools in tropical forest ecology: tree-ring measurements and stable isotope analyses. Tree-ring widths were measured to obtain long-term information on tree growth. Stable isotopes in the wood of tree rings were analysed to provide information on the environmental and physiological drivers of tree growth changes. This thesis is part of a larger project on the long-term changes in intact forests in the wet tropics (the TroFoClim project, led by Pieter Zuidema) and also includes the PhD theses of Mart Vlam and Peter Groenendijk. In this project, ~1400 trees of 15 species were examined that were collected in three forest sites distributed across the tropics (in Bolivia, Cameroon and Thailand).
For the assessment of long-term changes in growth and stable isotopes, it is important to understand shorter term fluctuations due to forest dynamics (i.e. gap formation), because these interfere with changes on a longer temporal scale. The formation of a gap in a closed canopy forest, after the death of a tree, can cause considerable environmental changes in the surrounding area, e.g. in light, nutrient and water availability. This can strongly affect the growth rates of the remaining trees. However, in most studies the environmental drivers of changes in tree growth after gap formation are not considered. In CHAPTER 2 I measured carbon isotope discrimination (Δ13C) in annual growth rings of Peltogyne cf.heterophylla, from a moist forest in North-eastern Bolivia, and evaluated the environmental drivers of growth responses after gap formation. Growth and Δ13C was compared between the seven years before and after gap formation. Forty-two trees of different sizes were studied, half of which grew close (<10m) to single tree-fall gaps; the other half grew more than 40 m away from gaps (control trees). I found that increased growth was mainly associated with decreased Δ13C suggesting that this response was driven by increased light availability and not by improved water availability. Interestingly, most small trees did not show a growth stimulation after gap formation. This result was hypothesized to be caused by an increased drought stress. However, the measurement of Δ13C showed that increased water stress is unlikely the cause for the absence of increased growth, but rather suggested that light conditions had not improved after gap formation. These results show that combining growth rates with changes in Δ13Cis a valuable tool to better understand the causes of temporal variation in tree growth.
An important potential driver of long-term changes in tree growth is climate change, e.g. global warming and altered annual precipitation. To understand the effect of climate change on tree growth, the availability of reliable data on historical climate is off course crucial. For the study areas in Bolivia and Thailand, previous studies have investigated the occurrence of temporal trends in temperature and precipitation. For the study area in Cameroon however, as well as for West and Central Africa in general, the availability of instrumental climate data is very restricted. This limits the possibility to relate past climatic variation to changes in tree growth and calls for proxies that allow reconstruction of past climatic conditions. In CHAPTER 3 I assessed the potential use of stable isotopes of oxygen (δ18O) in tree rings as a tool for the reconstruction of precipitation in tropical Africa. I measured δ18O in tree rings of five large Entandrophragmautiletreesfrom North-western Cameroon. A significant negative correlation was found between annual tree-ring δ18O values (averaged over the five individuals) and annual precipitation amount during 1930-2009 in large areas of West and Central Africa. I also found tree-ring δ18O to track sea surface temperatures (SST) in the Gulf of Guinea (1930-2009). These two results are related because rainfall variabilityin West and Central Africa is profoundly influenced by the SST of the tropical AtlanticOcean. Thus a high SST in the Gulf of Guinea is associated with high precipitation over large parts of West and Central Africa and recorded in tree rings by a relatively low δ18O value. On the other hand, dry years when SST is low, are recorded by relatively high tree-ring δ18O values. I also found a significant long-term increase of tree-ring δ18O values. This trend seems to be caused by lowered precipitation from 1970 to 1990 (the Sahel drought period). From 1860 to 1970, no significant long-term trend was observed in tree-ring δ18O values, suggesting no substantial change in precipitation amount occurred over this period.
Another potential driver of altered tree growth and biomass in intact tropical forests is the increase of anthropogenic nutrient depositions, especially nitrogen. The deposition of nitrogen has likely risen due to an increased industrialization and use of artificial N fertilizers in most tropical countries. Nitrogen can stimulate plant growth, as is well known from the positive effect of N fertilizer application on crop yields. Previous studies have shown that the stable isotope ratio of nitrogen (δ15N) increased during the last decennia in the wood of trees from Brazil and Thailand as well as in tree leaves from Panama. This increased δ15N has been interpreted as a signal that tropical nitrogen cycles have become more ‘open’ and ‘leaky’ during the last decades in response to increased anthropogenic nitrogen depositions. The underlying mechanism is that high rates of nitrogen deposition and high ambient nitrogen availability lead to an increased nitrification. This process can cause a gradual 15N-enrichment of soil nitrogen. In CHAPTER 4 I analysed changes in tree-ring δ15N values of 400 trees of six species from the three study sites. In the trees from Cameroon no long-term change of tree-ring δ15N values was found (1850-2005), even though NH3and NOxemissions seem to have increased strongly around the study area since 1970. Possibly, the very high precipitation at that site causes the local nitrogen cycle to be already very ‘leaky’, limiting the effect of additional nitrogen input on the δ15N signature of soil nitrogen. Alternatively, nitrogen input in this forest might be much lower than reconstructed NH3and NOxemissions suggest. For the study site in Bolivia, no significant change of tree-ring δ15N values was found (1875-2005), which is in line with the expected result for areas with a low anthropogenic nitrogen input. I found a marginally significant increased of δ15N values since 1950 in trees from Thailand, which confirms previous observations. This points to an effect of increased anthropogenic nitrogen deposition, which could have stimulated photosynthetic rates, if indeed nitrogen was limiting tree growth.
The most often hypothesized factor to cause a long-term increase of tree growth is the rise of atmospheric CO2. Since the onset of the industrial revolution (~1850) global atmospheric CO2concentration has increased by 40%. Elevated CO2can directly affect plants by increasing the activity, as well as the efficiency, of the CO2fixing enzyme rubisco and thereby increase photosynthetic rates. Potentially more important in plant communities subjected to periods of limited water availability (like a dry season) is that elevated CO2 can cause a reduction of stomatal conductance, which lowers evapo-transpiration and hence reduces water losses.This increases water-use efficiency (i.e. the amount of carbon gained through photosynthesis divided by the amount of water loss through transpiration) and might allow plants to extend their growth season and/or increase their photosynthetic activity during the hottest hours of the day when water-stress might be severe. Elevated atmospheric CO2is thus a very likely candidate to have stimulated tropical tree growth (also referred to as CO2fertilization), provided at least that plant growth was either carbon or water limited. In CHAPTER 5 I tested the CO2fertilization hypothesis by analysing growth-ring data of 1100 trees from the three study sites. The measurement of tree-ring widths allowed an assessment of historical growth rates, whereas stable carbon isotopes (δ13C) in the wood of the trees were used to obtain an estimate of the CO2concentration in the intercellular spaces in leaves (Ci) and of water-use efficiency (intrinsic water-use efficiency; iWUE). I used a sampling method that controls for ontogenetic (i.e. size developmental) changes in growth and δ13C. With this method, trees were compared across a fixed diameter (i.e. same ontogenetic stage). I chose two diameters: 8 cm (referring to small understorey trees) and 27 cm (referring to larger canopy trees). A mixed-effect model revealeda highly significant and exponential increase of Ciat each of the three sites, and in both understorey and canopy trees. Over the last 150 years Ciincreased by 43% and 53% for understorey and canopy trees respectively. Yet, the rate of increase in Ciwas consistently lower than that of atmospheric CO2. This ‘active’ response to elevated atmospheric CO2resulted in a significant and large increase of iWUE. Over the last 150 years, iWUE increased by 30% and 35% for understorey and canopy trees.A long-term increase of iWUE indicates either a proportional increase of net photosynthesis and/or a decrease of stomatal conductance and thus transpiration, both of which could have stimulated biomass growth. However, I found no increase of tree growth over the last 150 years in any of the sites. Although there are several possible explanations for these findings, I argue that it is most likely that tropical tree growth is generally not limited by water and carbon, but by a persistent nutrient limitation (e.g. of phosphates) and that this has prevented tropical trees to use the extra CO2for an acceleration of growth.
In this thesis I have studied environmental and physiological drivers of tree growth changes. I found evidence of decreased precipitation over the last decades at the study site in Cameroon (CHAPTER 3), a changed nitrogen cycle at the study site in Thailand (CHAPTER 4) and an overall change in the physiology of all tree species studied (increased iWUE; CHAPTER 5). One of the main findings of this thesis is however, that these changes have not led to a net change of tree growth over the last 150 years (CHAPTER 5). This is an important finding that could have two major implications. Firstly, the absence of a long-term growth stimulation suggests that the increase of iWUE is mainly driven by a reduced stomatal conductance, which likely leads to a reduced evaporative water loss. If trees across the tropics are reducing evapo-transpiration, this will change affect hydrological cycles, e.g. leading to a lower humidity, higher air temperatures and a reduced precipitation. Secondly, the absence of a growth stimulation over the last 150 years suggests that the carbon sink capacity of tropical forests is currently overestimated, e.g. by Dynamic Global Vegetation Models, which assume strong CO2fertilization effects and as such a high capacity of tropical forests to mitigate global warming. I anticipate that the planned Free Air Concentration Enrichment (FACE) experiments in the tropics will shed light on the reasons why increased CO2does not stimulate the growth rates of tropical trees. Furthermore, I argue that combining tree-ring measurements and stable isotope analyses together with permanent plot research is the most promising way to increase our understanding of the changes in tropical forests.
Forensic forest ecology : unraveling the stand history of tropical forests
Vlam, M. - \ 2014
Wageningen University. Promotor(en): Frits Mohren, co-promotor(en): Pieter Zuidema; P.J. Baker. - Wageningen : Wageningen University - ISBN 9789461739421 - 208
bosecologie - tropische bossen - geschiedenis - opstandsontwikkeling - bosopstanden - bosdynamiek - klimaatverandering - verstoring - forest ecology - tropical forests - history - stand development - forest stands - forest dynamics - climatic change - disturbance
Tropical forests are occasionally hit by intense disturbances like hurricanes or droughts that kill many trees. We found evidence for such intense disturbances in a tree-ring study on tropical forests in Bolivia, Cameroon and Thailand. To reconstruct past disturbances we applied ‘forensic forest ecology’, a combined analysis of age distributions and spatial distributions of trees. The study shows that all three forests carry a legacy of past disturbances. The process of recovery after past disturbance may explain recently reported increases in tree growth and forest biomass from long-term forest monitoring plots. This finding is in contradiction with the dominant paradigm that increases in forest biomass are the result of enhanced photosynthesis due to rising CO2 concentrations in the atmosphere. A more dominant role for past disturbances means that the compensating effect of tropical forests in global warming may be smaller than previously thought.
The sensitivity of tropical forests to climate variability and change in Bolivia
Seiler, C. - \ 2014
Wageningen University. Promotor(en): Pavel Kabat, co-promotor(en): Ronald Hutjes; Bart Kruijt. - Wageningen : Wageningen University - ISBN 9789461739230 - 157
tropische bossen - klimaatverandering - gevoeligheid - klimaat - remote sensing - koolstof - bolivia - tropical forests - climatic change - sensitivity - climate - remote sensing - carbon - bolivia