Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    The sensitivity of wet and dry tropical forests to climate change in Bolivia
    Seiler, C. ; Hutjes, R.W.A. ; Kruijt, B. ; Hickler, T. - \ 2015
    Journal of Geophysical Research: Biogeosciences 120 (2015)3. - ISSN 2169-8953 - p. 399 - 413.
    global vegetation models - soil respiration - terrestrial biosphere - plant geography - carbon-dioxide - amazon dieback - rain-forest - dynamics - water - temperature
    Bolivia's forests contribute to the global carbon and water cycle, as well as to global biodiversity. The survival of these forests may be at risk due to climate change. To explore the associated mechanisms and uncertainties, a regionally adapted dynamic vegetation model was implemented for the Bolivian case, and forced with two contrasting climate change projections. Changes in carbon stocks and fluxes were evaluated, factoring out the individual contributions of atmospheric carbon dioxide ([CO2]), temperature, and precipitation. Impacts ranged from a strong increase to a severe loss of vegetation carbon (cv), depending on differences in climate projections, as well as the physiological response to rising [CO2]. The loss of cv simulated for an extremely dry projection was primarily driven by a reduction in gross primary productivity, and secondarily by enhanced emissions from fires and autotrophic respiration. In the wet forest, less precipitation and higher temperatures equally reduced cv, while in the dry forest, the impact of precipitation was dominating. The temperature-related reduction of cv was mainly due to a decrease in photosynthesis and only to lesser extent because of more autotrophic respiration and less stomatal conductance as a response to an increasing atmospheric evaporative demand. Under an extremely dry projection, tropical dry forests were simulated to virtually disappear, regardless of the potential fertilizing effect of rising [CO2]. This suggests a higher risk for forest loss along the drier southern fringe of the Amazon if annual precipitation will decrease substantially.
    Loss of animal seed dispersal increases extinction risk in a tropical tree species due to pervasive negative density dependence across life stages
    Caughlin, T.T. ; Ferguson, J.M. ; Lichstein, J.W. ; Zuidema, P.A. ; Bunyavejchewin, S. ; Levey, D.J. - \ 2015
    Proceedings of the Royal Society. B: Biological Sciences 282 (2015)1798. - ISSN 0962-8452 - 9 p.
    spatial-patterns - rain-forest - recruitment - consequences - neighborhood - defaunation - habitat - uncertainty - diversity - abundance
    Overhunting in tropical forests reduces populations of vertebrate seed dispersers. If reduced seed dispersal has a negative impact on tree population viability, overhunting could lead to altered forest structure and dynamics, including decreased biodiversity. However, empirical data showing decreased animal-dispersed tree abundance in overhunted forests contradict demographic models which predict minimal sensitivity of tree population growth rate to early life stages. One resolution to this discrepancy is that seed dispersal determines spatial aggregation, which could have demographic consequences for all life stages. We tested the impact of dispersal loss on population viability of a tropical tree species, Miliusa horsfieldii, currently dispersed by an intact community of large mammals in a Thai forest. We evaluated the effect of spatial aggregation for all tree life stages, from seeds to adult trees, and constructed simulation models to compare population viability with and without animal-mediated seed dispersal. In simulated populations, disperser loss increased spatial aggregation by fourfold, leading to increased negative density dependence across the life cycle and a 10-fold increase in the probability of extinction. Given that the majority of tree species in tropical forests are animal-dispersed, overhunting will potentially result in forests that are fundamentally different from those existing now.
    Sapwood allocation in tropical trees: a test of hypotheses
    Schippers, P. ; Vlam, M. ; Zuidema, P.A. ; Sterck, F.J. - \ 2015
    Functional Plant Biology 42 (2015)7. - ISSN 1445-4408 - p. 697 - 709.
    temperate forest trees - global vegetation models - carbon allocation - shade-tolerance - rain-forest - biomass allocation - seasonal dynamics - use efficiency - climate-change - growth habits
    Carbon allocation to sapwood in tropical canopy trees is a key process determining forest carbon sequestration, and is at the heart of tree growth and dynamic global vegetation models (DGVM). Several allocation hypotheses exist including those applying assumptions on fixed allocation, pipe model, and hierarchical allocation between plant organs. We use a tree growth model (IBTREE) to evaluate these hypotheses by comparing simulated sapwood growth with 30 year tree ring records of the tropical long-lived tree Toona ciliata M. Roem. in Thailand. Simulated annual variation in wood production varied among hypotheses. Observed and simulated growth patterns matched most closely (r2 = 0.70) when hierarchical allocation was implemented, with low priority for sapwood. This allocation method showed realistic results with respect to reserve dynamics, partitioning and productivity and was the only one able to capture the large annual variation in tree ring width. Consequently, this method might also explain the large temporal variation in diameter growth and the occurrence of missing rings often encountered in other tropical tree species. Overall, our results show that sapwood growth is highly sensitive to allocation principles, and that allocation assumptions may greatly influence estimated carbon sequestration of tropical forests under climatic change.
    Phylogenetics of African Rinorea (Violaceae): Elucidating Infrageneric Relationships using Plastid and Nuclear DNA Sequences
    Velzen, R. van; Wahlert, G.A. ; Sosef, M.S.M. ; Onstein, R.E. ; Bakker, F.T. - \ 2015
    Systematic Botany 40 (2015)1. - ISSN 0363-6445 - p. 174 - 184.
    rain-forest - tropical forest - lepidoptera nymphalidae - diversity - cameroon - trees - gabon
    Rinorea is a pantropical genus of shrubs and small trees within the family Violaceae. The genus is particularly diverse in Africa where species are ecologically important as they are often abundant or even dominant in particular forest types and act as larval host plants for highly specialized Cymothoe butterflies. Despite their importance, species identification of African Rinorea is difficult and a taxonomic revision is needed. Previous phylogenetic studies have suggested that neotropical taxa are sister to a palaeotropical clade, with multiple independent dispersals to Madagascar, but these were based on plastid data only. We therefore present an updated phylogeny of Rinorea with increased sampling of African taxa, using plastid as well as nuclear DNA sequences. Phylogenetic relationships inferred from nuclear DNA data were generally congruent with those based on evidence from plastid haplotypes from earlier studies. Our increased taxonomic sampling also revealed previously undiscovered African Rinorea clades, some of which warrant further taxonomic study. Ancestral state reconstructions refute previous hypotheses about the evolution of morphological characters traditionally used for Rinorea infrageneric classification. In addition, some widespread species may comprise species complexes. It is clear that African Rinorea require comprehensive taxonomic revision; our contribution to understanding Rinorea infrageneric relationships will facilitate this task.
    Water-use advantage for lianas over trees in tropical seasonal forests
    Chen, Y.J. ; Cao, K.F. ; Schnitzer, S.A. ; Fan, Z.X. ; Zhang, J.L. ; Bongers, F. - \ 2015
    New Phytologist 205 (2015)1. - ISSN 0028-646X - p. 128 - 136.
    rain-forest - canopy trees - soil-water - sw china - aboveground biomass - secondary forest - eastern amazonia - stomatal control - stable-isotopes - woody-plants
    •Lianas exhibit peak abundance in tropical forests with strong seasonal droughts, the eco-physiological mechanisms associated with lianas coping with water deficits are poorly understood. •We examined soil water partitioning, sap flow, and canopy eco-physiological properties for 99 individuals of 15 liana and 34 co-occurring tree species in three tropical forests that differed in soil water availability. •In the dry season, lianas used a higher proportion of deep soil water in the karst forest (KF; an area with severe seasonal soil water deficit (SSWD)) and in the tropical seasonal forest (TSF, moderate SSWD), permitting them to maintain a comparable leaf water status than trees in the TSF or a better status than trees in the KF. Lianas exhibited strong stomatal control to maximize carbon fixation while minimizing dry season water loss. During the dry period, lianas significantly decreased water consumption in the TSF and the KF. Additionally, lianas had a much higher maximum photosynthetic rates and sap flux density in the wet season and a lower proportional decline in photosynthesis in the dry season compared with those of trees. •Our results indicated that access to deep soil water and strong physiological adjustments in the dry season together with active wet-season photosynthesis may explain the high abundance of lianas in seasonally dry forests.
    Nutrient resorption is associated with leaf vein density and growth performance of dipterocarp tree species
    Zhang, J.L. ; Zhang, S.B. ; Chen, Y.J. ; Zhang, Y.P. ; Poorter, L. - \ 2015
    Journal of Ecology 103 (2015)3. - ISSN 0022-0477 - p. 541 - 549.
    nitrogen-use-efficiency - life-span - spring growth - rain-forest - rhododendron-ferrugineum - acquisition strategies - phosphorus fractions - economics traits - foliar nutrients - evergreen shrub
    1.Nutrient resorption is important for the nutrient budget of plants, but little is known about which plant traits mediate nutrient resorption, how resorption efficiency is associated with other leaf traits and whether nutrient resorption has an impact on plant growth. 2.In this study, 17 dipterocarp tree species were compared in a common garden experiment. N and P resorption efficiencies were regressed against suites of traits associated with phloem transport capacity (i.e. leaf vein density; Dvein), leaf nutrient conservation traits (e.g. leaf mass per area; LMA) and species growth rate. 3.Across the dipterocarp species studied, N resorption efficiency (percentage N resorbed) was positively correlated with Dvein and leaf thickness. N resorption efficiency was also correlated with Dvein after considering phylogenetic effects. N resorption proficiency (N remaining in senesced leaves) was negatively correlated with Dvein, LMA, leaf thickness and palisade and spongy mesophyll thickness. Senesced-leaf N concentration was still negatively correlated with LMA and leaf thickness after considering phylogenetic effects. 4.N resorption efficiency was positively correlated with both height and diameter growth rates. After considering phylogenetic effect, N resorption efficiency was marginally correlated with diameter growth rate. Green-leaf N concentration was positively correlated with height growth rate after considering phylogenetic effect. 5.P resorption efficiency and proficiency were not related to any of the leaf morphological and anatomical traits, or to species growth rates. 6.Synthesis. These results indicate that higher phloem transport capacity of the dipterocarp species is positively correlated with greater N resorption efficiency and that N resorption proficiency is closely linked with leaf nutrient conservation traits. Growth rates of the dipterocarps are more likely governed by photosynthetic rates associated with green-leaf N concentration than N resorption rates per se. Although P is generally deficient in tropical soils, it appears that N rather than P availability is the key limiting factor for the growth of the dipterocarp species.
    The effects of drought and shade on the performance, morphology and physiology of Ghanaian tree species
    Amissah, L. ; Mohren, G.M.J. ; Kyereh, B. ; Poorter, L. - \ 2015
    PLoS ONE 10 (2015)4. - ISSN 1932-6203
    tropical forest - water relations - rain-forest - seedling establishment - desiccation-tolerance - carbon gain - trade-offs - photosynthetic acclimation - niche differentiation - relative importance
    In tropical forests light and water availability are the most important factors for seedling growth and survival but an increasing frequency of drought may affect tree regeneration. One central question is whether drought and shade have interactive effects on seedling growth and survival. Here, we present results of a greenhouse experiment, in which seedlings of 10 Ghanaian tree species were exposed to combinations of strong seasonal drought (continuous watering versus withholding water for nine weeks) and shade (5% irradiance versus 20% irradiance). We evaluated the effects of drought and shade on seedling survival and growth and plasticity of 11 underlying traits related to biomass allocation, morphology and physiology. Seedling survival under dry conditions was higher in shade than in high light, thus providing support for the “facilitation hypothesis” that shade enhances plant performance through improved microclimatic conditions, and rejecting the trade-off hypothesis that drought should have stronger impact in shade because of reduced root investment. Shaded plants had low biomass fraction in roots, in line with the trade-off hypothesis, but they compensated for this with a higher specific root length (i.e., root length per unit root mass), resulting in a similar root length per plant mass and, hence, similar water uptake capacity as high-light plants. The majority (60%) of traits studied responded independently to drought and shade, indicating that within species shade- and drought tolerances are not in trade-off, but largely uncoupled. When individual species responses were analysed, then for most of the traits only one to three species showed significant interactive effects between drought and shade. The uncoupled response of most species to drought and shade should provide ample opportunity for niche differentiation and species coexistence under a range of water and light conditions. Overall our greenhouse results suggest that, in the absence of root competition shaded tropical forest tree seedlings may be able to survive prolonged drought.
    Hyperdominance in Amazonian forest carbon cycling
    Fauset, S. ; Arets, E.J.M.M. ; Steege, H. ter; Pena Claros, M. ; Poorter, L. ; Levis, C. ; Toledo, M. - \ 2015
    Nature Communications 6 (2015). - ISSN 2041-1723
    tropical forests - rain-forest - experimental drought - species composition - economics spectrum - biomass - trees - allometry - productivity - mortality
    While Amazonian forests are extraordinarily diverse, the abundance of trees is skewed strongly towards relatively few ‘hyperdominant’ species. In addition to their diversity, Amazonian trees are a key component of the global carbon cycle, assimilating and storing more carbon than any other ecosystem on Earth. Here we ask, using a unique data set of 530 forest plots, if the functions of storing and producing woody carbon are concentrated in a small number of tree species, whether the most abundant species also dominate carbon cycling, and whether dominant species are characterized by specific functional traits. We find that dominance of forest function is even more concentrated in a few species than is dominance of tree abundance, with only ˜1% of Amazon tree species responsible for 50% of carbon storage and productivity. Although those species that contribute most to biomass and productivity are often abundant, species maximum size is also influential, while the identity and ranking of dominant species varies by function and by region.
    An assessment of the terrestrial mammal communities in forests of Central Panama, using camera-trap surveys
    Meyer, N.F.V. ; Esser, H.J. ; Moreno, R. ; Langevelde, F. van; Liefting, Y. ; Ros Oller, D. ; Vogels, C.B.F. ; Carver, A.D. ; Nielsen, C.K. ; Jansen, P.A. - \ 2015
    Journal for Nature Conservation 26 (2015). - ISSN 1617-1381 - p. 28 - 35.
    rain-forest - habitat fragmentation - conservation status - neotropical forest - atlantic forest - tayassu-pecari - abundance - biodiversity - landscape - density
    The Isthmus of Panama, part of the planet’s third largest megadiversity hotspot, and connecting the faunas of North and South America, has lost more than half of its forest due to agriculture and economicdevelopment. It is unknown to what degree the remaining forest, which is fragmented and subject topoaching, still supports the wildlife diversity found in intact forests. Here, we use camera-trap surveysto assess whether forests in Central Panama, the narrowest and most disturbed portion of the Isthmus,still have intact communities of medium- and large-bodied terrestrial mammals. During 2005–2014,we collected camera-trap survey data from 15 national parks and forest fragments on both sides ofthe Panama Canal, and compared these to similar data from two sites in the intact Darién NationalPark in Eastern Panama, the nearest available reference. We found that most sites in Central Panama– including some of the national parks – had lower mammal species richness and evenness than thereference sites, and less structurally-complex mammal communities. Forests in Central Panama had littleor no apex predators and large terrestrial frugivores, with the exception of two sites directly connectedto the reference site. Our results indicate that the terrestrial mammal community in forests of CentralPanama is currently degraded, even inside national parks. These data provide a baseline for evaluating the success of conservation efforts to prevent the Panamanian Isthmus to become a bottleneck for movement of aniamls
    15N in tree rings as a bio-indicator of changing nitrogen cycling in tropical forests: an evaluation at three sites using two sampling methods
    Sleen, J.P. van der; Vlam, M. ; Groenendijk, P. ; Anten, N.P.R. ; Bongers, F. ; Bunyavejchewin, S. ; Hietz, P. ; Pons, T.L. ; Zuidema, P. - \ 2015
    Frontiers in Plant Science 6 (2015). - ISSN 1664-462X
    rain-forest - natural-abundance - soil-nitrogen - isotope fractionation - wood deterioration - growth-rates - n deposition - dynamics - ecosystem - lowland
    Anthropogenic nitrogen deposition is currently causing a more than twofold increase of reactive nitrogen input over large areas in the tropics. Elevated N-15 abundance (delta N-15) in the growth rings of some tropical trees has been hypothesized to reflect an increased leaching of N-15-depleted nitrate from the soil, following anthropogenic nitrogen deposition over the last decades. To find further evidence for altered nitrogen cycling in tropical forests, we measured long-term delta N-15 values in trees from Bolivia, Cameroon, and Thailand. We used two different sampling methods. In the first, wood samples were taken in a conventional way: from the pith to the bark across the stem of 28 large trees (the "radial" method). In the second, delta N-15 values were compared across a fixed diameter (the "fixed-diameter" method). We sampled 400 trees that differed widely in size, but measured delta N-15 in the stem around the same diameter (20 cm dbh) in all trees. As a result, the growth rings formed around this diameter differed in age and allowed a comparison of delta N-15 values over time with an explicit control for potential size-effects on delta N-15 values. We found a significant increase of tree-ring delta N-15 across the stem radius of large trees from Bolivia and Cameroon, but no change in tree-ring delta N-15 values over time was found in any of the study sites when controlling for tree size. This suggests that radial trends of delta N-15 values within trees reflect tree ontogeny (size development). However, for the trees from Cameroon and Thailand, a low statistical power in the fixed-diameter method prevents to conclude this with high certainty. For the trees from Bolivia, statistical power in the fixed-diameter method was high, showing that the temporal trend in tree-ring delta N-15 values in the radial method is primarily caused by tree ontogeny and unlikely by a change in nitrogen cycling. We therefore stress to account for tree size before tree-ring delta N-15 values can be properly interpreted.
    CTFS-ForestGEO: A worldwide network monitoring forests in an era of global change
    Anderson-Teixeira, K.J. ; Davies, S.J. ; Bennett, A.C. ; Gonzalez-Akre, E.B. ; Muller-Landau, H.C. ; Wright, S.J. ; Abu Salim, K. ; Almeyda Zambrano, A.M. ; Jansen, P.A. ; Ouden, J. den - \ 2015
    Global Change Biology 21 (2015)2. - ISSN 1354-1013 - p. 528 - 549.
    tropical tree community - long-term nitrogen - rain-forest - neotropical forest - functional traits - spatial-patterns - el-nino - phylogenetic structure - seedling recruitment - dispersal limitation
    Global change is impacting forests worldwide, threatening biodiversity and ecosystem services including climate regulation. Understanding how forests respond is critical to forest conservation and climate protection. This review describes an international network of 59 long-term forest dynamics research sites (CTFS-ForestGEO) useful for characterizing forest responses to global change. Within very large plots (median size 25 ha), all stems =1 cm diameter are identified to species, mapped, and regularly recensused according to standardized protocols. CTFS-ForestGEO spans 25°S–61°N latitude, is generally representative of the range of bioclimatic, edaphic, and topographic conditions experienced by forests worldwide, and is the only forest monitoring network that applies a standardized protocol to each of the world's major forest biomes. Supplementary standardized measurements at subsets of the sites provide additional information on plants, animals, and ecosystem and environmental variables. CTFS-ForestGEO sites are experiencing multifaceted anthropogenic global change pressures including warming (average 0.61 °C), changes in precipitation (up to ±30% change), atmospheric deposition of nitrogen and sulfur compounds (up to 3.8 g N m-2 yr-1 and 3.1 g S m-2 yr-1), and forest fragmentation in the surrounding landscape (up to 88% reduced tree cover within 5 km). The broad suite of measurements made at CTFS-ForestGEO sites makes it possible to investigate the complex ways in which global change is impacting forest dynamics. Ongoing research across the CTFS-ForestGEO network is yielding insights into how and why the forests are changing, and continued monitoring will provide vital contributions to understanding worldwide forest diversity and dynamics in an era of global change.
    Time-dependent effects of climate and drought on tree growth in a Neotropical dry forest: Short-term tolerance vs. long-term sensitivity
    Mendivelso, H.A. ; Camarero, J.J. ; Gutierrez, E. ; Zuidema, P. - \ 2014
    Agricultural and Forest Meteorology 188 (2014). - ISSN 0168-1923 - p. 13 - 23.
    tropical forests - ring chronologies - rain-forest - santa-cruz - water-use - phenology - patterns - bolivia - precipitation - coordination
    We analyzed the effects of climate and drought on radial growth using dendrochronology in seven deciduous tree species coexisting in a Bolivian tropical dry forest subjected to seasonal drought. Precipitation, temperature and a multiscalar drought index were related to tree-ring width data at different time-scales (from one month to 42 years). Precipitation affected positively tree growth in all species, mainly during the wet season, while temperature affected it negatively in five species. Tree growth responses to precipitation and temperature were species-specific and peaked at short-time scales, specifically from one to nine months. At inter-annual scales tree growth always responded positively to less dry conditions at short-time scales, particularly from two to seven months, and also at long-time scales from six to 30 years. Tree growth was mainly sensitive to multi-annual droughts and such sensitivity differed among species. Our findings suggest that tree species of the studied tropical dry forest are predominantly sensitive in terms of growth reduction to long-lasting droughts. This time-dependency of growth responses to drought should be explicitly considered as an additional constraint of the community dynamics in evaluations of the future responses of tropical dry forests to climate warming. (C) 2014 Elsevier B.V. All rights reserved.
    Mycorrhizae support oaks growing in a phylogenetically distant neighbourhood
    Yguel, B. ; Courty, P.E. ; Jactel, H. ; Pan, X. ; Butenschoen, O. ; Murray, P.J. ; Prinzing, A. - \ 2014
    Soil Biology and Biochemistry 78 (2014). - ISSN 0038-0717 - p. 204 - 212.
    ectomycorrhizal community - niche conservatism - microbial biomass - species-diversity - quercus-robur - rain-forest - soil - trees - plants - fungi
    Host-plants may rarely leave their ancestral niche and in which case they tend to be surrounded by phylogenetically distant neighbours. Phylogenetically isolated host-plants might share few mutualists with their neighbours and might suffer from a decrease in mutualist support. In addition host plants leaving their ancestral niche might face a deterioration of their abiotic and biotic environment and might hence need to invest more into mutualist partners. We tested whether phylogenetic isolation of hosts from neighbours decreases or increases abundance and activity of their mutualists and whether mutualist activity may help to compensate deterioration of the environment. We study oak-hosts and their ectomycorrhizal fungi mutualists established in the litter layer formed by the phylogenetically closely or distantly related neighbourhood. We find that oaks surrounded by phylogenetically distant neighbours show increased abundance and enzymatic activity of ectomycorrhizal fungi in the litter. Moreover, oaks surrounded by phylogenetically distant neighbours also show delayed budburst but ectomycorrhizal fungi activity partly compensates this negative effect of phylogenetic isolation. This suggests decreased nutrient availability in a phylogenetically distant litter partly compensated by increased litter-degradation by ectomycorrhizal fungi activity. Most observed effects of phylogenetic isolation cannot be explained by a change in baseline soil fertility (as reflected by nutritional status of fresh oak litter, or soil microbial biomass and activity) nor by simple reduction of percentages of oak neighbours, nor by the presence of gymnosperms. Our results show that colonizing new niche represented by the presence of distantly related neighbours may delay plant phenology but may be supported by mycorrhizal mutualists. Studies on other host-plant species are required to generalize our findings. (C) 2014 Elsevier Ltd. All rights reserved.
    Modelling forest dynamics along climate gradients in Bolivia
    Seiler, C. ; Hutjes, R.W.A. ; Kruijt, B. ; Quispe, J. ; Añez, S. ; Arora, V.K. ; Melton, J.R. ; Hickler, T. ; Kabat, P. - \ 2014
    Journal of Geophysical Research: Biogeosciences 119 (2014)5. - ISSN 2169-8953 - p. 758 - 775.
    global vegetation model - tropical forest - terrestrial biosphere - carbon-dioxide - rain-forest - biomass - productivity - variability - drought - dieback
    Dynamic vegetation models have been used to assess the resilience of tropical forests to climate change, but the global application of these modeling experiments often misrepresents carbon dynamics at a regional level, limiting the validity of future projections. Here a dynamic vegetation model (Lund Potsdam Jena General Ecosystem Simulator) was adapted to simulate present-day potential vegetation as a baseline for climate change impact assessments in the evergreen and deciduous forests of Bolivia. Results were compared to biomass measurements (819 plots) and remote sensing data. Using regional parameter values for allometric relations, specific leaf area, wood density, and disturbance interval, a realistic transition from the evergreen Amazon to the deciduous dry forest was simulated. This transition coincided with threshold values for precipitation (1400 mm yr-1) and water deficit (i.e., potential evapotranspiration minus precipitation) (-830 mm yr-1), beyond which leaf abscission became a competitive advantage. Significant correlations were found between modeled and observed values of seasonal leaf abscission (R2 = 0.6, p
    Importance of bistatic SAR features from TanDEM-X for forest mapping and monitoring
    Schlund, M. ; Poncet, F. von; Hoekman, D.H. ; Kuntz, S. ; Schmullius, C. - \ 2014
    Remote Sensing of Environment 151 (2014)sp. issue. - ISSN 0034-4257 - p. 16 - 26.
    land-cover - southeast-asia - feature-selection - polarimetric sar - tropical-forest - decision tree - alos palsar - rain-forest - sir-c - classification
    Deforestation and forest degradation are one of the important sources for human induced carbon dioxide emissions and their rates are highest in tropical forests. For man-kind, it is of great importance to track land-use conversions like deforestation, e.g. for sustainable forest management and land use planning, for carbon balancing and to support the implementation of international initiatives like REDD + (Reducing Emissions from Deforestation and Degradation). SAR (synthetic aperture radar) sensors are suitable to reliably and frequently monitor tropical forests due to their weather independence. The TanDEM-X mission (which is mainly aimed to create a unique global high resolution digital elevation model) currently operates two X-band SAR satellites, acquiring interferometric SAR data for the Earth's entire land surface multiple times. The operational mission provides interferometric data as well as mono- and bistatic scattering coefficients. These datasets are homogenous, globally consistent and are acquired in high spatial resolution. Hence, they may offer a unique basic dataset which could be useful in land cover monitoring. Based on first datasets available from the TanDEM-X mission, the main goal of this research is to investigate the information content of TanDEM-X data for mapping forests and other land cover classes in a tropical peatland area. More specifically, the study explores the utility of bistatic features for distinguishing between open and closed forest canopies, which is of relevance in the context of deforestation and forest degradation monitoring. To assess the predominant information content of TanDEM-X data, the importance of information derived from the bistatic system is compared against the monostatic case, usually available from SAR systems. The usefulness of the TanDEM-X mission data, i.e. scattering coefficients, derived textural information and interferometric coherence is investigated via a feature selection process. The resulting optimal feature sets representing a monostatic and a bistatic SAR dataset were used in a subsequent classification to assess the added value of the bistatic TanDEM-X features in the separability of land cover classes. The results obtained indicated that especially the interferometric coherence significantly improved the separability of thematic classes compared to a dataset of monostatic acquisition. The bistatic coherence was mainly governed by volume decorrelation of forest canopy constituents and carries information about the canopy structure which is related to canopy cover. In contrast, the bistatic scattering coefficient had no significant contribution to class separability. The classification with coherence and textural information outperformed the classification with the monostatic scattering coefficient and texture by more than 10% and achieved an overall accuracy of 85%. These results indicate that TanDEM-X can serve as a valuable and consistent source for mapping and monitoring tropical forests.
    Learning from the past: Trends and dynamics in livelihoods of Bolivian forest communities
    Zenteno, M. ; Jong, W. de; Boot, R. ; Zuidema, P.A. - \ 2014
    Environmental Science & Policy 40 (2014). - ISSN 1462-9011 - p. 36 - 48.
    social-ecological systems - agricultural expansion - timber markets - rain-forest - amazon - deforestation - management - institutions - governance - complexity
    We use social ecological systems theory (SES) to analyse change in forest communities in the northern Bolivian Amazon. SES characterizes interdependent dynamics of social and ecological systems and we hypothesized it to be a useful frame to grasp dynamics of forest communities affected by changes in forest policies, regulations and institutions, as well as economic demands and conservation objectives. We analysed the long-term historical changes since the region became incorporated in the global tropical forest product value chain since the late 19th century and quantitatively analysed changes in 85 forest communities between 1997 and 2009. We collected information on 16 variables related to demographic, productive, and socio-economic characteristics. Results show that forest communities have experienced major changes and have adapted to these changes. Social thresholds, a key concept in SES, are consistent with multiple social economic forces experienced by forest communities. Detrimental feed-back effects of SES can be confronted when innovative exploration mechanisms, such as new productive chains are developed, or the agro-extractive cycles of current productive system are expanded. Competition among households, population growth and more profitable economic opportunities may threaten benign forms of forest products extraction that have persisted through various cycles of internal and external changes.
    Stimulating seedling growth in early stages of secondary forest succession: a modeling approach to guide tree liberation
    Kuijk, M. ; Anten, N.P.R. ; Oomen, R.J. ; Schieving, F. - \ 2014
    Frontiers in Plant Science 5 (2014). - ISSN 1664-462X - 13 p.
    natural regeneration - rain-forest - light interception - south kalimantan - carbon gain - costa-rica - canopy photosynthesis - subtropical forests - dipterocarp forest - tropical pasture
    Excessive growth of non-woody plants and shrubs on degraded lands can strongly hamper tree growth and thus secondary forest succession. A common method to accelerate succession, called liberation, involves opening up the vegetation canopy around young target trees. This can increase growth of target trees by reducing competition for light with neighboring plants. However, liberation has not always had the desired effect, likely due to differences in light requirement between tree species. Here we present a 3D-model, which calculates photosynthetic rate of individual trees in a vegetation stand. It enables us to examine how stature, crown structure, and physiological traits of target trees and characteristics of the surrounding vegetation together determine effects of light on tree growth. The model was applied to a liberation experiment conducted with three pioneer species in a young secondary forest in Vietnam. Species responded differently to the treatment depending on their height, crown structure and their shade-tolerance level. Model simulations revealed practical thresholds over which the tree growth response is heavily influenced by the height and density of surrounding vegetation and gap radius. There were strong correlations between calculated photosynthetic rates and observed growth: the model was well able to predict growth of trees in young forests and the effects of liberation there upon. Thus, our model serves as a useful tool to analyze light competition between young trees and surrounding vegetation and may help assess the potential effect of tree liberation.
    Changing drivers of species dominance during tropical forest succession
    Lohbeck, M.W.M. ; Poorter, L. ; Martinez-Ramos, M. ; Rodriguez-Valázquez, J. ; Breugel, M. van; Bongers, F. - \ 2014
    Functional Ecology 28 (2014)4. - ISSN 0269-8463 - p. 1052 - 1058.
    phylogenetic community structure - plant functional traits - leaf life-span - rain-forest - competitive-exclusion - secondary succession - relative importance - coastal california - assembly rules - diversity
    1. Deterministic theories predict that local communities assemble from a regional species pool based on niche differences, thus by plant functional adaptations. We tested whether functional traits can also explain patterns in species dominance among the suite of co-occurring species. 2. We predicted that along a gradient of secondary succession, the main driver of species dominance changes from environmental filtering in the relatively harsh (dry and hot) early successional conditions, towards increased competitive interactions and limiting similarity in later successional conditions (when light is limited). 3. We used the Kurtosis (K) (a measure of peakedness) of the functional trait distribution of secondary forest communities in high-diversity tropical rain forest in Chiapas, Mexico. The forests ranged 1-25 years in age, and we used eight functional leaf traits related to a plants' carbon, water and heat balance. We calculated the functional trait distribution based on species dominance, where trait values were weighted by species' relative basal area, as well as based on species presence, all species counting once. 'K-ratio' was subsequently computed by dividing kurtosis based on species dominance by kurtosis based on species presence. If the K-ratio is high, the dominant species are functionally similar and we interpreted this as environmentally driven functional convergence allowing species to become dominant. If the K-ratio is small, dominant species are a functionally dissimilar subset of the species present and we interpreted this as competitively driven functional divergence allowing species to become dominant. 4. We found that in early succession, dominant species represent a functionally narrow subset of species with similar traits, and in late succession, dominant species increasingly represent a wide subset of the species present. This trend was found for traits that reflect photosynthetic performance and light capture, and indicates increased competition for light with succession. No trend was found for traits that indicate defence against herbivory, suggesting no successional changes in herbivore pressure. 5. Synthesis. This is one of the first studies showing that drivers of species dominance change along a gradient of secondary succession. During the early successional time window we evaluated, the importance of environmental filtering as a driving force fades away rapidly, and the importance of niche partitioning for species dominance starts to emerge.
    Rinorea calcicola (Violaceae), an endangered new species from south-eastern Gabon
    Velzen, R. van; Wieringa, J.J. - \ 2014
    Phytotaxa 167 (2014)3. - ISSN 1179-3155 - p. 267 - 275.
    leguminosae-papilionoideae - rain-forest - cameroon - african
    Rinorea calcicola, endemic to south-eastern Gabon, is described as a new species. Its most striking characteristic is the laminate fruit, a character so far only known for R. zenkeri. Rinorea calcicola has a limited distribution and appears to be restricted to limestone outcroppings. We have assessed it as endangered—EN B1ab(ii,iii,iv,v)—according to IUCN criteria. Morphological affinities of R. calcicola are discussed, and a key to R. zenkeri and the Gabonese species of Rinorea with textured fruits is given. The new species is one of many endemic species from the same region, and we recommend that a study of their distribution and ecology is undertaken to inform conservation planning. Because of the presence of so many narrow endemic species in the region, we suggest that the area be considered for protected status.
    Potential of tree-ring analysis in a wet tropical forest: A case study on 22 commercial tree species in Central Africa
    Groenendijk, P. ; Sass, U.G.W. ; Bongers, F. ; Zuidema, P.A. - \ 2014
    Forest Ecology and Management 323 (2014). - ISSN 0378-1127 - p. 65 - 78.
    climate-growth relationships - rain-forest - pterocarpus-angolensis - age - periodicity - management - dynamics - reconstruction - precipitation - west
    Implementing sustainable forest management requires basic information on growth, ages, reproduction and survival of exploited tree species. This information is generally derived from permanent sample plots where individual trees are monitored. Accurately estimating growth rates and especially tree ages from plots is however challenging, as plots often contain only few individuals of the exploited species and monitoring periods cover only a fraction of the life-span of most trees. Alternatively, tree-ring analysis is increasingly used to obtain accurate age estimates and growth rates for tropical tree species, especially in regions with seasonally harsh conditions. However, for species from wet tropical forests (>4000 mm year-1 rainfall) few tree-ring studies exist. Under persistent high levels of rainfall, formation of distinct tree rings is uncertain due to the lack of strong seasonal variation in climate factors. Here we evaluated the potential of applying tree-ring analysis on commercial tree species in a wet tropical forest in Central-Africa. For this purpose we screened the wood anatomy of 22 tree species for the presence of tree-ring structures and, on a subset of five species, we assessed crossdating potential and evaluated the annual character of tree-ring formation by radiocarbon dating. A total of 14 of the 22 tree species showed distinct tree-ring boundaries. Radiocarbon proved annual tree-ring formation in four of the five tested species. Crossdating between trees was problematic for all species and impeded exactly dating each detected ring and building tree-ring chronologies. We also show that diameter growth rates vary strongly between and among species, with important consequences for the calculation of future timber yields. Tree-ring analysis can thus be applied on tree species growing in wet tropical forests to obtain growth rates. We argue that tree-ring analysis should actually be applied on more tree species from different areas to obtain accurate, site specific growth data. This data is urgently required to design and improve sustainable forest management practices.
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