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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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.
Pathways for resilience in Mediterranean cork oak land use systems
Acácio, V.C. ; Holmgren, M. - \ 2014
Annals of Forest Science 71 (2014)1. - ISSN 1286-4560 - p. 5 - 13.
stress-gradient hypothesis - eastern iberian peninsula - arid ecosystems - south-america - el-nino - regeneration - facilitation - restoration - persistence - landscapes
Context Loss of woodlands and degradation of vegetation and soil have been described for all Mediterranean-type ecosystems worldwide. In the Western Iberian Peninsula, overexploitation of evergreen cork oak land use systems has led to soil erosion, failures in oak recruitment, and loss of forests. Degraded and dry sites are quickly colonised by pioneer heathland rockrose (Cistus spp.) shrubs forming highly persistent patches. Aims Although traditionally shrublands have been considered as a transient successional state, we present evidence that they can represent persistent alternative states to former cork oak forests. Review trends and conclusions We first describe how Mediterranean vegetation evolved in the Iberian Peninsula and the role of fire and long-term human management as main disturbances. We then discuss alternative pathways through state-and-transition models indicating the ecological and land use variables that halt cork oak regeneration and recruitment and drive vegetation transitions towards persistent shrublands. Unless concerted management actions and restoration programmes are undertaken, the cork oak land use systems will not be sustainable
Effects of ENSO and temporal rainfall variation on the dynamics of successional communities in old-field succession of a seasonal tropical dry forest
Maza-Villalobos, S. ; Poorter, L. ; Martínez-Ramos, M. - \ 2013
PLoS One 8 (2013). - ISSN 1932-6203 - 12 p.
soil seed banks - deciduous forest - el-nino - carbohydrate storage - severe drought - woody-plants - land-use - tree - growth - climate
The effects of temporal variation of rainfall on secondary succession of tropical dry ecosystems are poorly understood. We studied effects of inter-seasonal and inter-year rainfall variation on the dynamics of regenerative successional communities of a tropical dry forest in Mexico. We emphasized the effects caused by the severe El Niño Southern Oscillation (ENSO) occurred in 2005. We established permanent plots in sites representing a chronosequence of Pasture (abandoned pastures, 0–1 years fallow age), Early (3–5), Intermediate (8–12), and Old-Growth Forest categories (n = 3 per category). In total, 8210 shrubs and trees 10 to 100-cm height were identified, measured, and monitored over four years. Rates of plant recruitment, growth and mortality, and gain and loss of species were quantified per season (dry vs. rainy), year, and successional category, considering whole communities and separating seedlings from sprouts and shrubs from trees. Community rates changed with rainfall variation without almost any effect of successional stage. Mortality and species loss rates peaked during the ENSO year and the following year; however, after two rainy years mortality peaked in the rainy season. Such changes could result from the severe drought in the ENSO year, and of the outbreak of biotic agents during the following rainy years. Growth, recruitment and species gain rates were higher in the rainy season but they were significantly reduced after the ENSO year. Seedlings exhibited higher recruitment and mortality rate than sprouts, and shrubs showed higher recruitment than trees. ENSO strongly impacted both the dynamics and trajectory of succession, creating transient fluctuations in the abundance and species richness of the communities. Overall, there was a net decline in plant and species density in most successional stages along the years. Therefore, strong drought events have critical consequences for regeneration dynamics, delaying the successional process and modifying the resilience of these systems.
Effects of interannual climate variability on tropical tree cover
Holmgren, M. ; Hirota, M. ; Nes, E.H. van; Scheffer, M. - \ 2013
Nature Climate Change 3 (2013). - ISSN 1758-678X - p. 755 - 758.
el-nino - critical transitions - shrub encroachment - south-america - woody cover - savanna - forest - determinants - ecosystems - africa
Climatic warming is substantially intensifying the global water cycle1 and is projected to increase rainfall variability2. Using satellite data, we show that higher climatic variability is associated with reduced tree cover in the wet tropics globally. In contrast, interannual variability in rainfall can have neutral or even positive effects on tree cover in the dry tropics. In South America, tree cover in dry lands is higher in areas with high year-to-year variability in rainfall. This is consistent with evidence from case studies suggesting that in these areas rare wet episodes are essential for opening windows of opportunity where massive tree recruitment can overwhelm disturbance effects, allowing the establishment of extensive woodlands. In Australia, wet extremes have similar effects, but the net effect of rainfall variability is overwhelmed by negative effects of extreme dry years. In Africa, effects of rainfall variability are neutral for dry lands. It is most likely that differences in herbivore communities and fire regimes contribute to regulating tree expansion during wet extremes. Our results illustrate that increasing climatic variability may affect ecosystem services in contrasting, and sometimes surprising, ways. Expansion of dry tropical tree cover during extreme wet events may decrease grassland productivity but enhance carbon sequestration, soil nutrient retention and biodiversity.
Trend change detection in NDVI time series: Effects of inter-annual variability and methodology
Forkel, M. ; Carvalhais, N. ; Verbesselt, J. ; Mahecha, M.D. ; Neigh, C. ; Reichstein, M. - \ 2013
Remote Sensing 5 (2013)5. - ISSN 2072-4292 - p. 2113 - 2144.
spectral vegetation indexes - satellite data - north-america - boreal forest - el-nino - alaska - modis - climate - disturbance - accuracy
Changing trends in ecosystem productivity can be quantified using satellite observations of Normalized Difference Vegetation Index (NDVI). However, the estimation of trends from NDVI time series differs substantially depending on analyzed satellite datase
Monitoring land cover changes in African protected areas in the 21st century
Gross, D. ; Dubois, G. ; Pekel, J.F. ; Mayaux, P. ; Holmgren, M. ; Prins, H.H.T. ; Rondini, C. ; Boitani, L. - \ 2013
Ecological Informatics 14 (2013). - ISSN 1574-9541 - p. 31 - 37.
tropical forests - el-nino - ecosystems - biodiversity - savannas - impact - fire - challenges - dynamics - soil
Africa is home to some of the most vulnerable natural ecosystems and species on the planet. Around 7000 protected areas seek to safeguard the continent's rich biodiversity, but many of them face increasing management challenges. Human disturbances permeating into the parks directly and indirectly affect the ecological functioning and integrity of protected areas. With the envisaged expansion of the protected area network and further expected population and economic growth in the region, the competition between nature conservation and resources demands is likely to increase. The regular monitoring of land cover in and around protected areas can support the early detection of conservation conflicts. In this paper, we evaluate the use of the annual time series of MODIS Land Cover (LC) type product between 2003 and 2009 to monitor land cover changes at continental scale. We use the mean classification confidence and change frequency as indicators to assess the temporal consistency of the MODIS LC classifier for accurately monitoring land cover changes. We discuss the perspectives and issues for an automated monitoring of land cover changes in African protected areas.
Decadal prediction skill in a multi-model ensemble
Oldenborgh, G.J. van; Doblas-Reyes, F.J. ; Wouters, B. ; Hazeleger, W. - \ 2012
Climate Dynamics 38 (2012)7-8. - ISSN 0930-7575 - p. 1263 - 1280.
coupled climate models - surface-temperature - precipitation trends - north-atlantic - time-scales - el-nino - ocean - variability - pacific - predictability
Decadal climate predictions may have skill due to predictable components in boundary conditions (mainly greenhouse gas concentrations but also tropospheric and stratospheric aerosol distributions) and initial conditions (mainly the ocean state). We investigate the skill of temperature and precipitation hindcasts from a multi-model ensemble of four climate forecast systems based on coupled ocean-atmosphere models. Regional variations in skill with and without trend are compared with similarly analysed uninitialised experiments to separate the trend due to monotonically increasing forcings from fluctuations around the trend due to the ocean initial state and aerosol forcings. In temperature most of the skill in both multi-model ensembles comes from the externally forced trends. The rise of the global mean temperature is represented well in the initialised hindcasts, but variations around the trend show little skill beyond the first year due to the absence of volcanic aerosols in the hindcasts and the unpredictability of ENSO. The models have non-trivial skill in hindcasts of North Atlantic sea surface temperature beyond the trend. This skill is highest in the northern North Atlantic in initialised experiments and in the subtropical North Atlantic in uninitialised simulations. A similar result is found in the Pacific Ocean, although the signal is less clear. The uninitialised simulations have good skill beyond the trend in the western North Pacific. The initialised experiments show some skill in the decadal ENSO region in the eastern Pacific, in agreement with previous studies. However, the results in this study are not statistically significant (p ˜ 0.1) by themselves. The initialised models also show some skill in forecasting 4-year mean Sahel rainfall at lead times of 1 and 5 years, in agreement with the observed teleconnection from the Atlantic Ocean. Again, the skill is not statistically significant (p ˜ 0.2). Furthermore, uninitialised simulations that include volcanic aerosols have similar skill. It is therefore still an open question whether initialisation improves predictions of Sahel rainfall. We conclude that the main source of skill in forecasting temperature is the trend forced by rising greenhouse gas concentrations. The ocean initial state contributes to skill in some regions, but variations in boundary forcings such as aerosols are as important in decadal forecasting.
Rainfall-tuned management facilitates dry forest recovery
Sitters, J. ; Holmgren, M. ; Stoorvogel, J.J. ; López, B.C. - \ 2012
Restoration Ecology 20 (2012)1. - ISSN 1061-2971 - p. 33 - 42.
difference vegetation index - el-nino - global desertification - semiarid grassland - tree establishment - african savannas - dynamics - restoration - ecosystems - patterns
Regeneration of original dry forests and shrublands in degraded arid and semiarid ecosystems can be a slow and difficult process. It has been hypothesized that restoration efforts during periods of increased water availability may potentially trigger shifts back to a high vegetation cover depending on several environmental factors that govern the response of vegetation to rainfall. Tuning restoration efforts to climate variability will likely become increasingly important under climate change conditions. The experiences evaluated here are a pioneering effort to reforest arid South American forests. We used a combination of field monitoring and remote sensing images to evaluate the long-term effects of seeding and herbivore control in local reforestation projects tuned to the forecasted rainy El Niño Southern Oscillation (ENSO) events of 1991/1992 and 1997/1998 in North Peru and to assess the regional response of vegetation to these rainy events at a regional scale. We found that managing seed availability in combination with seedling protection from herbivores only yielded persistent higher vegetation cover when implemented on sites without calcareous layers and relatively high water availability determined by the surrounding topography. Our study shows that management tuned to forecasted rainfall events is able to trigger a long-lasting shift toward higher vegetation cover. We provide a better insight in how environmental factors shape vegetation response to increased rainfall and discuss the implications for ecosystem resilience and restoration
Strict mast fruiting for a tropical dipterocarp tree: a demographic cost–benefit analysis of delayed reproduction and seed predation
Visser, M.D. ; Jongejans, E. ; Breugel, M. van; Zuidema, P.A. ; Chen, Y.Y. ; Kassim, A.R. ; Kroon, H. de - \ 2011
Journal of Ecology 99 (2011)4. - ISSN 0022-0477 - p. 1033 - 1044.
rain-forest - evolutionary ecology - spatial-patterns - resource-allocation - woody-plants - el-nino - recruitment - dispersal - dynamics - impact
1. Masting, the production of large seed crops at intervals of several years, is a reproductive adaptation displayed by many tree species. The predator satiation hypothesis predicts that starvation of seed predators between mast years and satiation during mast years decreases seed predation and thus enhances tree regeneration. 2. Mast fruiting comes at demographic costs such as missed reproduction opportunities and increased density-dependence of recruits, but it remains unknown if predator satiation constitutes a sufficiently large benefit for masting to evolve as a viable life-history strategy. So far, no studies have quantified the net fitness consequences of masting. 3. Using a long-term demographic data set of the dipterocarp Shorea leprosula in a Malaysian forest, we constructed stochastic matrix population models and performed a demographic cost–benefit analysis. 4. For observed values of mast frequency and seed predation rates, we show that strict masting strongly increases fitness compared with fruiting annually. Model results also show that the demographic costs of mast fruiting are very low compared to the demographic losses due to seed predation in a scenario of annual fruiting. Finally, we find that mast fruiting would still be selected for even at low levels of seed predation and when including additional costs such as decreased adult growth rates, limiting crop size and density-dependent seedling survival. 5. Synthesis. Our results are consistent with the predictions of the predator satiation hypothesis: mast fruiting increases fitness for a range of seed predation levels. Under seed predation pressure annually fruiting species are at a strong disadvantage and as a result a mast fruiting strategy may swiftly confer a fitness advantage. Our study shows that demographic modelling allows the weighing of fitness benefits and costs of life-history phenomena such as strict masting.
Rapid root extension during water pulses enhances establishment of shrub seedlings in the Atacama Desert
Leon, M.F. ; Squeo, F.A. ; Gutierrez, J.R. ; Holmgren, M. - \ 2011
Journal of Vegetation Science 22 (2011)1. - ISSN 1100-9233 - p. 120 - 129.
north-central chile - el-nino - semiarid ecosystems - tree establishment - resource pulses - sonoran desert - patterns - survival - dynamics - growth
Questions: (1) What is the water threshold for shrub seedling establishment in arid scrubland? (2) How do seedling root growth morphological traits affect the water threshold required for seedling establishment? Location: Arid scrubland, Atacama Desert, north-central Chile. Methods: We conducted a field experiment with nine native shrub species under a gradient of simulated rainfall to test if species with deep root architecture have higher seedling survival rates and establish more successfully during water pulses. Results: Seedling survival rate was very low; only 2% of the 12 150 planted seedlings survived the first summer drought. Seedling survival required at least 206 mm of water, which is twice the average rainfall and roughly equivalent to the precipitation during an ENSO (El Niño Southern Oscillation) event in this region. Seedling survival at the onset of the summer drought was best explained by leaf habit, root length and initial seedling size. However, only Senna cumingii seedlings survived through the first year. S. cumingii seedlings had distinctively longer roots than the other shrub species, enabling them to reach moister soil layers. Conclusions: Water conditions resembling rainy years enhance shrub seedling establishment in the Atacama Desert, but the effects of higher water availability are strongly dependent on the shrub species. Rapid and deep rooting appears to be a very important functional trait for successful first-year survival in this arid system where water availability is highly episodic.
Fire as a selective force in a Bornean tropical everwet forest
Slik, J.W.F. ; Breman, F.C. ; Bernard, C. ; Beek, M. van; Cannon, C.H. ; Eichhorn, K.A.O. ; Sidiyasa, K. - \ 2010
Oecologia 164 (2010)3. - ISSN 0029-8549 - p. 841 - 849.
rain-forest - topographic position - amazonian forest - tree diversity - eastern amazon - el-nino - drought - dynamics - savanna - biomass
Tree species rarely exposed to burning, like in everwet tropical forests, are unlikely to be fire adapted. Therefore, one could hypothesize that these species are affected equally by burning and that tree abundance changes are linked solely to fire behavior. Alternatively, if species do react differentially to burning, abundance changes should be linked to tree habitat preference and morphology. Using tree inventories from old-growth and adjacent burned Bornean forest in combination with a database on tree morphology and habitat preference, we test these alternative hypotheses by (1) determining whether species specific abundance changes after fire differ significantly from equal change, and (2) whether observed abundance changes are linked to species morphology and habitat preference. We found that of 196 species tested, 125 species showed an abundance change significantly different from that expected under our null model of equal change. These abundance changes were significantly linked to both tree morphology and habitat preference. Abundance declines were associated with slope or ridge preference, thin barks, and limited seed dormancy. Abundance increases were associated with high light preference, small adult stature, light wood, large leaves, small seeds and long seed dormancy. While species habitat preference and morphology explained observed abundance increases well, abundance declines were only weakly associated with them (R (2) similar to 0.09). This suggests that most tree mortality was random and everwet tropical tree species are poorly fire adapted. As fire frequencies are increasing in the everwet tropics, this might eventually result in permanently altered species compositions and even species extinctions.
Providing seasonal-to-interannual climate information for risk management and decision making
Goddard, L. ; Aitchellouche, Y. ; Baethgen, W. ; Dettinger, M. ; Graham, R. ; Hayman, P. ; Kadi, M. ; Martínez, R. ; Meinke, H.B. - \ 2010
Procedia Environmental Sciences 1 (2010). - ISSN 1878-0296 - p. 81 - 101.
incomes-transforming advice - australian drought policy - western united-states - el-nino - precipitation forecasts - multimodel ensembles - farm incomes - snow cover - prediction - rainfall
Much has been learned in the interpretation and use of climate information since the 1997/1998 El Niño event that garnered so much attention. Seasonal-to-interannual forecasts are now produced around the world. However, mismatches in their scales, specificity or communication (of forecast content and uncertainties) with decision-maker needs still hinder their use. More work is needed to improve a) the utility of models, b) access to observational and model/forecast data, c) understanding and communication of the opportunities and limitations of forecasts, and d) methods by which decision systems use climate predictions – both through modifications of decision systems and more tailored forecast information. This white paper discusses these issues and recent advances in providing climate information needed in effective climate risk management. The paper advocates for the establishment and/or strengthening of “chains of experts and communications” to better enable appropriate and effective dissemination, assimilation and further use and application of climate information
From Scientific Speculation to Effective Adaptive Management: A case study of the role of social marketing in promoting novel restoration strategies for degraded dry lands
Westley, F. ; Holmgren, M. ; Scheffer, M. - \ 2010
Ecology and Society 15 (2010)3. - ISSN 1708-3087 - p. 6 - 6.
chilean agrarian transformation - el-nino - tree establishment - conservation - ecosystems - mechanisms - vegetation - ecology - forests - climate
This article focuses on the role of social marketing, in particular the analysis of the motivations and capabilities of stakeholder groups, in encouraging acceptance of an innovative experimental approach to semiarid shrub land restoration in Chile. Controlled scientific experiments involving herbivory control during El Niño events have proved promising, but have not yet been introduced into ecosystem management approaches. Social marketing, as a lens for focusing on and understanding stakeholders’ motivations, provides a valuable framework in which strategies may be developed for diffusing promising scientific experiments into regional management contexts
Modeling the Spatial Distribution and Fruiting Pattern of a Key Tree Species in a Neotropical Forest: Methodology and Potential Applications
Caillaud, D. ; Crofoot, M.C. ; Scarpino, S.V. ; Jansen, P.A. ; Garzon-Lopez, C.X. ; Winkelhagen, A.J.S. ; Bohlman, S.A. ; Walsh, P.D. - \ 2010
PLoS One 5 (2010)11. - ISSN 1932-6203 - 10 p.
approximate bayesian computation - dipteryx-panamensis - movement patterns - habitat selection - tropical forest - seed dispersal - el-nino - behavior - panama - recruitment
Background - The movement patterns of wild animals depend crucially on the spatial and temporal availability of resources in their habitat. To date, most attempts to model this relationship were forced to rely on simplified assumptions about the spatiotemporal distribution of food resources. Here we demonstrate how advances in statistics permit the combination of sparse ground sampling with remote sensing imagery to generate biological relevant, spatially and temporally explicit distributions of food resources. We illustrate our procedure by creating a detailed simulation model of fruit production patterns for Dipteryx oleifera, a keystone tree species, on Barro Colorado Island (BCI), Panama. Methodology and Principal Findings - Aerial photographs providing GPS positions for large, canopy trees, the complete census of a 50-ha and 25-ha area, diameter at breast height data from haphazardly sampled trees and long-term phenology data from six trees were used to fit 1) a point process model of tree spatial distribution and 2) a generalized linear mixed-effect model of temporal variation of fruit production. The fitted parameters from these models are then used to create a stochastic simulation model which incorporates spatio-temporal variations of D. oleifera fruit availability on BCI. Conclusions and Significance - We present a framework that can provide a statistical characterization of the habitat that can be included in agent-based models of animal movements. When environmental heterogeneity cannot be exhaustively mapped, this approach can be a powerful alternative. The results of our model on the spatio-temporal variation in D. oleifera fruit availability will be used to understand behavioral and movement patterns of several species on BCI.
Wood Density as a Conservation Tool: Quantification of Disturbance and Identification of Conservation-Priority Areas in Tropical Forests
Slik, J.W.F. ; Bernard, C.S. ; Breman, F.C. ; Beek, M. van; Salim, A. ; Sheil, D. - \ 2008
Conservation Biology 22 (2008)5. - ISSN 0888-8892 - p. 1299 - 1308.
tree species composition - rain-forest - dipterocarp forests - central-america - el-nino - diversity - borneo - kalimantan - indonesia - fire
Inventories of tree species are often conducted to guide conservation efforts in tropical forests. Such surveys are time consuming, demanding of expertise, and expensive to perform and interpret. Approaches to make survey efforts simpler or more effective would be valuable. In particular, it would be good to be able to easily identify areas of old-growth forest. The average density of the wood of a tree species is closely linked to its successional status. We used tree inventory data from eastern Borneo to determine whether wood density can be used to quantify forest disturbance and conservation importance. The average density of wood in a plot was significantly and negatively related to disturbance levels, with plots with higher wood densities occurring almost exclusively in old-growth forests. Average wood density was unimodally related to the diversity of tree species, indicating that the average wood density in a plot might be a better indicator of old-growth forest than species diversity. In addition, Borneo endemics had significantly heavier wood than species that are common throughout the Malesian region, and they were more common in plots with higher average wood density. We concluded that wood density at the plot level could be a powerful tool for identifying areas of conservation priority in the tropical rain forests of Southeast Asia.
Pulse driven loss of top-down control: The critical-rate hypothesis
Scheffer, M. ; Nes, E.H. van; Holmgren, M. ; Hughes, T. - \ 2008
Ecosystems 11 (2008)2. - ISSN 1432-9840 - p. 226 - 237.
exploitation ecosystems - climate-change - coral-reefs - el-nino - stability - shifts - photosynthesis - cyanobacteria - productivity - degradation
In systems ranging from lakes and woodlands to coral reefs, the long-term ecosystem state may often be determined largely by rare extreme events such as wet ENSO years, droughts, or disease outbreaks. Such events can flip these systems into a contrasting state that represents either an alternative attractor or a transient that is slow enough to persist even if the frequency of events that push the system to this state is low. Here we show that escape from herbivores is a mechanism that can play a role in several state shifts of this kind. This can happen if plants become less susceptible to herbivory as they grow. Using a model we show that, surprisingly, this mechanism can lead to a situation where a brief resource pulse for plants may invoke a persistent shift to a high biomass state whereas gradual enrichment to the same resource level is insufficient to allow such a change. This counterintuitive phenomenon occurs if the numerical response of herbivores is sufficiently slow to allow the plants to use the resource pulse to escape to a safe biomass at which herbivory is reduced. Our results imply that rates of environmental change can sometimes be more important than their magnitude. This has many ramifications. On the conceptual side, our findings suggest that key mechanisms that regulate long-term ecosystem dynamics are easily missed by the traditional focus of modelers on equilibria. A more practical corollary is that increased climatic variability may have more profound effects in some ecosystems than gradual change in conditions.
'If only it would rain': Farmers' perceptions of rainfall and drought in semi-arid central Tanzania
Slegers, M.F.W. - \ 2008
Journal of Arid Environments 72 (2008)11. - ISSN 0140-1963 - p. 2106 - 2123.
perceived behavioral-control - african rainfall - climate-change - el-nino - ethiopia - anthropology - environment - framework - hazards - food
Farmers in semi-arid East Africa prioritize drought as their major productivity-reducing problem, while scientists identify soil degradation as a major threat. The question that needs to be addressed is how farmers perceive drought. Insight into farmers¿ perceptions of drought could be the missing link towards increased and sustainable production. During the period from 2004 to 2006, a study was conducted in Goima Ward, Tanzania. The study included questionnaires with mainly open-ended questions (N=120), in-depth interviews, group discussions, field visits, picture assignment and 1-day seminar with farmers, extension workers, government officials, and scientists. Farmers saw drought in a broader perspective than that of deficient rainfall. Drought indicators were area-specific. Farmers recognized differences in drought vulnerability between soil types, location and state of land, land management practices and farmer types. Farmers perceived an increased drought risk. Farmers¿ perceptions can be related to the scientific concept of agricultural drought. Since the amount of rainfall cannot be influenced, the efficiency of the rains can be a focal point for addressing farmers¿ concerns about drought. Any viable strategy to reduce farmers¿ vulnerability to drought and to improve productivity should be integrated into farmers¿ existing strategies to adapt to and cope with ecological insecurity
Estimating annual rainfall threshold for establishment of tree species in water-limited ecosystems using tree-ring data
Lopez, B.C. ; Holmgren, M. ; Sabate, S. ; Gracia, C.A. - \ 2008
Journal of Arid Environments 72 (2008)5. - ISSN 0140-1963 - p. 602 - 611.
el-nino - semiarid ecosystems - atacama desert - south-america - enso events - climate - vegetation - demography - australia - oscillation
In arid and semi-arid ecosystems, water availability is discontinuous, highly variable, and characterized by discrete pulse events separated by long periods of limited resource availability. Plant recruitment in these ecosystems is also episodic and dependent on the water available during and after these discrete rainfall events. Precipitation thresholds for plant establishment have been estimated mainly for herbaceous plants and tree seedlings, but extrapolation of short-term results based on seedlings to natural tree populations is difficult. Nevertheless, estimations of water availability thresholds for tree recruitment are essential for successful policies on forest conservation and restoration. We propose a methodology to estimate precipitation thresholds for adult tree populations using tree-ring series and precipitation data. We used this methodology with two Prosopis species from South America: Prosopis pallida and Prosopis chilensis. Results indicate a precipitation threshold of around 85 mm for the establishment of P. pallida trees, whereas the threshold for P. chilensis is likely to be much higher.
The persistence and conservation of Borneo's mammals in lowland rain forests managed for timber: observations, overviews and opportunities.
Meijaard, E. ; Sheil, D. - \ 2008
Ecological Research 23 (2008)1. - ISSN 0912-3814 - p. 21 - 34.
tropical forests - east kalimantan - protected areas - habitat disturbance - sun bears - el-nino - indonesia - malaysia - wildlife - diversity
Lowland rainforests on Borneo are being degraded and lost at an alarming rate. Studies on mammals report species responding in various ways to habitat changes that occur in commercial forestry concessions. Here we draw together information on the relationship between the ecological, evolutionary, and biogeographic characteristics of selected Bornean non-volant mammals, and their response to timber harvesting and related impacts. Only a minority of species show markedly reduced densities after timber harvesting. Nonetheless there are many grounds for concern as various processes can, and often do, reduce the viability of wildlife populations. Our review of what we know, and of current understanding, helps predict mammalian dynamics and subsequent mammal-induced ecosystem changes in logged forests. We identify groups of mammal species that, although largely unstudied, are unlikely to tolerate the impacts associated with timber harvesting. On a positive note we find and suggest many relatively simple and low-cost ways in which concession management practices might be modified so as to improve the value of managed forests for wildlife conservation. Improving forest management can play a vital role in maintaining the rich biodiversity of Borneo's tropical rain forests
Reduced herbivory during simulated ENSO rainy events increases native herbaceous plants in semiarid Chile
Manrique, R. ; Gutierrez, J.R. ; Holmgren, M. ; Squeo, F.A. - \ 2007
Plant Ecology 191 (2007)1. - ISSN 1385-0237 - p. 21 - 31.
el-nino - community structure - annual forbs - vegetation - california - grassland - dynamics - impacts - establishment - variability
El Niño Southern Oscillation (ENSO) events have profound consequences for the dynamics of terrestrial ecosystems. Since increased climate variability is expected to favour the invasive success of exotic species, we conducted a field experiment to study the effects that simulated rainy ENSO events in combination with herbivores and shade have on the composition of a semiarid herbaceous community in north-central Chile. We hypothesized that water pulses, such as those associated with rainy ENSO events could trigger significant changes in the relative abundance of exotic and native herbaceous species. Specifically, we predicted an increase in native grasses and a reduction in the abundance of exotic species, especially prostrate forbs, if water pulses were combined with reduced herbivory. We found that herbivory by small mammals, especially introduced European rabbits (Oryctolagus cuniculus) and hares (Lepus europaeus), have an overwhelming effect on species abundance and composition in this semiarid herbaceous community. Herbivore exclusion produced an overall increase in herb density and biomass mostly due to the extraordinary growth of tall native grasses (especially Bromus berterianus) that outcompeted small prostrate forbs (both native and exotic ones), and small exotic grasses (Koeleria pleoides, Schismus arabicus). Our results suggest that it might be possible to enhance the recovery of native grasses by applying efficient herbivore control during rainy years such as those associated with ENSO events although a negative consequence would be the loss of small native forbs, which greatly contribute to the richness of herbaceous communities in semiarid ecosystems.
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