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Savannas are characterized by the coexistence of two contrasting plant life‐forms: woody and herbaceous vegetation. During the last decades, there has been a global trend of an increase in woody cover and the spread of shrubs and trees into areas that were previously dominated by grasses. This process, termed bush encroachment, is associated with severe losses of ecosystem functions and typically difficult to reverse. It is assumed to be an example of a critical transition between two alternative stable states. Overgrazing due to unsustainable rangeland management has been identified as one of the main causes of this transition, as it can trigger several self‐reinforcing feedback loops. However, the dynamic role of grazing within such feedback loops has received less attention. We used a set of coupled differential equations to describe the competition between shrubs and grasses, as well as plant biomass consumption via grazing and browsing. Grazers were assumed to receive a certain level of care from farmers, so that grazer densities emerge dynamically from the combined effect of vegetation abundance and farmer support. We quantified all self‐reinforcing and self‐dampening feedback loops at play and analyzed their relative importance in shaping system (in‐)stability. Bistability, the presence of a grass dominated and a shrub dominated state, emerges for intermediate levels of farmer support due to positive feedback that arises from competition between shrubs and grasses and from herbivory. We furthermore demonstrate that disturbances, such as drought events, trigger abrupt transitions from the grass dominated to the shrub dominated state and that the system becomes more susceptible to disturbances with increasing farmer support. Our results thus highlight the potential of interaction networks in combinations with feedback loop analysis for improving our understanding of critical transitions in general, and bush encroachment in particular. [ABSTRACT FROM AUTHOR]

Savannas are characterized by water scarcity and degradation, making them highly vulnerable to increased uncertainties in water availability resulting from climate change. This poses a significant threat to ecosystem services and rural livelihoods that depend on them. In addition, the lack of consensus among climate models on precipitation change makes it difficult for land managers to plan for the future. Therefore, Savanna rangeland management needs to develop strategies that can sustain Savanna resilience and avoid tipping points under an uncertain future climate.Our study aims to analyse the impacts of climate change and rangeland management on degradation in Savanna ecosystems of southern Africa, providing insights for the management of semi‐arid Savannas under uncertain conditions worldwide. To achieve this, we simulated the effects of projected changes in temperature and precipitation, as predicted by 10 global climate models, on water resources and vegetation (cover, functional diversity, tipping points (transition from grass‐dominated to shrub‐dominated vegetation)). We simulated three different rangeland management options (herbivore communities dominated by grazers, by browser and by mixed feeders), each with low and high animal densities, using the ecohydrological model EcoHyD.Our results identified intensive grazing as the primary contributor to the increased risk of degradation in response to changing climatic conditions across all climate change scenarios. This degradation encompassed a reduction in available water for plant growth within the context of predicted climate change. It also entails a decline in the overall vegetation cover, the loss of functionally important plant species and the inefficient utilization of available water resources, leading to earlier tipping points.Synthesis and applications. Our findings underscore that, in the face of climate uncertainty, farmers' most effective strategy for securing their livelihoods and ecosystem stability is to integrate browsers and apply management of mixed herbivore communities. This management approach not only significantly delays or averts tipping points but also maintained greater plant functional diversity, fostering a more robust and resilient ecosystem that acts as a vital buffer against adverse climatic conditions. [ABSTRACT FROM AUTHOR]

Drylands occur worldwide and are particularly vulnerable to climate change because dryland ecosystems depend directly on soil water availability that may become increasingly limited as temperatures rise. Climate change will both directly impact soil water availability and change plant biomass, with resulting indirect feedbacks on soil moisture. Thus, the net impact of direct and indirect climate change effects on soil moisture requires better understanding. We used the ecohydrological simulation model SOILWAT at sites from temperate dryland ecosystems around the globe to disentangle the contributions of direct climate change effects and of additional indirect, climate change-induced changes in vegetation on soil water availability. We simulated current and future climate conditions projected by 16 GCMs under RCP 4.5 and RCP 8.5 for the end of the century. We determined shifts in water availability due to climate change alone and due to combined changes of climate and the growth form and biomass of vegetation. Vegetation change will mostly exacerbate low soil water availability in regions already expected to suffer from negative direct impacts of climate change (with the two RCP scenarios giving us qualitatively similar effects). By contrast, in regions that will likely experience increased water availability due to climate change alone, vegetation changes will counteract these increases due to increased water losses by interception. In only a small minority of locations, climate change-induced vegetation changes may lead to a net increase in water availability. These results suggest that changes in vegetation in response to climate change may exacerbate drought conditions and may dampen the effects of increased precipitation, that is, leading to more ecological droughts despite higher precipitation in some regions. Our results underscore the value of considering indirect effects of climate change on vegetation when assessing future soil moisture conditions in water-limited ecosystems. [ABSTRACT FROM AUTHOR]

Questions: Soil resource heterogeneity influences the outcome of plant–plant interactions and, consequently, species co‐existence and diversity patterns. The magnitude and direction of heterogeneity effects vary widely, and the processes underlying such variations are not fully understood. In this study, we explored how and under what resource conditions small‐scale heterogeneity modulates grassland plant diversity. Location: Oderhänge Mallnow, Potsdam, Brandenburg, Germany. Methods: We expanded the individual‐based plant community model (IBC‐grass) to incorporate dynamic below‐ground resource maps, simulating spatial heterogeneity of resource availability. Empirical centimeter‐scale data of soil C/N ratio were integrated into the model, accounting for both configurational and compositional heterogeneity. We then analyzed the interplay between small‐scale heterogeneity and resource availability on the interaction and co‐existence of plant species and overall diversity. Results: Our results showed significant differences between the low‐ and high‐resource scenarios, with both configurational and compositional heterogeneity having a positive effect on species richness and Simpson's diversity, but only under low‐resource conditions. As compositional heterogeneity in the fine‐scale C/N ratio increased, we observed a positive shift in Simpson's diversity and species richness, with the highest effects at the highest level of variability tested. We observed little to no effect in nutrient‐rich scenarios, and a shift to negative effects at the intermediate resource level. The study demonstrates that site‐specific resource levels underpin how fine‐scale heterogeneity influences plant diversity and species co‐existence, and partly explains the divergent effects recorded in different empirical studies. Conclusions: This study provides mechanistic insights into the complex relationship between resource heterogeneity and diversity patterns. It highlights the context‐dependent effects of small‐scale heterogeneity, which can be positive under low‐resource, neutral under high‐resource, and negative under intermediate‐resource conditions. These findings provide a foundation for future investigations into small‐scale heterogeneity–diversity relationships, contributing to a deeper understanding of the processes that promote species co‐existence in plant communities. [ABSTRACT FROM AUTHOR]

Under a changing climate, soil fungal communities will increasingly be subject to periods of heat stress. These periods can affect the performance of individual fungi and their competition for space and resources. Competition between fungi is strongly controlled by the exudation of inhibitory compounds, resulting in different competitive outcomes that range from overgrowth of the inferior competitor to a deadlock, where the competing fungi inhibit each other. As heat stress can alter the competitive outcome between fungi, the community composition can also change strongly. So far, a general understanding of the mechanisms that drive the competitive outcome between fungi under heat stress is still missing. However, this understanding is essential to assess important community functions, such as decomposition or mediation of plant nutrition, which strongly depend on the fungal community composition. Here, we used a partial differential equation (PDE) model simulating two fungal competitors in a two‐dimensional space, to mechanistically explain the observed change of fungal competition under heat stress. The model describes mycelial growth, the production and secretion of antifungal compounds and the synthesis of heat shock proteins of interacting colonies. We found a heat stress‐induced lag phase favouring the accumulation of antifungal compounds and the build‐up of inhibitor fields. This led to a qualitative change of the competitive outcome, reducing the occurrence of overgrowth by two thirds. The changes in competitive outcome favoured slower growing species, which benefit more strongly from the additional time during a stress‐induced lag to build up a defence or block territory that would otherwise be quickly claimed by faster competitors. Our work is an important step towards understanding how environmental changes may lead to qualitative changes in competitive outcomes. Our results show the importance of explicitly including species interactions into studies of climate change effects. [ABSTRACT FROM AUTHOR]

1. The complex, nonlinear response of dryland systems to grazing and climatic variations is a challenge to management of these lands. Predicted climatic changes will impact the desertification of drylands under domestic livestock production. Consequently, there is an urgent need to understand the response of drylands to grazing under climate change. 2. We enhanced and parameterized an ecohydrological savanna model to assess the impacts of a range of climate change scenarios on the response of a semi-arid African savanna to grazing. We focused on the effects of temperature and CO2 level increase in combination with changes in inter- and intra-annual precipitation patterns on the long-term dynamics of three major plant functional types. 3. We found that the capacity of the savanna to sustain livestock grazing was strongly influenced by climate change. Increased mean annual precipitation and changes in intra-annual precipitation pattern have the potential to slightly increase carrying capacities of the system. In contrast, decreased precipitation, higher interannual variation and temperature increase are leading to a severe decline of carrying capacities owing to losses of the perennial grass biomass. 4. Semi-arid rangelands will be at lower risk of shrub encroachment and encroachment will be less intense under future climatic conditions. This finding holds in spite of elevated levels of atmospheric CO2 and irrespective of changes in precipitation pattern, because of the drought sensitivity of germination and establishment of encroaching species. 5. Synthesis and applications. Changes in livestock carrying capacities, both positive and negative, mainly depend on the highly uncertain future rainfall conditions. However, independent of the specific changes, shrub encroachment becomes less likely and in many cases less severe. Thus, managers of semi-arid rangelands should shift their focus from woody vegetation towards perennial grass species as indicators for rangeland degradation. Furthermore, the resulting reduced competition from woody vegetation has the potential to facilitate ecosystem restoration measures such as re-introduction of desirable plant species that are only little promising or infeasible under current climatic conditions. On a global scale, the reductions in standing biomass resulting from altered degradation dynamics of semi-arid rangelands can have negative impacts on carbon sequestration. [ABSTRACT FROM AUTHOR]

We analyzed relative sensitivities of small- and medium-sized carnivores to livestock husbandry (stocking rates and predator control) in Kalahari, South Africa, rangelands at a regional scale. We monitored small carnivores using track counts on 22 Kalahari farms across a land-use gradient ranging from low to high stocking rates and also interviewed each farm manager to identify farmers' perception of small carnivores as potential predators for livestock. We recorded 12 species of small- and medium-sized carnivores across 22 Kalahari farms. Stocking rate was the most important driving variable for local carnivore abundance. Abundance of all species was lowest on farms where stocking rate was high. Most farm managers perceived medium-sized carnivores, in particular, African wildcat (Felis silvestris lybica), blackbacked jackal (Canis mesomelas), and caracal (Caracal caracal ), as potential predators of livestock. Multiple regression analysis shows that blackbacked jackal, African wildcat, and caracal were negatively affected by predator control measures, whereas bat-eared fox (Otocyon megalotis), cape fox (Vulpes chama), and small-spotted genet (Genetta genetta) were positively affected. Our results show a need for expanding research and conservation activities toward small- and medium-sized carnivores in southern African savannah rangelands. We, therefore, suggest developing a monitoring program combining passive tracking with indigenous knowledge of local Khoisan Bushmen to monitor carnivore populations, and we recommend additional predator removal experiments that manipulate predator densities. [ABSTRACT FROM AUTHOR]

1. Sustainable land use under climate change requires detailed knowledge of the system dynamics. This applies particularly for the management of domestic livestock in semi-arid and arid grazing systems, where the risk of degradation is high and likely climate change may have a strong impact. A suitable way to assess potential future trends of these complex systems is through the application of simulation models. 2. We reviewed 41 models published between 1995 and 2005 simulating semi-arid and arid livestock grazing systems. The models were categorized according to the model aim and type, their temporal and spatial scale, and several indicators of model complexity. Additionally, we developed a list of model requirements for adequately simulating the effects of climate change. Based on these requirements, we evaluated the potential of current models to simulate impacts of climate change and determine important shortcomings. 3. Three general model types could be distinguished, namely state and transition, difference and differential equation, and rule-based models. Over time, we found that the number of models aiming to improve management strategies increased, while there were fewer models that aimed to understand system dynamics. This was accompanied by a trend to simplify model descriptions of hydrological relationships. 4. Important shortcomings of current models included the impact of increased CO2 levels on plant productivity and the ability to resolve changes in intra-annual precipitation patterns. The consideration of both external drivers is crucial under climate change, hence sustainable long-term decision making is currently lacking important information. 5. Synthesis and applications. Sustainable livestock management in semi-arid and arid systems requires knowledge about effects of future climate change to adjust livestock density adequately. Producers’ experiences from current weather conditions are not necessarily transferable to future conditions, thus models could help to support management. However, an analysis of current models has shown that few existing models are able to assess the impacts of the predicted climate change. Therefore, we call for the development of new dynamic grazing models that provide land managers with the necessary tools to face the threat of future climate changes. [ABSTRACT FROM AUTHOR]

Abstract: A global ecological restoration agenda has led to ambitious programs in environmental policy to mitigate declines in biodiversity and ecosystem services. Current restoration programs can incompletely return desired ecosystem service levels, while resilience of restored ecosystems to future threats is unknown. It is therefore essential to advance understanding and better utilize knowledge from ecological literature in restoration approaches. We identified an incomplete linkage between global change ecology, ecosystem function research, and restoration ecology. This gap impedes a full understanding of the interactive effects of changing environmental factors on the long‐term provision of ecosystem functions and a quantification of trade‐offs and synergies among multiple services. Approaches that account for the effects of multiple changing factors on the composition of plant traits and their direct and indirect impact on the provision of ecosystem functions and services can close this gap. However, studies on this multilayered relationship are currently missing. We therefore propose an integrated restoration agenda complementing trait‐based empirical studies with simulation modeling. We introduce an ongoing case study to demonstrate how this framework could allow systematic assessment of the impacts of interacting environmental factors on long‐term service provisioning. Our proposed agenda will benefit restoration programs by suggesting plant species compositions with specific traits that maximize the supply of multiple ecosystem services in the long term. Once the suggested compositions have been implemented in actual restoration projects, these assemblages should be monitored to assess whether they are resilient as well as to improve model parameterization. Additionally, the integration of empirical and simulation modeling research can improve global outcomes by raising the awareness of which restoration goals can be achieved, due to the quantification of trade‐offs and synergies among ecosystem services under a wide range of environmental conditions. [ABSTRACT FROM AUTHOR]

Maintaining the resilience and functionality of savannas is key to sustaining the ecosystem services they provide. This maintenance is largely dependent on the resilience of savannas to stressors, such as prolonged droughts. The resilience to drought is largely determined by the interaction of herbivores and the functional composition of vegetation. So far, our understanding and ability to predict the response of savannas to drought under different types of rangeland use and as a function of vegetation composition are still limited.In this study, we used the ecohydrological, spatially‐explicit savanna model EcoHyD to determine if the resilience of a savanna rangeland towards prolonged droughts can be enhanced by the choice of rangeland use type (grazer‐dominated, mixed‐feeders or browser‐dominated) and animal density. We evaluated the ability of a Namibian savanna system to withstand droughts and recover from droughts based on its perennial grass cover and the overall species composition.Generally, we determined a low resilience under high grazer densities. Most importantly, we found that functional diversification of herbivores and plants acted as resilience insurance against droughts, leading to greater resistance and recovery of perennial grasses. In particular, a higher proportion of herbivores allowed for higher resilience, probably also due to a short‐term switch to more drought‐resistant or unpalatable species.In this case, herbivore diversification was of high self‐regulatory value by reestablishing trophic complexity, reducing the need for additional management interventions.Synthesis and applications: Savanna systems will be more resistant to drought if (i) a dense perennial grass cover is maintained, protecting the topsoil from heat‐induced water losses and erosion, encompassing functionally important species that are particularly well adapted to water stress and that are palatable, if (ii) the grazing pressure is adjusted to the productivity of the system, and (iii) the herbivore community includes browsers. [ABSTRACT FROM AUTHOR]

Changing climatic conditions and unsustainable land use are major threats to savannas worldwide. Historically, many African savannas were used intensively for livestock grazing, which contributed to widespread patterns of bush encroachment across savanna systems. To reverse bush encroachment, it has been proposed to change the cattle‐dominated land use to one dominated by comparatively specialized browsers and usually native herbivores. However, the consequences for ecosystem properties and processes remain largely unclear. We used the ecohydrological, spatially explicit model EcoHyD to assess the impacts of two contrasting, herbivore land‐use strategies on a Namibian savanna: grazer‐ versus browser‐dominated herbivore communities. We varied the densities of grazers and browsers and determined the resulting composition and diversity of the plant community, total vegetation cover, soil moisture, and water use by plants. Our results showed that plant types that are less palatable to herbivores were best adapted to grazing or browsing animals in all simulated densities. Also, plant types that had a competitive advantage under limited water availability were among the dominant ones irrespective of land‐use scenario. Overall, the results were in line with our expectations: under high grazer densities, we found heavy bush encroachment and the loss of the perennial grass matrix. Importantly, regardless of the density of browsers, grass cover and plant functional diversity were significantly higher in browsing scenarios. Browsing herbivores increased grass cover, and the higher total cover in turn improved water uptake by plants overall. We concluded that, in contrast to grazing‐dominated land‐use strategies, land‐use strategies dominated by browsing herbivores, even at high herbivore densities, sustain diverse vegetation communities with high cover of perennial grasses, resulting in lower erosion risk and bolstering ecosystem services. [ABSTRACT FROM AUTHOR]

Morphological plasticity is a striking characteristic of plants in natural communities. In the context of foraging behavior particularly, root plasticity has been documented for numerous species. Root plasticity is known to mitigate competitive interactions by reducing the overlap of the individuals' rhizospheres. But despite its obvious effect on resource acquisition, plasticity has been generally neglected in previous empirical and theoretical studies estimating interaction intensity among plants. In this study, we developed a semi-mechanistic model that addresses this shortcoming by introducing the idea of compensatory growth into the classical-zone-of influence (ZOI) and field-of-neighborhood (FON) approaches. The model parameters describing the belowground plastic sphere of influence (PSI) were parameterized using data from an accompanying field experiment. Measurements of the uptake of a stable nutrient analogue at distinct distances to the neighboring plants showed that the study species responded plastically to belowground competition by avoiding overlap of individuals' rhizospheres. An unexpected finding was that the sphere of influence of the study species Bromus hordeaceus could be best described by a unimodal function of distance to the plant's center and not with a continuously decreasing function as commonly assumed. We employed the parameterized model to investigate the interplay between plasticity and two other important factors determining the intensity of competitive interactions: overall plant density and the distribution of individuals in space. The simulation results confirm that the reduction of competition intensity due to morphological plasticity strongly depends on the spatial structure of the competitive environment. We advocate the use of semi-mechanistic simulations that explicitly consider morphological plasticity to improve our mechanistic understanding of plant interactions. [ABSTRACT FROM AUTHOR]

Summary: Priming, an inducible stress defence strategy that prepares an organism for an impending stress event, is common in microbes and has been studied mostly in isolated organisms or populations. How the benefits of priming change in the microbial community context and, vice versa, whether priming influences competition between organisms, remain largely unknown. In this study, we grew different isolates of soil fungi that experienced heat stress in isolation and pairwise competition experiments and assessed colony extension rate as a measure of fitness under priming and non‐priming conditions. Based on this data, we developed a cellular automaton model simulating the growth of the ascomycete Chaetomium angustispirale competing against other fungi and systematically varied fungal response traits to explain similarities and differences observed in the experimental data. We showed that competition changes the priming benefit compared with isolated growth and that it can even be reversed depending on the competitor's traits such as growth rate, primeability and stress susceptibility. With this study, we transfer insights on priming from studies in isolation to competition between species. This is an important step towards understanding the role of inducible defences in microbial community assembly and composition. [ABSTRACT FROM AUTHOR]

Ecological restoration increasingly aims at improving ecosystem multifunctionality and making landscapes resilient to future threats, especially in biodiversity hotspots such as Mediterranean‐type ecosystems. Plants and their traits play a major role in the functioning of an ecosystem. Therefore, successful restoration towards long‐term multifunctionality requires a fundamental mechanistic understanding of this link under changing climate. An integrated approach of empirical research and simulation modelling with a focus on plant traits can allow this understanding.Based on empirical data from a large‐scale restoration project in a Mediterranean‐type ecosystem in Western Australia, we developed and validated the spatially explicit simulation model Modelling Ecosystem Functions and Services based on Traits (ModEST), which calculates coupled dynamics of nutrients, water and individual plants characterised by functional traits. We then simulated all possible combinations of eight plant species with different levels of diversity to assess the role of plant diversity and traits on multifunctionality, the provision of six ecosystem functions that can be linked to ecosystem services, as well as trade‐offs and synergies among the functions under current and future climatic conditions.Our results show that multifunctionality cannot fully be achieved because of trade‐offs among functions that are attributable to sets of traits that affect functions differently. Our measure of multifunctionality was increased by higher levels of planted species richness under current, but not future climatic conditions. In contrast, single functions were differently impacted by increased plant diversity and thus the choice and weighting of these functions affected multifunctionality. In addition, we found that trade‐offs and synergies among functions shifted with climate change due to different direct and indirect (mediated via community trait changes) effects of climate change on functions.Synthesis and application. With our simulation model Modelling Ecosystem Functions and Services based on Traits (ModEST), we show that restoration towards multifunctionality might be challenging not only under current conditions but also in the long‐term. However, once ModEST is parameterised and validated for a specific restoration site, managers can assess which target goals can be achieved given the set of available plant species and site‐specific conditions. It can also highlight which species combinations can best achieve long‐term improved multifunctionality due to their trait diversity. [ABSTRACT FROM AUTHOR]

Aim: It is widely accepted that biodiversity is influenced by both niche-related and spatial processes from local to global scales. Their relative importance, however, is still disputed, and empirical tests are surprisingly scarce at the global scale. Here, we compare the importance of area (as a proxy for pure spatial processes) and environmental heterogeneity (as a proxy for niche-related processes) for predicting native mammal species richness world-wide and within biogeographical regions. Location: Global. Time period: We analyse a spatial snapshot of richness data collated by the International Union for Conservation of Nature. Major taxa studied: All terrestrial mammal species, including possibly extinct species and species with uncertain presence. Methods: We applied a spreading dye algorithm to analyse how native mammal species richness changes with area and environmental heterogeneity. As measures for environmental heterogeneity, we used elevation ranges and precipitation ranges, which are well-known correlates of species richness. Results: We found that environmental heterogeneity explained species richness relationships better than did area, suggesting that niche-related processes are more prevalent than pure area effects at broad scales. Main conclusions: Our results imply that niche-related processes are essential to understand broad-scale species--area relationships and that habitat diversity is more important than area alone for the protection of global biodiversity. [ABSTRACT FROM AUTHOR]

Manipulative experiments typically show a decrease in dryland biocrust cover and altered species composition under climate change. Biocrust‐forming lichens, such as the globally distributed Diploschistes diacapsis, are particularly affected and show a decrease in cover with simulated climate change. However, the underlying mechanisms are not fully understood, and long‐term interacting effects of different drivers are largely unknown due to the short‐term nature of the experimental studies conducted so far.We addressed this gap and successfully parameterised a process‐based model for D. diacapsis to quantify how changing atmospheric CO2, temperature, rainfall amount and relative humidity affect its photosynthetic activity and cover. We also mimicked a long‐term manipulative climate change experiment to understand the mechanisms underlying observed patterns in the field.The model reproduced observed experimental findings: warming reduced lichen cover, whereas less rainfall had no effect on lichen performance. This warming effect was caused by the associated decrease in relative humidity and non‐rainfall water inputs, which are major water sources for biocrust‐forming lichens. Warming alone, however, increased cover because higher temperatures promoted photosynthesis during early morning hours with high lichen activity. When combined, climate variables showed non‐additive effects on lichen cover, and effects of increased CO2 levelled off with decreasing levels of relative humidity.Synthesis. Our results show that a decrease in relative humidity, rather than an increase in temperature, may be the key factor for the survival of the lichen D. diacapsis under climate change and that effects of increased CO2 levels might be offset by a reduction in non‐rainfall water inputs in the future. Because of a global trend towards warmer and drier air and the widespread global distribution of D. diacapsis, this will affect lichen‐dominated dryland biocrust communities and their role in regulating ecosystem functions worldwide. [ABSTRACT FROM AUTHOR]

•Climate change will very likely affect most forests in Amazonia during the course of the 21st century, but the direction and intensity of the change are uncertain, in part because of differences in rainfall projections. In order to constrain this uncertainty, we estimate the probability for biomass change in Amazonia on the basis of rainfall projections that are weighted by climate model performance for current conditions. •We estimate the risk of forest dieback by using weighted rainfall projections from 24 general circulation models (GCMs) to create probability density functions (PDFs) for future forest biomass changes simulated by a dynamic vegetation model (LPJmL). •Our probabilistic assessment of biomass change suggests a likely shift towards increasing biomass compared with nonweighted results. Biomass estimates range between a gain of 6.2 and a loss of 2.7 kg carbon m−2 for the Amazon region, depending on the strength of CO2 fertilization. •The uncertainty associated with the long-term effect of CO2 is much larger than that associated with precipitation change. This underlines the importance of reducing uncertainties in the direct effects of CO2 on tropical ecosystems. [ABSTRACT FROM AUTHOR]