Considerable uncertainty surrounds the fate of Amazon rainforests in response to
climate change.
Here, carbon (C) flux predictions of five terrestrial biosphere models (Community
Land Model version ...3.5 (CLM3.5), Ecosystem Demography model version 2.1 (ED2),
Integrated BIosphere Simulator version 2.6.4 (IBIS), Joint UK Land Environment
Simulator version 2.1 (JULES) and Simple Biosphere model version 3 (SiB3)) and a
hydrodynamic terrestrial ecosystem model (the
Soil–Plant–Atmosphere (SPA) model) were evaluated against
measurements from two large-scale Amazon drought experiments.
Model predictions agreed with the observed C fluxes in the control plots of both
experiments, but poorly replicated the responses to the drought treatments. Most
notably, with the exception of ED2, the models predicted negligible reductions
in aboveground biomass in response to the drought treatments, which was in
contrast to an observed c. 20% reduction at both sites.
For ED2, the timing of the decline in aboveground biomass was accurate, but the
magnitude was too high for one site and too low for the other.
Three key findings indicate critical areas for future research and model
development. First, the models predicted declines in autotrophic respiration
under prolonged drought in contrast to measured increases at one of the sites.
Secondly, models lacking a phenological response to drought introduced bias in
the sensitivity of canopy productivity and respiration to drought. Thirdly, the
phenomenological water-stress functions used by the terrestrial biosphere models
to represent the effects of soil moisture on stomatal conductance yielded
unrealistic diurnal and seasonal responses to drought.
The tropics are predicted to become warmer and drier, and understanding the sensitivity of tree species to drought is important for characterizing the risk to forests of climate change. This study ...makes use of a long-term drought experiment in the Amazon rainforest to evaluate the role of leaf-level water relations, leaf anatomy and their plasticity in response to drought in six tree genera.
The variables (osmotic potential at full turgor, turgor loss point, capacitance, elastic modulus, relative water content and saturated water content) were compared between seasons and between plots (control and through-fall exclusion) enabling a comparison between short- and long-term plasticity in traits. Leaf anatomical traits were correlated with water relation parameters to determine whether water relations differed among tissues.
The key findings were: osmotic adjustment occurred in response to the long-term drought treatment; species resistant to drought stress showed less osmotic adjustment than drought-sensitive species; and water relation traits were correlated with tissue properties, especially the thickness of the abaxial epidermis and the spongy mesophyll.
These findings demonstrate that cell-level water relation traits can acclimate to long-term water stress, and highlight the limitations of extrapolating the results of short-term studies to temporal scales associated with climate change.
Considerable uncertainty surrounds the impacts of anthropogenic climate change on the composition and structure of Amazon forests. Building upon results from two large‐scale ecosystem drought ...experiments in the eastern Brazilian Amazon that observed increases in mortality rates among some tree species but not others, in this study we investigate the physiological traits underpinning these differential demographic responses. Xylem pressure at 50% conductivity (xylem‐P50), leaf turgor loss point (TLP), cellular osmotic potential (πo), and cellular bulk modulus of elasticity (ε), all traits mechanistically linked to drought tolerance, were measured on upper canopy branches and leaves of mature trees from selected species growing at the two drought experiment sites. Each species was placed a priori into one of four plant functional type (PFT) categories: drought‐tolerant versus drought‐intolerant based on observed mortality rates, and subdivided into early‐ versus late‐successional based on wood density. We tested the hypotheses that the measured traits would be significantly different between the four PFTs and that they would be spatially conserved across the two experimental sites. Xylem‐P50, TLP, and πo, but not ε, occurred at significantly higher water potentials for the drought‐intolerant PFT compared to the drought‐tolerant PFT; however, there were no significant differences between the early‐ and late‐successional PFTs. These results suggest that these three traits are important for determining drought tolerance, and are largely independent of wood density—a trait commonly associated with successional status. Differences in these physiological traits that occurred between the drought‐tolerant and drought‐intolerant PFTs were conserved between the two research sites, even though they had different soil types and dry‐season lengths. This more detailed understanding of how xylem and leaf hydraulic traits vary between co‐occuring drought‐tolerant and drought‐intolerant tropical tree species promises to facilitate a much‐needed improvement in the representation of plant hydraulics within terrestrial ecosystem and biosphere models, which will enhance our ability to make robust predictions of how future changes in climate will affect tropical forests.
Associations between hydraulic traits and drought tolerance have been inferred for tropical tree species based on abundances in ecosystems with contrasting soils or precipitation, but it has never been experimentally demonstrated, as we have done, on species growing in direct competition in the Amazon rainforest. Our results show that when exposed to experimental drought, species with higher mortality rates had higher vulnerability to cavitation, turgor loss point, and leaf osmotic potential compared to species with lower mortality rates. These patterns were spatially conserved between tropical forests with contrasting soil types—clay versus sandy soil.
High levels of species diversity hamper current understanding of how tropical forests may respond to environmental change. In the tropics, water availability is a leading driver of the diversity and ...distribution of tree species, suggesting that many tropical taxa may be physiologically incapable of tolerating dry conditions, and that their distributions along moisture gradients can be used to predict their drought tolerance. While this hypothesis has been explored at local and regional scales, large continental-scale tests are lacking. We investigate whether the relationship between drought-induced mortality and distributions holds continentally by relating experimental and observational data of drought-induced mortality across the Neotropics to the large-scale bioclimatic distributions of 115 tree genera. Across the different experiments, genera affiliated to wetter climatic regimes show higher drought-induced mortality than dry-affiliated ones, even after controlling for phylogenetic relationships. This pattern is stronger for adult trees than for saplings or seedlings, suggesting that the environmental filters exerted by drought impact adult tree survival most strongly. Overall, our analysis of experimental, observational, and bioclimatic data across neotropical forests suggests that increasing moisture-stress is indeed likely to drive significant changes in floristic composition.
Version 5 of the Community Land Model (CLM5) introduces the plant hydraulic stress (PHS) configuration of vegetation water use, which is described and compared with the corresponding parameterization ...from CLM4.5. PHS updates vegetation water stress and root water uptake to better reflect plant hydraulic theory, advancing the physical basis of the model. The new configuration introduces prognostic vegetation water potential, modeled at the root, stem, and leaf levels. Leaf water potential replaces soil potential as the basis for stomatal conductance water stress, and root water potential is used to implement hydraulic root water uptake, replacing a transpiration partitioning function. Point simulations of a tropical forest site (Caxiuanã, Brazil) under ambient conditions and partial precipitation exclusion highlight the differences between PHS and the previous CLM implementation. Model description and simulation results are contextualized with a list of benefits and limitations of the new model formulation, including hypotheses that were not testable in previous versions of the model. Key results include reductions in transpiration and soil moisture biases relative to a control model under both ambient and exclusion conditions, correcting excessive dry season soil moisture stress in the control model. PHS implements hydraulic gradient root water uptake, which allows hydraulic redistribution and compensatory root water uptake and results in PHS utilizing a larger portion of the soil column to buffer shortfalls in precipitation. The new model structure, which bases water stress on leaf water potential, could have significant implications for vegetation‐climate feedbacks, including increased sensitivity of photosynthesis to atmospheric vapor pressure deficit.
Key Points
An updated soil‐plant‐atmosphere continuum model based on hydraulic theory is implemented in the Community Land Model (version 5)
Prognostic leaf water potential replaces soil matric potential as the basis for stomatal conductance water stress
Prognostic root water potential is used to implement hydraulic root water uptake, replacing a “soil wilting point” approach
At least one climate model predicts severe reductions of rainfall over Amazonia during this century. Long-term throughfall exclusion (TFE) experiments represent the best available means to ...investigate the resilience of the Amazon rainforest to such droughts. Results are presented from a 7 yr TFE study at Caxiuanã National Forest, eastern Amazonia. We focus on the impacts of the drought on tree mortality, wood production and above-ground biomass. Tree mortality in the TFE plot over the experimental period was 2.5% yr⁻¹ compared with 1.25% yr⁻¹ in a nearby control plot experiencing normal rainfall. Differences in stem mortality between plots were greatest in the largest (> 40 cm diameter at breast height (dbh)) size class (4.1 % yr⁻¹ in the TFE and 1.4% yr⁻¹ in the control). Wood production in the TFE plot was 30% lower than in the control plot. Together, these changes resulted in a loss of 37.8 ± 2.0 Mg carbon (C) ha⁻¹ in the TFE plot (2002-2008), compared with no change in the control. These results are remarkably consistent with those from another TFE (at Tapajós National Forest), suggesting that eastern Amazonian forests may respond to prolonged drought in a predictable manner.
A rich understanding of the productivity, carbon and nutrient cycling of terrestrial ecosystems is essential in the context of understanding, modelling and managing the future response of the ...biosphere to global change. This need is particularly acute in tropical ecosystems, home to over 60% of global terrestrial productivity, over half of planetary biodiversity, and hotspots of anthropogenic pressure. In recent years there has been a surge of activity in collecting data on the carbon cycle, productivity, and plant functional traits of tropical ecosystems, most intensively through the Global Ecosystems Monitoring network (GEM). The GEM approach provides valuable insights by linking field-based ecosystem ecology with the needs of Earth system science. In this paper, we review and synthesize the context, history and recent scientific output from the GEM network. Key insights have emerged on the spatial and temporal variability of ecosystem productivity and on the role of temperature and drought stress on ecosystem function and resilience. New work across the network is now linking carbon cycling to nutrient cycling and plant functional traits, and subsequently to airborne remote sensing. We discuss some of the novel emerging patterns and practical and methodological challenges of this approach, and examine current and possible future directions, both within this network and as lessons for a more general terrestrial ecosystem observation scheme.
•The GEM network describes the productivity, metabolism and carbon cycle of mainly tropical forests and savannas.•Measurements include tree growth, litterfall, reproductive production, root growth, stem respiration, soil respiration and leaf gas exchange.•It has yielded new insights into the factors that control the spatial and seasonal variation of productivity, and the impacts of drought and logging.•Coupled with plant functional traits data, it is opening new opportunities to measure ecosystem function through remote sensing.
Dry periods are predicted to become more frequent and severe in the future in some parts of the tropics, including Amazonia, potentially causing reduced productivity, higher tree mortality and ...increased emissions of stored carbon. Using a long-term (12 year) through-fall exclusion (TFE) experiment in the tropics, we test the hypothesis that trees produce leaves adapted to cope with higher levels of water stress, by examining the following leaf characteristics: area, thickness, leaf mass per area, vein density, stomatal density, the thickness of palisade mesophyll, spongy mesophyll and both of the epidermal layers, internal cavity volume and the average cell sizes of the palisade and spongy mesophyll. We also test whether differences in leaf anatomy are consistent with observed differential drought-induced mortality responses among taxa, and look for relationships between leaf anatomy, and leaf water relations and gas exchange parameters. Our data show that trees do not produce leaves that are more xeromorphic in response to 12 years of soil moisture deficit. However, the drought treatment did result in increases in the thickness of the adaxial epidermis (TFE: 20.5 ± 1.5 µm, control: 16.7 ± 1.0 µm) and the internal cavity volume (TFE: 2.43 ± 0.50 mm
cm
, control: 1.77 ± 0.30 mm
cm
). No consistent differences were detected between drought-resistant and drought-sensitive taxa, although interactions occurred between drought-sensitivity status and drought treatment for the palisade mesophyll thickness (P = 0.034) and the cavity volume of the leaves (P = 0.025). The limited response to water deficit probably reflects a tight co-ordination between leaf morphology, water relations and photosynthetic properties. This suggests that there is little plasticity in these aspects of plant anatomy in these taxa, and that phenotypic plasticity in leaf traits may not facilitate the acclimation of Amazonian trees to the predicted future reductions in dry season water availability.
In the next few decades, climate of the Amazon basin is expected to change, as a result of deforestation and rising temperatures, which may lead to feedback mechanisms in carbon (C) cycling that are ...presently unknown. Here, we report how a throughfall exclusion (TFE) experiment affected soil carbon dioxide (CO₂) production in a deeply weathered sandy Oxisol of Caxiuanã (Eastern Amazon). Over the course of 2 years, we measured soil CO₂ efflux and soil CO₂ concentrations, soil temperature and moisture in pits down to 3 m depth. Over a period of 2 years, TFE reduced on average soil CO₂ efflux from 4.3±0.1 μmol CO₂ m⁻² s⁻¹ (control) to 3.2±0.1 μmol CO₂ m⁻² s⁻¹ (TFE). The contribution of the subsoil (below 0.5 m depth) to the total soil CO₂ production was higher in the TFE plot (28%) compared with the control plot (17%), and it did not differ between years. We distinguished three phases of drying after the TFE was started. The first phase was characterized by a translocation of water uptake (and accompanying root activity) to deeper layers and not enough water stress to affect microbial activity and/or total root respiration. During the second phase a reduction in total soil CO₂ efflux in the TFE plot was related to a reduction of soil and litter decomposers activity. The third phase of drying, characterized by a continuing decrease in soil CO₂ production was dominated by a water stress-induced decrease in total root respiration. Our results contrast to results of a drought experiment on clay Oxisols, which may be related to differences in soil water retention characteristics and depth of rooting zone. These results show that large differences exist in drought sensitivity among Amazonian forest ecosystems, which primarily seem to be affected by the combined effects of texture (affecting water holding capacity) and depth of rooting zone.
Along with total radiation received, the proportion of diffuse to direct solar radiation can influence forest photosynthesis and carbon cycling. However, tropical diffuse radiation regimes are poorly ...described, and to date there are few or no site-based or regional diffuse radiation datasets. The relationship between cloud fraction and diffuse solar radiation was investigated using data from two sites in western and eastern Amazonia. Radiation regimes for diffuse and total radiation were characterised for each site, and the variation in clear sky diffuse radiation fraction between wet and dry season demonstrated and quantified, as well as the dependence of diffuse radiation on cloud amount. Using high frequency measurements of diffuse and total solar radiation data from the two sites, and estimated top of the canopy clear-sky radiation, a number of alternative models to predict diffuse radiation fraction from cloud fraction were formulated and tested. Results showed that cloud fraction can be approximated using the relationship between observed and calculated top of canopy radiation, after which diffuse radiation can then be predicted from cloud fraction. We also demonstrate that satellite cloud data (from the International Satellite Cloud Climatology Project) can be used as inputs to the diffuse radiation model to provide estimates of annual and monthly diffuse radiation proportion.
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