Antonio Carlos Lola da Costa

Effect of 7 yr of experimental drought on vegetation dynamics and biomass storage of an eastern Amazonian rainforest

*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 c. 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.

Variação espacial e temporal da precipitação no estado do Pará.

Estudos sobre a climatologia das precipitacoes no Estado do Para sao essenciais para o planejamento das atividades agricolas. A variacao da precipitacao anual e sazonal no Estado do Para foi analisada com base em series historicas de 23 anos (1976-1998) de dados diarios de chuva. A analise foi realizada para 31 localidades do Estado do Para, sendo os resultados representados em mapas com a utilizacao de tecnicas de sistemas de informacoes geograficas (SIG). A variabilidade da precipitacao anual e sazonal foi caracterizada com base no coeficiente de variacao e no indice de variabilidade interanual relativo. A variacao desses coeficientes para a precipitacao anual no Estado do Para foi de 15 a 30%. As caracteristicas mensais da estacao chuvosa, em termos de inicio, fim e duracao, foram determinadas utilizando-se o criterio proposto por KASSAM (1979). A variacao entre as datas de plantio precoces e tardias corresponderam aos decendios identificados pelos dias julianos 309319 e 353363, respectivamente.

Confronting model predictions of carbon fluxes with measurements of Amazon forests subjected to experimental drought

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 fate of assimilated carbon during drought: impacts on respiration in Amazon rainforests.

Interannual variations in CO2 exchange across Amazonia, as deduced from atmospheric inversions, correlate with El Niño occurrence. They are thought to result from changes in net ecosystem exchange and fire incidence that are both related to drought intensity. Alterations to net ecosystem production (NEP) are caused by changes in gross primary production (GPP) and ecosystem respiration (Reco). Here, we analyse observations of the components of Reco (leaves, live and dead woody tissue, and soil) to provide first estimates of changes in Reco during short-term (seasonal to interannual) moisture limitation. Although photosynthesis declines if moisture availability is limiting, leaf dark respiration is generally maintained, potentially acclimating upwards in the longer term. If leaf area is lost, then short-term canopy-scale respiratory effluxes from wood and leaves are likely to decline. Using a moderate short-term drying scenario where soil moisture limitation leads to a loss of 0.5 m2 m−2 yr−1 in leaf area index, we estimate a reduction in respiratory CO2 efflux from leaves and live woody tissue of 1.0 (±0.4) t C ha−1 yr−1. Necromass decomposition declines during drought, but mortality increases; the median mortality increase following a strong El Niño is 1.1% (n=46 tropical rainforest plots) and yields an estimated net short-term increase in necromass CO2 efflux of 0.13–0.18 t C ha−1 yr−1. Soil respiration is strongly sensitive to moisture limitation over the short term, but not to associated temperature increases. This effect is underestimated in many models but can lead to estimated reductions in CO2 efflux of 2.0 (±0.5) t C ha−1 yr−1. Thus, the majority of short-term respiratory responses to drought point to a decline in Reco, an outcome that contradicts recent regional-scale modelling of NEP. NEP varies with both GPP and Reco but robust moisture response functions are clearly needed to improve quantification of the role of Reco in influencing regional-scale CO2 emissions from Amazonia.

The linkages between photosynthesis, productivity, growth and biomass in lowland Amazonian forests

Understanding the relationship between photosynthesis, net primary productivity and growth in forest ecosystems is key to understanding how these ecosystems will respond to global anthropogenic change, yet the linkages among these components are rarely explored in detail. We provide the first comprehensive description of the productivity, respiration and carbon allocation of contrasting lowland Amazonian forests spanning gradients in seasonal water deficit and soil fertility. Using the largest data set assembled to date, ten sites in three countries all studied with a standardized methodology, we find that (i) gross primary productivity (GPP) has a simple relationship with seasonal water deficit, but that (ii) site-to-site variations in GPP have little power in explaining site-to-site spatial variations in net primary productivity (NPP) or growth because of concomitant changes in carbon use efficiency (CUE), and conversely, the woody growth rate of a tropical forest is a very poor proxy for its productivity. Moreover, (iii) spatial patterns of biomass are much more driven by patterns of residence times (i.e. tree mortality rates) than by spatial variation in productivity or tree growth. Current theory and models of tropical forest carbon cycling under projected scenarios of global atmospheric change can benefit from advancing beyond a focus on GPP. By improving our understanding of poorly understood processes such as CUE, NPP allocation and biomass turnover times, we can provide more complete and mechanistic approaches to linking climate and tropical forest carbon cycling.

Threshold Responses to Soil Moisture Deficit by Trees and Soil in Tropical Rain Forests: Insights from Field Experiments

Many tropical rain forest regions are at risk of increased future drought. The net effects of drought on forest ecosystem functioning will be substantial if important ecological thresholds are passed. However, understanding and predicting these effects is challenging using observational studies alone. Field-based rainfall exclusion (canopy throughfall exclusion; TFE) experiments can offer mechanistic insight into the response to extended or severe drought and can be used to help improve model-based simulations, which are currently inadequate. Only eight TFE experiments have been reported for tropical rain forests. We examine them, synthesizing key results and focusing on two processes that have shown threshold behavior in response to drought: (1) tree mortality and (2) the efflux of carbon dioxdie from soil, soil respiration. We show that: (a) where tested using large-scale field experiments, tropical rain forest tree mortality is resistant to long-term soil moisture deficit up to a threshold of 50% of the water that is extractable by vegetation from the soil, but high mortality occurs beyond this value, with evidence from one site of increased autotrophic respiration, and (b) soil respiration reaches its peak value in response to soil moisture at significantly higher soil moisture content for clay-rich soils than for clay-poor soils. This first synthesis of tropical TFE experiments offers the hypothesis that low soil moisture–related thresholds for key stress responses in soil and vegetation may prove to be widely applicable across tropical rain forests despite the diversity of these forests.

Variation in stem mortality rates determines patterns of above-ground biomass in Amazonian forests: implications for dynamic global vegetation models

Abstract Understanding the processes that determine above‐ground biomass (AGB) in Amazonian forests is important for predicting the sensitivity of these ecosystems to environmental change and for designing and evaluating dynamic global vegetation models (DGVMs). AGB is determined by inputs from woody productivity [woody net primary productivity (NPP)] and the rate at which carbon is lost through tree mortality. Here, we test whether two direct metrics of tree mortality (the absolute rate of woody biomass loss and the rate of stem mortality) and/or woody NPP, control variation in AGB among 167 plots in intact forest across Amazonia. We then compare these relationships and the observed variation in AGB and woody NPP with the predictions of four DGVMs. The observations show that stem mortality rates, rather than absolute rates of woody biomass loss, are the most important predictor of AGB, which is consistent with the importance of stand size structure for determining spatial variation in AGB. The relationship between stem mortality rates and AGB varies among different regions of Amazonia, indicating that variation in wood density and height/diameter relationships also influences AGB. In contrast to previous findings, we find that woody NPP is not correlated with stem mortality rates and is weakly positively correlated with AGB. Across the four models, basin‐wide average AGB is similar to the mean of the observations. However, the models consistently overestimate woody NPP and poorly represent the spatial patterns of both AGB and woody NPP estimated using plot data. In marked contrast to the observations, DGVMs typically show strong positive relationships between woody NPP and AGB. Resolving these differences will require incorporating forest size structure, mechanistic models of stem mortality and variation in functional composition in DGVMs.

Ecosystem respiration and net primary productivity after 8–10 years of experimental through-fall reduction in an eastern Amazon forest

Background: There is much interest in how the Amazon rainforest may respond to future rainfall reduction. However, there are relatively few ecosystem-scale studies to inform this debate. Aims: We described the carbon cycle in a 1 ha rainforest plot subjected to 8–10 consecutive years of ca. 50% through-fall reduction (TFR) and compare these results with those from a nearby, unmodified control plot in eastern Amazonia. Methods: We quantified the components of net primary productivity (NPP), autotrophic (R a) and heterotrophic respiration, and estimate gross primary productivity (GPP, the sum of NPP and R a) and carbon-use efficiency (CUE, the ratio of NPP/GPP). Results: The TFR forest exhibited slightly lower NPP but slightly higher R a, such that forest CUE was 0.29 ± 0.04 on the control plot but 0.25 ± 0.03 on the TFR plot. Compared with four years earlier, TFR plot leaf area index and small tree growth recovered and soil heterotrophic respiration had risen. Conclusions: This analysis tested and extended the key findings of a similar analysis 4 years earlier in the TFR treatment. The results indicated that, while the forest recovered from extended drought in some respects, it maintained higher overall R a relative to the undroughted control, potentially causing the droughted forest to act as a net source of CO2.

After more than a decade of soil moisture deficit, tropical rainforest trees maintain photosynthetic capacity, despite increased leaf respiration.

Abstract Determining climate change feedbacks from tropical rainforests requires an understanding of how carbon gain through photosynthesis and loss through respiration will be altered. One of the key changes that tropical rainforests may experience under future climate change scenarios is reduced soil moisture availability. In this study we examine if and how both leaf photosynthesis and leaf dark respiration acclimate following more than 12 years of experimental soil moisture deficit, via a through‐fall exclusion experiment (TFE) in an eastern Amazonian rainforest. We find that experimentally drought‐stressed trees and taxa maintain the same maximum leaf photosynthetic capacity as trees in corresponding control forest, independent of their susceptibility to drought‐induced mortality. We hypothesize that photosynthetic capacity is maintained across all treatments and taxa to take advantage of short‐lived periods of high moisture availability, when stomatal conductance (g s) and photosynthesis can increase rapidly, potentially compensating for reduced assimilate supply at other times. Average leaf dark respiration (R d) was elevated in the TFE‐treated forest trees relative to the control by 28.2 ± 2.8% (mean ± one standard error). This mean R d value was dominated by a 48.5 ± 3.6% increase in the R d of drought‐sensitive taxa, and likely reflects the need for additional metabolic support required for stress‐related repair, and hydraulic or osmotic maintenance processes. Following soil moisture deficit that is maintained for several years, our data suggest that changes in respiration drive greater shifts in the canopy carbon balance, than changes in photosynthetic capacity.

Plasticity in leaf-level water relations of tropical rainforest trees in response to experimental drought

Summary 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.