Topografia e densidade da madeira modulam o crescimento sazonal na Amazônia Central

Global climatic changes are widely recognized as the greatest current environmental impact, but the effects and mechanisms by which climate affects the carbon balance of tropical forests are uncertain. Growth seasonality has been studied in an attempt to understand this. Here we investigated how hydro-topographic gradients and wood density modulate intra-annual growth of trees in a terra-firme forest in central Amazonian Brazil. A total of 1,879 trees and palms were monitored with dendrometers every two months in nine 1ha plots. Growth varied as a function of terrain height above the nearest drainage, soil texture and wood specific gravity across time. Increases in girth were mostly associated with raising rainfall during the transition from dry to wet season. At the end of the wet season, trees in clayey soils and vertically far from the water table had greater growth increments than trees in slopes and sandy valleys. However, at the start of the dry season, higher growth rates occurred in valleys and slopes with sandy soils. Additionally, trees with high specific gravity woods maintained low intra-annual girth growth variability, with increment increases in the transition period from dry to wet season, while trees with low specific gravity woods had a broad range of growth variability. These differences suggest that a trade-off between wood growth investment and hydraulic safety may be driving physiological responses along topographically-driven hydrological gradients. Tree stem growth is driven by the rainfall increase in the transition from dry to wet seasons, and not by greater irradiation or greater amount of rainfall as recent studies have suggested. Moreover, the most favorable conditions for growth move across the landscape along time, in association with topography, tracking the best conditions of soil-water availability. Valleys around terra-firme streams may benefit from some level of drought, therefore if dry-seasons become longer, the balance of growth may shift in favor of these areas. This would mean a lower net effect of droughts over the landscape than expected from drought experiments, which were conducted in upland areas far from places with a superficial water-table.Mudanças climáticas são reconhecidas como um dos maiores impactos ambientais sobre o estoque de carbono das florestas tropicais, mas os efeitos e os mecanismos pelos quais o clima afeta o balanço de carbono das florestas tropicais ainda são incertos. A sazonalidade de crescimento tem sido estudada visando compreender isto. Neste estudo foi investigado como o gradiente hidro-topográfico e a densidade da madeira modulam o crescimento intra-anual das árvores em uma floresta de terra-firme na Amazônia Central, Brasil. Foram monitoradas 1879 árvores e palmeiras com dendrômetros, a cada dois meses em nove parcelas de 1ha. O crescimento variou em função da distância vertical da drenagem mais próxima, da textura do solo e densidade da madeira ao longo do tempo. O aumento em circunferência foi principalmente associado com o aumento das chuvas no período de transição da estação seca para chuvosa. No final da estação chuvosa, as árvores situadas em solos argilosos e verticalmente mais distantes do lençol freático tiveram maior crescimento do que as árvores nas vertentes e nos baixios. No entanto, no começo da estação seca, as maiores taxas de crescimento ocorreram nos baixios e nas vertentes com solos arenosos. Além disso, árvores de alta densidade da madeira mantiveram uma baixa variabilidade de crescimento intra-anual em circunferência, com incremento aumentando no período de transição da estação seca para chuvosa, enquanto as árvores de baixa densidade da madeira tivera alta variabilidade de crescimento. Estas diferenças sugerem que o balanço entre o investimento em crescimento de madeira e a prevenção ao embolismo podem direcionar as respostas fisiológicas ao longo de gradientes hidro-topográficos. O crescimento em circunferência é direcionado pelo aumento de chuvas no período de transição da estação seca para chuvosa, o qual não é o período de maior irradiação e nem com maior quantidade de chuvas, como recentes estudos tem sugerido. Além disso, as condições mais favoráveis para o crescimento variam na topografia ao longo do tempo, de acordo com as melhores condições de água disponível no solo. Os baixios podem ser beneficiados por um período de seca, com o balanço de crescimento em favor destas áreas. Isto poderia significar um menor efeito negativo das secas ao longo da paisagem do que esperado por experimentos, os quais foram conduzidos em áreas distantes do lençol-freático

Climate seasonality limits leaf carbon assimilation and wood productivity in tropical forests

The seasonal climate drivers of the carbon cycle in tropical forests remain poorly known, although these forests account for more carbon assimilation and storage than any other terrestrial ecosystem. Based on a unique combination of seasonal pan-tropical data sets from 89 experimental sites (68 include aboveground wood productivity measurements and 35 litter productivity measurements), their associated canopy photosynthetic capacity (enhanced vegetation index, EVI) and climate, we ask how carbon assimilation and aboveground allocation are related to climate seasonality in tropical forests and how they interact in the seasonal carbon cycle. We found that canopy photosynthetic capacity seasonality responds positively to precipitation when rainfall is  < 2000 mm yr⁻¹ (water-limited forests) and to radiation otherwise (light-limited forests). On the other hand, independent of climate limitations, wood productivity and litterfall are driven by seasonal variation in precipitation and evapotranspiration, respectively. Consequently, light-limited forests present an asynchronism between canopy photosynthetic capacity and wood productivity. First-order control by precipitation likely indicates a decrease in tropical forest productivity in a drier climate in water-limited forest, and in current light-limited forest with future rainfall  < 2000 mm yr⁻¹

Data and R-code from 'Mode of death and mortality risk factors in Amazon trees'. Nature communications. 2020

The carbon sink capacity of tropical forests is substantially affected by tree mortality. However, the main drivers of tropical tree death remain largely unknown. Here we present a pan-Amazonian assessment of how and why trees die, analysing over 120,000 trees representing > 3800 species from 189 long-term RAINFOR forest plots. While tree mortality rates vary greatly Amazon-wide, on average trees are as likely to die standing as they are broken or uprooted—modes of death with different ecological consequences. Species-level growth rate is the single most important predictor of tree death in Amazonia, with faster-growing species being at higher risk. Within species, however, the slowest-growing trees are at greatest risk while the effect of tree size varies across the basin. In the driest Amazonian region species-level bioclimatic distributional patterns also predict the risk of death, suggesting that these forests are experiencing climatic conditions beyond their adaptative limits. These results provide not only a holistic pan-Amazonian picture of tree death but large-scale evidence for the overarching importance of the growth–survival trade-off in driving tropical tree mortality

WagnerClimateSeasonalityLimits.pdf

The seasonal climate drivers of the carbon cycle in tropical forests remain poorly known, although these forests account for more carbon assimilation and storage than any other terrestrial ecosystem. Based on a unique combination of seasonal pan-tropical data sets from 89 experimental sites (68 include aboveground wood productivity measurements and 35 litter productivity measurements), their associated canopy photosynthetic capacity (enhanced vegetation index, EVI) and climate, we ask how carbon assimilation and aboveground allocation are related to climate seasonality in tropical forests and how they interact in the seasonal carbon cycle. We found that canopy photosynthetic capacity seasonality responds positively to precipitation when rainfall is  < 2000 mm yr⁻¹ (water-limited forests) and to radiation otherwise (light-limited forests). On the other hand, independent of climate limitations, wood productivity and litterfall are driven by seasonal variation in precipitation and evapotranspiration, respectively. Consequently, light-limited forests present an asynchronism between canopy photosynthetic capacity and wood productivity. First-order control by precipitation likely indicates a decrease in tropical forest productivity in a drier climate in water-limited forest, and in current light-limited forest with future rainfall  < 2000 mm yr⁻¹

WagnerClimateSeasonalityLimitsSupplementalTablesandFigures.pdf

The seasonal climate drivers of the carbon cycle in tropical forests remain poorly known, although these forests account for more carbon assimilation and storage than any other terrestrial ecosystem. Based on a unique combination of seasonal pan-tropical data sets from 89 experimental sites (68 include aboveground wood productivity measurements and 35 litter productivity measurements), their associated canopy photosynthetic capacity (enhanced vegetation index, EVI) and climate, we ask how carbon assimilation and aboveground allocation are related to climate seasonality in tropical forests and how they interact in the seasonal carbon cycle. We found that canopy photosynthetic capacity seasonality responds positively to precipitation when rainfall is  < 2000 mm yr⁻¹ (water-limited forests) and to radiation otherwise (light-limited forests). On the other hand, independent of climate limitations, wood productivity and litterfall are driven by seasonal variation in precipitation and evapotranspiration, respectively. Consequently, light-limited forests present an asynchronism between canopy photosynthetic capacity and wood productivity. First-order control by precipitation likely indicates a decrease in tropical forest productivity in a drier climate in water-limited forest, and in current light-limited forest with future rainfall  < 2000 mm yr⁻¹