Amazon forest structure mediates drought impacts on carbon uptake and evapotranspiration

Abstract:

Tropical forests account for over 25% of the total carbon stocks and about 66% of species in terrestrial ecosystems, and provide a significant water source to the atmosphere through evapotranspiration. Yet, the future of the terrestrial carbon and water cycles remains highly uncertain according to Earth System Models (ESMs). Most ESMs represent climate change and increasing CO2 effects on tropical ecosystems, but they rarely account for the heterogeneity of forest canopy structure at local and regional scales. Here we investigate how structural diversity of the Amazon forests affects the spatial variability of water use efficiency and mortality risk during severe droughts. We integrate a comprehensive small-footprint airborne lidar sampling data set (430,000 ha) collected over the entire Brazilian Amazon into the Ecosystem Demography Model (ED2). ED2 is a process-based terrestrial biosphere model that accounts for vertical and horizontal heterogeneity of forest canopies and its effect on water and carbon cycles, and has been previously shown to realistically represent the magnitude and seasonality of evapotranspiration, productivity and respiration. Directly assimilating forest structure from airborne lidar observations allows us to account for the diversity of forest ages since last disturbance, and multiple disturbance types that occur across the Amazon biome. We simulate the forest dynamics using meteorological reanalysis data for 1980–2019 to quantify changes in evapotranspiration, gross primary productivity, and carbon balance during severe and increasingly hot drought events. To quantify the role of forest structure heterogeneity, we compare the results initialized with observed lidar forest structure with the simulated vegetation dynamics from a simplified distribution of forest structure typical of old-growth forests. Results suggest that decreases in productivity and evapotranspiration at early drought stages are the strongest in intermediate-size canopies, whereas old-growth forests show longer periods of minimal response to drought conditions, indicating that forest structure is an important driver of tropical forest resistance to droughts. Our analysis highlights the need to consider feedbacks between structural diversity of tropical forests and climate extremes.