PNNL

Abstract

Tropical forest tree mortality is increasing, in part due to more severe droughts. Yet our understanding of how tree traits and life strategies are linked to drought stress has been limited due to a scarcity of measurements. The BIONTE (BIOmass and NuTrient Experiment) project near Manaus, Brazil boasts one of the largest single sap flow (referred to as transpiration) installations in the world. Sensors distributed in 90 canopy trees in BIONTE have been used for the measurements since June 2022, covering a wood density gradient. The 2023 El Niño event resulted in a strong drought in the Central Amazon that provided a unique opportunity to evaluate how water availability impacts tree transpiration. An interpretable machine learning framework was used to study the complex interactions between transpiration and multiple environmental variables such as soil water availability and vapor pressure deficit (VPD), and how these interactions vary with wood density and species. We found varying responses of transpiration from different trees during the El Niño drought. Transpiration generally increased with temperature, with stronger effects in wetter areas and in trees with low to medium wood density. However, this response was modulated by stomatal sensitivity to VPD, which constrained transpiration under high atmospheric demand, particularly in intermediate-moisture area. The inflection in transpiration rate at high temperatures (>32oC) underscores the role of stomatal and hydraulic regulation in limiting water loss and protecting trees from excessive evaporative demand. Analysis of soil water contribution to transpiration revealed unimodal patterns in wetter area, with peak contributions near 0.45?cm³?cm?³ of surface soil water and declining or flat responses beyond that threshold, suggesting a shift from water- to energy-limited transpiration. In contrast, drier areas exhibited limited transpiration sensitivity to soil water conditions and minimal trait-based variation in VPD responses, indicating supply-limited conditions. Even though trees with higher wood density are generally more resilient, this study shows diverse tree resilience to drought, prompting further investigation into the specific traits and dynamics between environmental variables in regulating transpiration and other physiological processes in trees.