Ecohidrologia de Espécies Arbóreas de Terra Firme na Amazônia

Abstract

The Amazon is the largest tropical forest in the world with great importance in the global biogeochemical cycles. Under climate change scenarios, a mechanistic understanding of the water cycle from individual trees to landscapes is needed to predict changes in the forest structure and function. In the Amazon basin, an estimated 25-50% of precipitation is recycled back to the atmosphere through forest transpiration. At the leaf level, transpiration flux is a function of vapor pressure deficit (VPD) and stomatal conductance (gs), according to Fick’s laws of diffusion. Also, leaf temperature (Tleaf), net radiation, and soil moisture are often considered important. In this study, we present in situ field observations of environmental (direct solar radiation, air temperature, and VPD) and physiological (sap velocity, stomatal conductance, and leaf water potential) variables and their correlations with Tleaf during the 2015-2016 ENSO and the regular seasons 2017 and 2018. In order to observe the interactions between physiological variables and fast-changing environmental conditions, we collected a high temporal frequency data (15-60 min) in two primary rainforest sites one located in the Central Amazon within the limits of the Experimental Station of Tropical Forestry (ZF-2) near the K-34 tower site, and another in the Eastern Amazon within the limits of the Tapajós National Forest at the the K-67 tower site. Our study shows that the interactions between the observed environmental and physiological variables can be explained by the hysteresis phenomena. The temporal difference between the peak of stomatal conductance (late morning to midday) and the peak of VPD (early afternoon) is one of the major regulators of the sap velocity hysteresis patterns. Also, for the first time, the hysteresis patterns between Tleaf and Tair were described. During the 2015-2016 ENSO, the differences between Tleaf and air temperature (Tair) reached almost 8°C, and generally, Tleaf was higher than Tair during the morning period to early afternoon, and lower than Tair during the late afternoon and night. With the leaf temperature data, it was possible to calculate the true VPD (ΔVPD), which is the pressure gradient between the substomatal cavity and the boundary layer of the air near the leaf surface. The complexity of the observed physiological and environmental variables that can affect the transpiration dynamics reinforces the importance of detailed analyzes during periods of climatic anomalies such as El Niño. The presented data can provide new perspectives for the improvement of current Earth System Models (ESMs).