Assessing belowground hydrologic controls on vegetation activity across Amazônia

Abstract:

Inadequate understanding of the mechanisms that control the behavior of tropical forests hinders model predictions of forest response to an increase in drought intensity and frequency. Research on drought-induced forest mortality highlights hydraulic failure as a key mechanism behind tree death, and water access seem to influence which plants survive such drought events. Climatic variables such as precipitation and radiation have long been evoked to explain large scale vegetation characteristics and dynamics in Amazônia. However, climate alone cannot explain several vegetation patterns observed at the landscape to regional scales, such as fine scale variability in forest structure and plant functional traits. Access to groundwater along topographic gradients has emerged as an important environmental driver for vegetation, with valleys being wetter than hills under the same climate. We assessed the influence of hydrologic gradients, nested within the climate gradient, on forest productivity and response to atmospheric droughts across Amazônia. We present results on the analyses of spatiotemporal decoupling between precipitation and groundwater, which makes groundwater an independent axis of water access variability influencing plant water uptake and drought response. We also show precipitation and groundwater controls on forest dynamics using the Enhanced Vegetation Index (EVI) both for average years and for the droughts of 2005, 2010 and 2015. Our work highlights the importance of considering belowground water when assessing the response of tropical forests to drought.