Disentangle the drivers of spatio-temporal variations in vegetation optical depth over tropical forests with ecosystem hydrodynamics modeling
Abstract
Tropical forests are facing increasing water stress under climate change and the accompanying intensification of hydrological cycles. Vegetation optical depth (VOD), a microwave-band remote sensing product that reflects vegetation canopy water content (CWC), has been widely used to monitor and understand plant water stress and drought-related biomass changes. However, interpretation of the spatio-temporal variations in VOD is still challenging because a variety of distinctive physiological and biophysical processes regulate CWC and thus VOD, including dew formation, rainfall interception, plant water stress, and biomass changes. Here, we leverage recent advances in the cohort-based Ecosystem Demography model that allows for explicit process-based simulation of CWC (ED-2.2-hydro) to disentangle the drivers of spatio-temporal variations in AMSR-E X-band VOD and to improve our understanding in the coupled carbon-water dynamics for tropical forests. We simulate vegetation dynamics for tropical forests across a large rainfall gradient (from rainforest to savanna) in Brazil over 2003 to 2010 when full-year AMSR-E VOD data are available after spinning up the model for 400 years. After evaluating the simulated canopy structure and plant hydraulics, we then compare simulated leaf surface water, leaf internal water, and wood internal water with VOD observations from diurnal to seasonal and inter-annual time scales. Specifically, we aim to understand the roles of leaf surface water, plant water stress, phenology, and woody biomass changes in explaining VOD variability. Results over four flux tower sites suggest (1) that VOD is linearly scaled with the simulated CWC when we apply our best estimated penetration depth; (2) that leaf surface water contributes significantly to diurnal variations in VOD, which has been largely ignored in previous analyses; and (3) that plant water stress and phenology drive seasonal and inter-annual variations in VOD while biomass changes mostly explain spatial variations. Our results demonstrate the complexity in VOD variability and the need to consider multiple ecophysiological processes when using VOD data to infer changes in ecosystem carbon and water dynamics in tropical forests.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMGC019..08X
- Keywords:
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- 0426 Biosphere/atmosphere interactions;
- BIOGEOSCIENCES;
- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 1615 Biogeochemical cycles;
- processes;
- and modeling;
- GLOBAL CHANGE;
- 1655 Water cycles;
- GLOBAL CHANGE