PNNL

Abstract

Topographic heterogeneity and lateral subsurface flow at the hillslope scale of 1km or less may have outsized impacts on tropical forest distribution through their impacts on water availability to plants. However, vegetation dynamics and finer-scale hydrologic processes such as subsurface lateral flow are not concurrently represented in Earth system models, impeding our ability to simulate the influence of soil hydrology and groundwater dynamics on vegetation. In this study, we integrate the Energy Exascale Earth System Model (E3SM) Land Model (ELM), in a configuration that includes the Functionally-Assembled Terrestrial Ecosystem Simulator (FATES), with a three-dimensional hydrology model (ParFlow) to explicitly resolve hillslope topography and subsurface flow for better understanding of the processes that drive tropical forest dynamics. Numerical experiments are performed to understand how hydrologic processes at the hillslope scale modulate water available to plants and how vegetation structures vary along the gradients of water availability at Barro Colorado Island (BCI), Panama. Our results show groundwater table depth (WTD) plays a large role in governing aboveground biomass (AGB). Using random forest (RF) models, the domain-wide simulated AGB and WTD can be well predicted by static topographic attributes including surface elevation (DEM), slope and convexity. The accuracy of the RF model can be further improved by including variables (e.g., soil moisture and ground water status) influenced by physical processes. Different model representations of mortality due to hydraulic failure can change the dominant topographic driver for the simulated AGB. However, the topographic attributes and measured soil moisture do not have predictive power for the observed AGB in the well-drained 50-ha plot, possibly due to features such as nutrient status that are not included in this study. The new coupled modeling capability may be used in future research for modeling and understanding the diverse impact of local heterogeneity, biodiversity, nutrient availability, as well as anthropogenic factors on the structure of plant communities in basins with different hydroclimate regimes.