Dry season soil moisture dynamics along secondary forest succession in Panama: impacts of soil hydraulic properties vs. tree root profile

Abstract:

Understanding age-related behaviors of tropical forest is crucial for improving predictions of ecosystem dynamics along secondary forest succession. Observational evidence shows that mature tropical forest (>= 80-year-old) can maintain high volumetric soil water content in shallow soil layers throughout the dry season, while young tropical forest (< 25-year-old) and C4 grass cannot, but the underlying mechanism is not clear. We hypothesize that this difference could be attributed to age-driven changes in either soil hydraulic properties such as saturated hydraulic conductivity in the shallow soil layers, or root system characteristics which control deep soil water uptake. To test this hypothesis, we use the ecosystem demography model Functionally Assembled Terrestrial Ecosystem Simulator (FATES) implemented in the Energy Exascale Earth System Model (E3SM) Land Model (ELM), ELM-FATES, which has the capabilities needed to fully explore the interactions among canopy structure, forest succession, hydrology, and biogeochemical cycling. Furthermore, the advanced selective logging module in ELM-FATES allows for a more realistic representation of forest harvest. We conduct numerical experiments at two sites in Panama with distinct land cover types (a mature forest and a C4 grass). We use comprehensive measurements of carbon and water fluxes at these two sites to calibrate key parameters of hydraulic properties and root profiles in ELM-FATES. We find that the fraction of deeper roots is crucial for capturing the observed soil moisture dynamics during the dry season as deeper roots allow trees to extract water from deep soil depths rather than from surface layers. These results highlight the importance of belowground components to understand the soil moisture and vegetation dynamics in tropical forests.