Monitor and Constrain Tropical Ecosystem Sensitivity to Moisture (MACROCOSM) with Experimental Throughfall Exclusion in a Tropical Moist Forest

Authors

Xiangtao Xu¹* ([email protected]), Xavier Comas², Xi Yang³, Tana Wood⁴, Charlie Koven⁵, Maria Uriarte⁶, Vincent Humphrey⁷, Chris Smith-Martin⁶

Institutions

¹Cornell University, Ithaca, NY; ²Florida Atlantic University–Boca Raton, FL; ³University of Virginia, Charlottesville, VA; ⁴International Institute of Tropical Forestry, Luquillo, PR; ⁵Lawrence Berkeley National Laboratory, Berkeley, CA; ⁶Columbia University, New York, NY; ⁷Swiss Federal Institute of Technology in Zürich, Switzerland

Abstract

Ecosystem sensitivity to water availability is pivotal to understanding resilience of tropical forests under a changing hydroclimatic regime. However, large knowledge gaps remain in how plants respond to moisture changes and how these responses in turn regulate ecosystem water dynamics, resulting in considerable uncertainty in ecohydrological predictions across state-of-the-art Earth system models. This project aims to improve mechanistic understanding of plant-mediated ecohydrology by monitoring and modeling ecosystem dynamics at a new manipulative throughfall exclusion (TFE) experiment at Luquillo Long-Term Ecological Research station, PR, where annual rainfall exceeds 3500 mm. Particularly, the team focuses on investigating: (1) the coupling between soil moisture and vegetation water content; (2) the relatively unconstrained plant ecohydrological processes such as leaf angle responses to water stress and root hydraulic redistribution; and (3) the implications of site-level manipulative experiment results on interpreting large-scale remote sensing observations and modeling long-term vegetation dynamics in the tropics.

At ecosystem scale, researchers combine state-of-the-art remote sensing techniques, especially microwave remote sensing such as global navigation satellite systems–based vegetation optical depth measurement and ground-penetration radar, and soil moisture and water potential sensors at different depths to address the challenge. These new nondestructive measurements can continuously track subdaily patterns of both above- and belowground water dynamics and characterize fine-scale heterogeneity. At organism and tissue scale, the team monitors vegetation structural and hydraulic responses to drought by tracking leaf angle and canopy structure changes using terrestrial laser scanning and plant transpiration and root hydraulic redistribution using stem and root sap-flow sensors. Individual-level drought responses will be related to plant ecophysiological and hydraulic trait measurements available at the same site to quantify the role of canopy and roots in ecosystem sensitivity to moisture.

The team has finished the first field campaign, configured prototypes for in situ sensors, and started to collect pretreatment ecohydrological and forest structure data. The field data will be assimilated into ED2.2-hydro and E3SM Land Model–FATES to explore implications of tropical ecosystem sensitivity to moisture constrained by the TFE experiment. Overall, the project is on track to develop an integrated, scale-aware, and predictive understanding of tropical ecosystem responses to hydroclimatic changes.