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


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


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


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.