B116-0018
Risky trees in safe waters? Drought-sensitive trees survive on deep water access in a tropical forest community

Wednesday, 16 December 2020
Poster
Rutuja Chitra-Tarak, Los Alamos National Laboratory, Earth and Environmental Sciences, Los Alamos, NM, United States; Smithsonian Environmental Research Center, Edgewater, NM, United States, Chonggang Xu, Los Alamos National Laboratory, Los Alamos, NM, United States, Brett Wolfe, Louisiana State University, School of Renewable Natural Resources, Baton Rouge, United States, Norbert Kunert, Smithsonian Conservation Biology Institute, Conservation Ecology Center, Front Royal, United States, Matteo Detto, Princeton University, Princeton, United States, Joseph Wright S., Smithsonian Tropical Research Institute, Gamboa, Panama, Joseph Zailaa, California State University, Los Angeles, Department of Biological Sciences, Los Angeles, CA, United States, Lawren Sack, University of California Los Angeles, Ecology and Evolutionary Biology, Los Angeles, CA, United States, Nathan McDowell, Pacific Northwest National Laboratory, Richland, WA, United States, Brent D Newman, LANL, Los Alamos, NM, United States, Kristina J Anderson-Teixeira, Smithsonian Institution, Smithsonian Conservation Biology Institute & Smithsonian Tropical Research Institute, Front Royal, VA, United States, Laurent Ruiz, Indian Institute of Science, Indo-French Cell for Water Sciences, Bangalore, India; INRA Rennes, UMR1069, Rennes Cedex, France and Sean McMahon, Smithsonian Tropical Research Institute, Balboa, Panama; Smithsonian Environmental Research Center Edgewater, Edgewater, United States
Abstract:
Among all the mechanisms plants employ to deal with drought, deep-water access is arguably the most commonly invoked, but under-studied mechanism. Above-ground hydraulic traits such as vulnerability to embolism (Ψ88, stem) and safety margins (Ψmin - Ψ88, stem) are predictors of drought-induced mortality risk globally, but deep-water access may delay plants from reaching those critical thresholds. Here, using a novel data-model approach, we test the role of deep-water access in the drought-tolerance of a hyper-diverse tropical moist forest community (Barro Colorado Island, Panama). We inversely modelled Effective Rooting Depth (ERD) for large trees of 32 canopy tree species by linking species growth dynamics (6 censuses, 1990-2015) with (i) vapor pressure deficit (VPD), (ii) soil water dynamics (Ψsoil, z) by depth (z) in the whole-soil column—obtained by locally parameterizing and calibrating a hydrological water balance model (ELM-FATES)—and, (iii) new data on leaf hydraulic vulnerability curves (Kleaf vs. Ψleaf). We evaluated ERD by leveraging existing isotopic data (δ2H) on trees and soil, and assessed coordination of ERD with stem hydraulic traits and mortality rates. To estimate the time spent below critical hydraulic threshold (Ψ88, leaf) we counted the number of days that soil water potential (Ψsoil, z in the depth (z) matching species ERD was lower than Ψ88, leaf. Across species, deeper ERD was associated with higher stem hydraulic conductivity (Kstem), greater vulnerability to xylem embolism (Ψ88, stem), and narrower safety margins, (Ψmin - Ψ88, stem), that is, traits associated with rapid water use and greater drought sensitivity, or the ‘fast’, aka ‘risky’, end of the ‘fast-slow’ water economics spectrum. Yet, deeper ERD was associated with lower mortality rates over 35 years (1985-2015), a period that included droughts of a variety of intensity and duration. Time spent below the critical hydraulic threshold (Ψ88, leaf) decreased with ERD indicating that ERD buffers drought-stress, and thereby enables survival despite ‘high risk’ hydraulic traits. The role of deep-water access in alleviating mortality of drought-sensitive trees has important implications for modeling the future of tropical forests under climate change.
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