July 02, 2021
Risky Trees in Safe Waters?
Hydraulically-vulnerable trees survive on deep-water access during droughts in a tropical forest
In a rainforest of Barro Colorado Island, Panama, scientists from Los Alamos National Laboratory and other institutions as part of the Next Generation Ecosystem Experiment (NGEE)-Tropics developed and tested the first inverse model of trees’ rooting depths that is integrated with plant physiology. Deep-rooted tree species had water transport systems that were likely to fail when water stressed. However, across a variety of drought conditions, deep-rooted species were less dehydrated and survived better than shallow-rooted tree species, especially among evergreen trees. This emphasizes the need to incorporate drought exposure risk in evaluating tree drought resilience.
Rooting depths are a critical unknown for modeling forest response to droughts, which are projected to intensify. Due to challenges in measuring rooting or water-sourcing depths, researchers have relied on above-ground traits to assess the likelihood of drought-induced tree mortality. The models developed through this research will allow wider integration of rooting depths and drought exposure in drought resilience studies.
Deep-water access is arguably the most effective, but under-studied, mechanism that trees employ to survive during drought. Functional traits such as the degree of vulnerability of trees’ water-conduits to blockage due to air-entry (embolism) can predict mortality risk at given levels of dehydration, but deep-water access may delay tree dehydration. Here, scientists tested the role of deep-water access in enabling survival within a diverse tropical forest community in Panama using a novel data-model approach.
Scientists inversely estimated the effective rooting depth (ERD, as the average depth of water extraction), for 29 canopy species by linking diameter growth dynamics (1990–2015) to vapor pressure deficit, water potentials in the whole-soil column, and leaf hydraulic vulnerability curves. They validated ERD estimates against existing isotopic data of potential water-access depths.
Across species, deeper ERD was associated with higher maximum stem hydraulic conductivity, greater vulnerability to xylem embolism, narrower safety margins, and lower mortality rates during extreme droughts over 35 years (1981–2015) among evergreen species. Species exposure to water stress declined with deeper ERD indicating that trees compensate for water stress-related mortality risk through deep-water access.
The role of deep-water access in mitigating mortality of hydraulically-vulnerable trees has important implications for our predictive understanding of forest dynamics under current and future climates.
Los Alamos National Lab
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
This research was supported as part of the Next Generation Ecosystem Experiments-Tropics (NGEE-Tropics), funded by the Office of Biological and Environmental Research in the U.S. Department of Energy’s (DOE) Office of Science. Funding from the following sources also supported this research: the U.S. National Science Foundation, the U.S. Department of Agriculture, and the Institute of Research for Development (IRD) France.
Chitra-Tarak, R., et al. "Hydraulically-Vulnerable Trees Survive on Deep-Water Access during Droughts in a Tropical Forest." New Phytologist 231 (5), 1798–1813 (2021). https://doi.org/10.1111/nph.17464.