PNNL

The Science

Drought impacts tropical forests across the globe, but scientists do not fully understand what controls tree responses to drought. Researchers measured hydraulic traits for 27 tree species across a rainfall gradient in Panama, leveraged historical forest censuses to examine how these traits varied across sites, and determined which traits explained moisture requirements and mortality. They found that hydraulic traits sorted into two main groups. The first group included traits associated with plant water status and the second group included mainly leaf associated traits. The researchers found that safety from leaf wilting plays an important role in tree mortality, while the ratio of leaf area to sapwood area informs tree moisture needs.

The Impact

These findings identify key traits that influence tree responses to drought. This contributes to our understanding of how tropical forests respond to drier climates. In addition, these data can be used to parameterize and validate models to predict the future of tropical forests under climate change. This has implications for understanding biodiversity, community dynamics, and biogeochemical cycles.  

Summary

Intensified droughts are affecting tropical forests across the globe. However, the underlying mechanisms of tree drought response and mortality are poorly understood. Hydraulic traits and hydraulic safety margins—the extent to which plants buffer themselves from water stress thresholds—provide insights into species-specific drought vulnerability. This study investigated the degree that tree hydraulic traits varied across the Isthmus of Panama rainfall gradient and the relationships between hydraulic traits and species-specific optimal moisture and mortality rates. Researchers found strong coordination among traits, with a network analysis revealing two major groups of correlated traits. One group included plant water status, leaf wilting point, stem water storage, stem density, hydraulic safety margins, and mortality rate. The second group has leaf mass per area, leaf dry matter content, hydraulic architecture (leaf area to sapwood area ratio), and species-specific optimal moisture. These results demonstrated that while species with greater safety from turgor loss had lower mortality rates, only hydraulic architecture explained species’ moisture dependency. Species with a greater leaf area to sapwood area ratio were associated with drier sites and reduced their dry season transpirational demand via deciduousness.

PNNL Contact

Nate McDowell, Pacific Northwest National Laboratory, nate.mcdowell@pnnl.gov

Funding

This work was supported by the U.S. Department of Energy's (DOE’s) Atmospheric System Research, an Office of Science Biological and Environmental Research program; Pacific Northwest National Laboratory is operated for DOE by the Battelle Memorial Institute under contract DE-AC05-76RL01830. Collection of the data was supported by the Atmospheric Radiation Measurement (ARM) user facility, a DOE user facility managed by the Biological and Environmental Research program.