Tropical forests store enormous amounts of carbon and harbor most of the world's terrestrial biodiversity, but scientists fear that increasingly severe droughts threaten their ability to survive and continue absorbing carbon from the atmosphere. A new study published today in Nature offers unexpected hope, however, and shows that tropical trees may be more adaptable to drought than scientists had previously believed.
Researchers from Columbia University, the University of Minnesota, Uppsala University, and the University of Tasmania measured eleven hydraulic traits—the properties that determine how trees transport and conserve water—in nearly 300 trees representing 18 tropical species across Puerto Rico's major climate zones, which range from ever-wet tropical forests (which receive 4,000 millimeters of rain per year) to dry tropical forests (which receive 1,000 millimeters of rain per year). The work was led by Chris Smith-Martin, a former postdoctoral researcher in the lab of Columbia University professor Maria Uriarte. Smith-Martin is now an assistant professor at the University of Minnesota. The team found that 17 of the 18 tree species adjusted their hydraulic traits to match local water availability, becoming more drought-resistant in drier forests.
For years, ecologists assumed these hydraulic traits were largely fixed within species, since studies in temperate forests had found little variation. This study shows the opposite: Tropical tree species exhibit substantial variation within species, allowing many of them to thrive across very different rainfall conditions.
“The prevailing view has been that these critical drought-resistance traits don't change much within species,” said senior author María Uriarte, professor of Ecology, Evolution and Environmental Biology at Columbia University. “We found exactly the opposite. Most tropical tree species can adjust their water transport systems to local conditions, suggesting they have greater capacity to cope with increasingly severe droughts than we previously recognized.”
The researchers measured traits related to both drought tolerance (a tree's ability to survive water shortages) and drought avoidance (strategies that conserve water before damage sets in). Trees in drier forests consistently showed greater resistance to hydraulic failure and larger safety margins against drought stress, whereas trees in the wetter forests tended to have higher water-storing capacity.
Current ecosystem and climate models generally assume hydraulic traits stay constant within species. This study suggests that building tree flexibility into prediction models could sharpen our understanding of forest resilience, carbon storage, and how species composition shifts as the climate changes.
“This study shows that we need to think about adaptation not only in terms of which tree species are present in a forest, but also how individual species adjust to local environments,” Uriarte said. “That flexibility could prove critical as droughts become more frequent and severe.”
