Changes to leaves and vessels allow trees to continue to carry water to their crowns during times of drought.
Bold climbers and researchers have challenged a major hypothesis in the biology of tree drought.
In the rainforests of the island of Borneo, Southeast Asia, the water transportation systems of a colossal and dominant tree family are no more damaged than smaller trees in the face of droughtthe researchers report on July 2 in Science. This finding contradicts the idea that gravity makes these giants more vulnerable to drought by preventing hydration of their upper branches and crowns. The study cannot confirm that this is true for all tree families. But the findings can help inform conservation efforts, the researchers say.
The path from roots to leaves becomes longer in larger trees. Water traveling up a tree’s vessels, or xylem, encounters more resistance the higher it goes, says functional plant ecologist Arne Scheire, now at the Southeast Asian Rainforest Research Partnership in Kota Kinabalu, Malaysia. And thanks to gravity, longer xylems have a more choked water potential, or a propensity for water to be sucked up. when the leaves of the plant release water.
But no experiments verify that larger trees suffer from these pressures in drought, Scheire says.
So Scheire, then at the University of Exeter in England, and his colleagues ventured into the lush, humid rainforest of Malaysia’s Kabili Sepilok Forest Reserve. There, the incomprehensibly tall crowns of trees from the Dipterocarpaceae family dominate the canopy. The researchers teamed up with professional climbers, waking up before dawn to collect samples of leaves, trunks and branches at different tree heights throughout the day. The 38 trees studied included five species, measuring between 7.7 meters and 71 meters tall, more than three-quarters of the height of the Statue of Liberty. They measured 25 traits related to trees’ water transport.
On average, taller trees start with larger xylem widths at their bases – a strategy that compensates for increased strength with height, the team found. And the taller leaves have adjusted their dehydration limit to adapt to less water being sucked in.
These adaptations apparently helped during a severe drought which lasted from 2023 to 2024. The team found that there was no height-related decline in growth rate among the sampled trees, suggesting that drought responses were independent of tree height.
A competing theory already cast doubt on river transport harming height. He predicted that larger trees would have adaptations like those presented in the study, says Julieta Rosell, a functional ecologist at the National Autonomous University of Mexico in Mexico City, who was not involved in the work. Interestingly, she says, the theory “motivated these researchers to test the upper parts of large trees… This has never been done before.”
Large trees can store enormous amounts of aboveground carbon and contribute to ecosystems by producing seeds and providing habitats for many tropical organisms. Because this family of trees is not bigger sensitivity to droughtthis “could be a good long-term place to sequester carbon” in Southeast Asia, Scheire says.
For Rosell, the results encourage rethinking the nature of trees.
“They do things all the time, constantly making changes to their anatomy,” she says. “And it gives a different perspective to the trees because they seem so calm.”