For decades, ecologists assumed that tall trees face an inherent disadvantage during drought. The logic seemed straightforward: gravity adds roughly 0.1 megapascals of tension to the water column for every 10 meters of height, and longer pathways create more resistance to water flow. Taller trees, the thinking went, should be closer to hydraulic failure, air bubbles blocking their water-transport vessels, when soil moisture runs low.
A study published July 2 in Science by an international team led by Cardiff University and the University of Exeter now overturns that assumption, at least for the tallest flowering trees on Earth.
“Understanding tall trees is vital because the tallest 1% of trees store more than half of above-ground carbon in forests,” said Paulo Bittencourt, the study’s lead author.
Scaling giants
The researchers focused on dipterocarps, a family of trees that dominates the rainforests of Southeast Asia and includes species that can exceed 100 meters. They recruited professional tree climbers who used double-rope techniques to scale 38 trees in the Kabili-Sepilok Forest Reserve and Danum Valley in Malaysian Borneo, ranging from just over 7 meters to 71 meters in height.
At multiple points along each trunk, the climbers collected samples and made direct measurements. The team analyzed xylem vessel anatomy, the microscopic pipes that carry water, measured leaf water potentials with a pressure chamber, and determined embolism vulnerability using a Pneumatron device. Automated dendrometer bands strapped to the trunks recorded growth rates at 30-minute intervals, capturing the trees’ response to the severe 2023-2024 El Niño drought.
The results were unambiguous. “The water-transport system of tropical giants called dipterocarps has evolved to defy the effects of gravity and resist water stress,” reported a companion Nature News & Views article published July 13.
Trees of all heights showed the same hydraulic safety margins, approximately 0.4 megapascals between their operating water potential and the point at which leaves wilt. Their vulnerability to embolism (the P50 value, the pressure at which 50% of xylem conductivity is lost) showed no correlation with height. During the El Niño drought, taller trees suffered no greater growth reductions than shorter ones did, relative to their size.
Two tricks
The study identified two mechanisms that allow dipterocarps to compensate for height. First, taller trees have wider xylem vessels near their base, essentially larger-diameter pipes that reduce frictional resistance over longer distances. Second, their leaves are adapted to greater water stress: the turgor loss point, the pressure at which leaves begin to wilt, shifts to more negative values in taller trees, so they can withstand lower water potentials before wilting.
“The hydraulic systems of very tall Dipterocarp trees are perfectly evolved for their height,” said senior author Lucy Rowland of the University of Exeter. “They should not suffer more than small Dipterocarp trees exposed to the same drought conditions.”
The finding provides experimental support for a revised theoretical model proposed by Anfodillo et al. in 2024, which predicted that trees could compensate for height through anatomical adjustments. Before this study, no one had been able to test the model in the uppermost reaches of the tallest tropical trees.
Several caveats apply. The results are specific to one tree family in one region; whether they hold for other families in other climates remains to be tested, and the team is expanding its work to the Amazon. The tallest tree in the sample was 71 meters, but some dipterocarps exceed 100 meters, whether compensation holds at those extreme heights is not directly tested. And only a single drought event was studied; repeated or more intense droughts could produce different results.
Nevertheless, the study challenges a foundational assumption embedded in some global vegetation models, which currently predict higher drought mortality for tall trees. If the compensation mechanism is widespread, those models may need revision.
“Our results show that trees are doing things all the time, making changes in their anatomy all the time,” said Julieta Rosell of the National Autonomous University of Mexico, who was not involved in the study. “And that gives a different perspective to trees because they seem so quiet.”
Source: Bittencourt, P., Scheire, A., Jotan, P. et al. “Height does not impair the hydraulic system of the tallest tropical Dipterocarp trees.” Science 393(6806), 60-64 (2026). DOI: 10.1126/science.aea9013