The extraordinary height of the world's tallest tropical trees does not make them more vulnerable to drought than smaller trees, a new study has revealed.
The research overturns a widely held and untested scientific belief that the higher a tree grows, the harder it becomes to move water from the soil to its leaves – potentially increasing its vulnerability to drought stress.
The international team, led by researchers at Cardiff University and the University of Exeter, studied five species of dipterocarps in the rainforests of Malaysian Borneo, observing how the trees adapted their woody anatomies as they grew to ensure a regulated water supply from root to leaf.
Their findings, published in the journal Science, reveal the tallest rainforest trees suffer no more than smaller trees during drought events, challenging decades of ecological theory which the team says must now be re-evaluated.
Understanding tall trees is vital, because the tallest 1% store more than half of above-ground carbon in forests. These trees are therefore rare and play an important role in the planet's rainforests capacity to combat climate change by storing and absorbing carbon.
"As a tree becomes taller, keeping its leaves hydrated becomes more challenging. Existing predictions suggest an impaired hydraulic system places them at higher risk of dying due to drought. That prediction is included in some models of climate-change impacts, and our study suggests this may not be correct," explains lead author Dr Paulo Bittencourt from Cardiff University's School of Earth and Environmental Sciences.
Reaching heights of up to 100 metres tall, Dipterocarps dominate the rainforests of southeast Asia, storing vast amounts of carbon and helping to protect and shape their surrounding ecosystems.
The researchers witnessed how these colossal trees protected themselves against the effects of drought by widening their internal water‑carrying vessels and adjusting their leaf physiology.
"Trees contain lots of thin, hollow vessels and – like drinking through a straw – they draw water upwards by creating low pressure at the top," said the study's co-author Professor Lucy Rowland from the University of Exeter.
"Unlike drinking straws, these vessels have evolved intricate adaptations that can maintain the water in liquid form, even under the extreme low pressures required to move it above 10 metres.
"However, a widely accepted theory suggests that in tall trees, the sheer length of vessels and the effects of gravity limit water transport, photosynthesis and growth.
"Our results challenge this by showing that the hydraulic systems of very tall Dipterocarp trees are perfectly evolved for their height, and should not suffer more than small Dipterocarp trees exposed to the same drought conditions."
The researchers examined Dipterocarp trees ranging from 7 to 71 metres tall in Malaysian Borneo, and measured a variety of characteristics at multiple positions along each tree.
They found taller trees compensate for their height in various ways, including water-carrying vessels that grow wider nearer the ground and leaves which have adapted to withstand greater water stress before wilting.
They also measured trunk growth rates before, during and after the strong El Niño drought period of 2023-2024.
Co-author Palasiah Jotan, a Malaysian PhD student studying in The Czech University of Life Sciences, added: "As a Malaysian researcher co-authoring this study, showing that even the tallest of these trees are hydraulically resilient to drought is a finding I hope will strengthen the case for protecting these forests under a changing climate."
The research team included Sabah Forestry Department (Malaysia), the UK Centre for Ecology & Hydrology and the University of Aberdeen, as well as institutions from the Czech Republic, Spain, Brazil and the USA.
More research is now needed to investigate the hydraulic systems and drought resilience of other tall trees and so we are expanding this work to test its generality across the tropics, repeating, for example, in the giant Amazon trees.
The study was funded by the Natural Environment Research Council.
