Malaysia's tallest rainforest trees keep pumping water to their highest branches even during drought, thanks to a hidden hydraulic adaptation, scientists found

For years, one idea has quietly shaped the way climate scientists in the US and elsewhere think about forests and drought. The theory is that the taller a tree grows, the harder it has to work to pull water from its roots all the way up to its topmost leaves, leaving giant trees more vulnerable when the rains stop. But a new study turns that assumption on its head.

In the study, ‘Height does not impair the hydraulic system of the tallest tropical Dipterocarp trees,’ published in Science, researchers from Cardiff University and the University of Exeter examined five species of dipterocarps, a family of rainforest giants that dominate the rainforests of Malaysian Borneo. They found that the tallest of these trees is not more vulnerable to drought than its shorter neighbors. The same study reports that the trees have silently built their own internal workaround for the physical challenge of moving water to such extreme heights.

Why this matters beyond the rainforest
It’s not only of interest to botanists. The University of Exeter says the tallest 1% of trees on the planet hold more than half of all the carbon held above ground in forests, making them a major piece of the puzzle when it comes to slowing climate change. According to Science's own reporting on the study, the assumption that trees are more fragile in droughts because they’re tall has gone straight into the climate models scientists use to predict how much carbon forests will keep locked up as the planet continues to warm. So getting this detail wrong has real consequences.

The old theory, and why it made sense on paper
Imagine drinking water through a straw. Now imagine that straw going up more than 20 stories high. As a tree grows taller, the path water must travel from root to leaf becomes longer, and the added pull of gravity makes it harder for the tree to keep its highest leaves hydrated, the Science study notes. That’s the basic physics that led scientists to predict that the tallest trees would be the first to suffer once a drought set in.

In the paper, the authors point out that trees move water under extremely low pressure, and that the conventional view has long been that vessel length and gravity should limit transport, photosynthesis, and growth as trees get taller. Their results challenge that expectation by showing the hydraulic systems of very tall dipterocarps are “perfectly evolved” for their height.

What the researchers actually did
The Science study notes that the team tested that assumption by measuring dipterocarp trees ranging in height from about 23 feet to about 233 feet in the Kabili-Sepilok Forest Reserve in Malaysia on the island of Borneo. The same paper also tracked how each tree’s internal water-transport system changed as it grew taller, and measured trunk growth rates before, during, and after the severe El Niño drought of 2023 and 2024, one of the most intense dry spells the region has experienced in recent years.

What they found
The results even surprised the scientists involved. The University of Exeter said the tallest trees did not suffer any more growth loss than smaller trees in the drought, suggesting that being tall does not necessarily put them at a disadvantage. In the Science paper, the authors wrote that the trees compensated for their tallness with larger water-conducting vessels located closer to the ground. They also altered their leaves to withstand greater water stress before wilting, fully compensating for the height disadvantage.

A theory that may need a rewrite
The finding challenges a long-standing assumption in forest ecology that has shaped researchers’ thinking about which trees are most at risk as droughts become more frequent and severe. The paper published in Science argues that a tree’s risk of damage from drought may have more to do with the shade and microclimate of its immediate canopy than its height.

The research also had a personal touch. This shows that even the tallest of these trees can withstand drought, adding to the argument for preserving Borneo’s rainforests as the climate continues to change, according to co-author Palasiah Jotan, a Malaysian doctoral student at the University of Exeter.

What this means, and doesn't mean, for the US
It’s a big shift for American readers, given how often wildfire risk, drought, and forest die-off are in the headlines here. It’s worth being precise about what the study does and doesn’t show. According to the Science paper, the research focused specifically on dipterocarp rainforest trees in South-east Asia, not American species such as California's redwoods or giant sequoias, so it would be premature to assume the exact same mechanism applies to US forests. But what the study does provide is clear evidence that height and drought susceptibility are not necessarily linked, a distinction that is important for the way climate scientists everywhere refine their models.

The bigger picture
With droughts becoming more frequent with climate change, knowing which trees are really at risk, and why, is important for conservation and the accuracy of the carbon accounting behind climate forecasts. It turns out that some of the tallest trees on the planet may have already evolved a solution to a problem scientists had long assumed was working against them.