

Rainforest in Malaysian Borneo. Photo by Rhett A. Butler.

Forests worldwide are at “equally high risk” to die-off from drought conditions, warns a new study published this week in the journal Nature.

The study, conducted by an international team of scientists, assessed the specific physiological effects of drought on 226 tree species at 81 sites in different biomes around the world. It found that 70 percent of the species sampled are particularly vulnerable to reduction in water availability. With drought conditions increasing around the globe due to climate change and deforestation, the research suggests large swathes of the world’s forests — and the services they afford — may be approaching a tipping point.

Water is critical to trees, transporting nutrients, providing stabilizing, and serving as a medium for the metabolic processes that generate the energy needed for a tree to survive. Mechanically, water moves through plants via their xylem, a tissue that can be compared to a system of tubes. Transpiration or release of water from a plant’s leaves keeps the system moving. But when water availability is insufficient, the process begins to break down, having substantial impacts on the health of a tree. While this has long been observed, until recently the exact mechanism that triggers drought stress in forests was poorly understood. The new study argues that “hydraulic failure” may be a key factor. Effectively, insufficient water availably leads a tree to start pulling air bubbles — called gas emboli — into its xylem impeding the flow of water. Hydraulic failure is akin to attempting to drink through a broken straw — air bubbles significantly reduce the amount of liquid that reaches the top of the straw.



Beech forest in New Zealand. Photo by Rhett A. Butler.

The researchers found that a wide range of trees are susceptible to “hydraulic failure”.

“We show that 70% of 226 forest species from 81 sites worldwide operate with narrow… hydraulic safety margins against injurious levels of drought stress and therefore potentially face long-term reductions in productivity and survival if temperature and aridity increase as predicted for many regions across the globe,” the authors write. “Safety margins are largely independent of mean annual precipitation, showing that there is global convergence in the vulnerability of forests to drought, with all forest biomes equally vulnerable to hydraulic failure regardless of their current rainfall environment.”

The results provide insight on why drought-induced forest die-off is occurring in a range of forest types, including tropical rainforests which are not typically considered at risk of drought. Over the past 15 years, forests in Borneo and the Amazon have suffered from widespread drought-induced decline. Drought stress is often accompanied by increased incidence of fires, either from natural sources like lightning or human activities like burning for cattle pasture or plantation establishment.



Forest in Malaysian Borneo. Photo by Rhett A. Butler.

The findings provide an ominous warning on the condition and future of forests, according to Bettina Engelbrecht, a biologist at Germany’s University of Bayreuth.

“Angiosperm trees in all forest biomes have converged on a risky strategy, operating at extremely narrow safety margins,” Engelbrecht wrote in a commentary accompanying the Nature study. “This implies that these trees are already, under current conditions, on the verge of injurious levels of water availability, and that even a minor increase in drought intensity will induce levels of xylem embolism that will impair growth and lead to tree death.”

“The suggestion that all forests are on the brink of succumbing to drought, and may already be responding to climate change, is supported by observations of increased drought-induced forest die-offs and tree mortality in many ecosystems.”

The research also has implications for efforts to combat climate change by relying on forests to sequester more carbon from the atmosphere. Dying forests release, rather than absorb, carbon.

“These studies sound a warning bell that we can expect to see forest diebacks become more widespread, more frequent and more severe — and that no forests are immune,” Engelbrecht continued. “The ramifications of this scenario are diverse and, in many respects, dire: forest mortality will be accompanied by changes in species composition, changes in ecosystem function and losses of services and biodiversity.”

CITATION: Brendan Choat et al. Global convergence in the vulnerability of forests to drought. Nature (2012) doi:10.1038/nature11688

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