Study: Drought Pushing World's Trees to Their Limits

November 21, 2012 - News Release

Most of the world’s trees – including rainforest species – live close to their “safety threshold” for coping with water stress, leaving them vulnerable to effects of droughts caused by climate change.

That’s the finding of a new study published today in the journal Nature by an international research team, including a University of Guelph professor.

Hafiz Maherali of the Department of Integrative Biology and 23 scientists from 10 countries set out to predict how climate change will affect forests.

Scientists know that rising global temperatures are affecting forest ecosystems by increasing rainfall in some regions and increasing drought in others. One of the main causes of tree dieback in regions experiencing drought is hydraulic failure: drought stress creates air blocks in the plant’s vascular system — similar to an embolism in the human circulatory system — and makes it harder for a tree to supply water to its leaves for photosynthesis.

Plants differ dramatically in their resistance to the problem. Without a clear understanding of how species and environments tolerate drought, researchers find it difficult to predict the effects of climate change. In this first-ever worldwide study, the researchers examined all existing measurements of plant embolism resistance in forest species.

“We wanted to know how resistant plants around the world are to embolism, which species are more resistant, and how much stress a plant is experiencing relative to its resistance threshold,” said Maherali, whose previous research has examined plant hydraulic systems and survival.

They discovered that 70 per cent of 226 forest species from 81 sites worldwide live within narrow “safety margins” against potentially deadly levels of drought stress.

Those safety margins are largely independent of rainfall, making all forest types equally vulnerable to drought. Trees seem to favour a “risky” hydraulic strategy that maximises growth against the risk of death.

“This explains why there are patterns of tree dieback in various ecosystems and not just in areas that are drier,” Maherali said. “It shows how finely closely matched a tree species is to its environment.”

With such a slim “safety margin,” plant productivity and survival could be dramatically affected by increased drought.

The study may help pinpoint which tree species will likely persist and which will suffer and potentially disappear.

“This is not a doom-and-gloom scenario,” Maherali said. “Drought will most probably trigger changes in the species composition of a community; species could either evolve, acclimate or migrate to a more suitable climate and be replaced with species that are more resistant. The most important implication of these findings is that it will help us predict which forests are most likely to experience change.”

The new study was organized through the ARC-NZ Research Network for Vegetation Function in Australia, and was headed by Brendan Choat of the University of Western Sydney and Steven Jansen of Ulm University in Germany.

Prof. Hafiz Maherali
Department of Integrative Biology
519-824-4120, Ext. 52767

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