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Risky business: Drought pushes tree plumbing to its limits

Beneath the bark of world’s trees pulses a finely tuned vascular system that transports billions of litres of water to the skies every day. This plant hydraulic system depends on a unique but unstable mechanism that is continuously challenged by environmental stress. New research in the current issue of Nature, has found most trees, even those in rainforests, operate very close to their hydraulic safety threshold leaving them highly vulnerable to droughts of increased severity.

An international team of 24 plant scientists organized via the ARC-NZ Research Network for Vegetation Function at Macquarie University gathered together a global data synthesis examining all existing measurements of plant embolism resistance in forest species.

Dr Brendan Choat from the University of Western Sydney and Dr Steven Jansen from Ulm University in Germany led the analysis.

One of the scientists from the Macquarie research team, Dr Sean Gleason, said the project provided a unique opportunity.

“Scientists from institutions all over the world worked together to achieve a global view of how climate change might affect the Earth’s plant species,” he said.

Drought is one of the major forces shaping our forest ecosystems. Over the last century, drought has been responsible for many incidences of large-scale forest dieback around the world. To make effective predictions of how the forest landscape may change in future, we need to first understand how plants work. One of the main problems that plants face during drought is to keep their ‘plumbing’ working. In order to take up carbon dioxide for photosynthesis and to cool their leaves, plants must transpire very large amounts of water every day. This water is absorbed from the soil and transported through a network of thread-like pipes that connect the roots to the leaves.

But this hydraulic pipeline is vulnerable and can break down during drought because of the development of air embolism. As soil dries, the water in these pipes comes under a large tension that can cause breakage of the liquid threads inside the plumbing system. So called “cavitation” of the liquid continuum inside the plant vascular system causes an air blockage, similar to the embolisms that can block the human circulatory system. As drought stress increases, gas accumulates in the system until the plant desiccates and dies.

Vulnerability to embolism is known to be one of the main factors determining drought effects on trees. However, plants vary dramatically in their tolerance of drought induced embolism making predictions of how forests might be altered more difficult.

As expected, species growing in wet forests were less resistant to embolism than those growing in arid areas. However, when vulnerability to embolism was compared to the moisture conditions typical for each species it emerged that most trees currently operate very close to their hydraulic safety threshold leaving them highly vulnerable to drought.

While plants vary greatly in their embolism resistance, their vulnerability to drought is the same across all forest types. Seventy percent of 226 forest species from 81 sites in the worldwide study operate with narrow hydraulic safety margins against potentially deadly levels of drought stress. The team found safety margins are largely independent of mean annual precipitation, illustrating global convergence in the vulnerability of forests to drought, with all forest types equally vulnerable to hydraulic failure regardless of their current rainfall environment.

The findings provide insight into why drought-induced forest decline is occurring not only in arid regions but also in wet forests not normally considered at drought risk. Trees take a ‘risky’ hydraulic strategy in a trade-off that balances growth with protection against the risk of mortality.

For trees, and the planet, the consequences of longer droughts and higher temperatures are potentially dramatic. For example, rapid forest collapse via drought could convert the world’s tropical forests from a net carbon sink into a large carbon source during this century.

However, the results of the study do not necessarily point to forest Armageddon. A forest may respond to climate change in a number of ways. For instance, some species may be able to evolve quickly enough to keep pace with a changing climate in one location, while others may spread into new locations, tracking their preferred conditions. Survival is largely dependent on species having enough time to respond to changes in the environment. The new dataset will be useful to better predict the balance between a declining or a healthy forest. It will also provide a better understanding of which species are likely to persist and which are likely to suffer and potentially disappear.

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