New Findings on How Trees Impact Droughts

With root systems spanning for dozens of meters underground and a hefty amount of leaf transpiration, we already know that trees impact local, regional and global water cycles. However, how different tree species act in the event of drought has been unknown until recently.

William R. L. Anderegg from the University of Utah and his team published a paper called “Plant functional traits and climate influence drought intensification and land-atmosphere feedbacks” in the Proceedings of the National Academy of Science. The researchers asked an intriguing question: “Instead of looking at how drought affects the forest, how might this go the other way around?” – as Anderegg had put it.


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The team collected data on the water, heat, and carbon distribution and flow at 40 sites from across the world. Afterward, they compared their data to local forest profiles. The premise was that the dominant tree species could somehow be associated with drought patterns. Researchers looked at the traits that could be related to intensified droughts. The results revealed that tree physiology could directly affect drought intensification and that various tree species handle drought conditions in distinctive ways. The study sheds light on different strategies of water usage and their consequences on the immediate environment of the trees.

2014 drought affects at the Fresno Harlen Ranch, Fresno, California. Photo: Cynthia Mendoza, USDA, public domain.

2014 drought affects at the Fresno Harlen Ranch, Fresno, California. Photo: Cynthia Mendoza, USDA, public domain.

Cooler climate species – especially conifers such as pines or cedars – can slow down their internal water circulation to keep the surrounding soil moist for as long as possible. On the other hand, some deciduous trees in warm climates, such as oaks in California and the Mediterranean tend to quickly use up all the water that is available to them. This results in additional loss of soil moisture, and consequently an increase in local temperatures and further aggravation of drought conditions. What’s also interesting that locations with diverse tree species mitigated drought conditions more efficiently. “That suggests it’s not just the dominant trees that matter, but having a diversity of tree species,” Anderegg said.

The study’s findings could have several practical implications. Understanding how vegetation affects regional weather is becoming increasingly important under the burden of climate change. Droughts are particularly challenging and essential to predict since they can induce a myriad of issues, from food and water shortages to devastating wildfires. Studies like this one are crucial to help predict and possibly mitigate the consequences – especially since preserving and planting forests are seen as one of the immediately available resources for buffering climate change.

Non-diverse tree populations like this orange grove can potentially make drought conditions worse. Photo: Cynthia Mendoza, USDA, Fresno Harlen Ranch in Fresno, CA, 2014, public domain.

Non-diverse tree populations like this orange grove can potentially make drought conditions worse. Photo: Cynthia Mendoza, USDA, Fresno Harlen Ranch in Fresno, CA, 2014, public domain.

The fact that non-diverse tree populations may worsen drought conditions automatically advocates for the planting of diverse forests instead of monoculture or low diversity forests. Also, the authors suggest that hydraulic transport mechanisms should be included in climate modelingas a mechanistic link between soil moisture and plant transpiration, all in the hope of predicting the land-atmosphere feedbacks and the potential climate extremes more accurately. They also report that “fortunately, large-scale hydraulic-enabled models are actively being developed.”

Resources

The Study

Anderegg, W. R. L. et al. (2019) Plant functional traits and climate influence drought intensification and land-atmosphere feedbacks. PNAS July 9, 2019.. https://doi.org/10.1073/pnas.190474711

Articles

Renault, M. (2019) Some trees can make droughts worse. Popular Science. June 24, 2019.

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