Researchers model optimal amount of rainfall for plants

CORVALLIS, Ore. – Researchers have determined what could be considered a “Goldilocks” climate for rainfall use by plants: not too wet and not too dry.

But those landscapes are likely to shrink and become less productive in the future through climate change, said Stephen Good, a hydrologist at Oregon State University and lead author on a recent study, published in the journal Nature Ecology and Evolution: A mesic maximum in biological water use demarcates biome sensitivity to aridity shifts.

Good led a research team that used models to determine the optimal amount of rainfall for the plants to be most productive in ecosystems.

“We found that the fraction of rainfall an ecosystem can use is largest at intermediate climates,” said Good, assistant professor in OSU’s College of Agricultural Sciences. “We’re seeing through climate change that wet landscapes are getting wetter and dry landscapes are getting drier. Ecosystems in both of these types of climate will use less of their rainfall for growth.”

The study is based on measuring transpiration – the process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. Transpiration is essentially evaporation of water from plant leaves, Good said.

“When you think of water as a resource, transpiration is the total amount of water a plant is able to use,” Good said. “It’s the part of the water cycle that is most linked to productivity of plants, ecosystems and agricultural systems. If environments could become more extreme, then plants could be less productive.”

Georgianne Moore, study coauthor and associate professor in the Department of Ecosystem Science and Management at Texas A&M University, said the researchers determined a “mesic maximum”—the middle of the curve between the two ends of the climate spectrum.

“The fraction that occurs in a climate in that range is the balance where the plants can use slightly more moisture than what they usually get, which makes plants optimize the use of water for beneficial growth. You could call it the sweet spot,” she said.

This new information can help scientists determine where optimal transpiration is in a given climate and how different management practices might change that, i.e., replacing older trees with younger trees or replacing grassland with cropland.

The researchers used data from real landscapes, and they also modeled the outcomes using a novel mathematical approach. The model shows how rainfall is partitioned between plant water use, evaporation, runoff and interception.

“The projection very closely matches what was observed in field stations around the globe,” Moore said. “We produced a map based on the model to show what parts of the world will fall off the curve and no longer use their rainfall as efficiently for beneficial growth.”

If these ecosystems don’t get irrigated or have another option to get water, their production can be expected to decline, said Moore, who holds a doctorate in environmental sciences from OSU.

“It’s going to be a wet gets wetter, dry gets drier world,” she said. “There could be big consequences, as it will affect forests, grasslands, savannas and deserts. Scientists can now use the model we have to help make predictions about the future of these plant communities.”

The study was also coauthored by Diego Miralles of Ghent University in Belgium. The research was funded by NASA, the U.S. Department of Energy Office of Science and the European Research Council.

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