Previously unknown mechanism causes increased forest water use

Researchers have discovered a previously unknown mechanism that causes increased water use in forests, advancing our understanding of forest water cycles and highlighting threats to plants from water stress, as happens from acid rain.

A study in Science Advances reports how the mechanism works: When fossil fuels are burned, sulfuric and nitric acid eventually fall back to earth in rain and snow, causing acidification of the soil. When that happens, a significant amount of calcium washes out of the soil, causing calcium deficiency and more water use in plants.

Calcium plays a unique role in plant cells by regulating the minute pores, called stomata, in leaves or stems. Without enough calcium, plants can't close these pores, and their water use increases. The plants pump more water through transpiration, the process of water movement through a plant and its evaporation from leaves, to meet the calcium demand.

Lixin Wang of Indiana University-Purdue University Indianapolis, senior author of the NSF-funded research, said, "When plants use a lot of water, there will be less water for people. It also means that these plants are very sensitive to drought. If a drought comes, and plants can't close their stomata, they are subject to high levels of mortality due to water stress."

Traditionally, forest water use was considered a function of meteorological factors, species composition and soil water availability. The impacts of soil biogeochemistry on large-scale forest water use had not been investigated.

"The effects of acid rain on water quality and ecological systems have been known for many years, but acid rain's influence on the water cycle was largely unknown," says Ingrid Padilla, a program director in NSF's Division of Earth Sciences. "This study provides key evidence that acid rain also affects the movement of water between soil and the atmosphere and impacts the amount of water available for humans."