A forest has been watered by acid rain may be less able to quench its thirst.
Those are the results from a decades-long experiment in the Appalachian Mountains, where the U.S. Forest Service since 1989 has been dousing a 34-hectare patch of forest with an acidifying ammonium sulfate fertilizer three times a year. The chemical served as a proxy for acid rain, which is created when sulfur and nitrogen-containing compounds released by industrial activities, agriculture and the burning of fossil fuels acidify raindrops.
In most of the years from 1989 to 2012, the acidified forest soaked up around 5 percent more water than a neighboring, 39-hectare untreated area — and up to 10 percent more in two of the years, researchers report July 31 in Science Advances. Levels of calcium, a nutrient plants need to retain water, in the water that permeates the soil of the acidified forest also declined over the study period, which could explain the forest’s water-guzzling.
“We didn’t expect that plants actually will respond so strongly to acidification,” says ecohydrologist Lixin Wang of Indiana University-Purdue University Indianapolis. That may be cause for concern, he says: Thirstier plants could contribute to droughts or leave less water available for people and other animals.
Scientists have previously looked at acid rain’s effects on trees and small plots, but tests of acidification on an entire watershed are rare. The team, which included a Forest Service scientist, used the agency’s data to calculate the forest’s water uptake by subtracting how much flowed out of the forest in streams from how much water fell as precipitation. The trends in water use were also reflected in soil moisture data collected during the study.
It’s not clear if other types of forest, with different trees or types of soil, would show the same results. Fenrow Experimental Forest near Parsons, W. Va., where the study was conducted, grows from a silty loam of sandstone and shale and is packed with deciduous trees: cherry, maple, oak, and others.
Still, “this is not a small [amount] of water we’re talking about,” says Matthew Lanning, an ecohydrologist in Wang’s lab. It adds up to around extra 13.6 million liters of water per year on average used by the treated watershed, Lanning says.
The acid in acid rain can dissolve minerals in the soil and deplete its calcium. Previous research has shown that calcium regulates the opening and closing of guard cells around the stomata, pores through which plants release water. Without enough calcium, plants can’t control this water loss as well.
Still, how the scientists calculated the trees’ water use gives Dymond some pause. “They’re very simple calculations,” she says, “and this simplicity could mask a piece of the story.” For instance, the 5 percent increase in water use with acidification may be within the natural range of variation.
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