Climate Change Increases Runoff In Eastern U.S. Forests

Runoff may be as much as 17 percent higher in forests of the eastern United States when models also account for changes in soil nitrogen levels and atmospheric ozone exposure.

"Failure to consider the effects of nitrogen limitation and ozone on photosynthesis can lead us to underestimate regional runoff," said Benjamin Felzer, an ecosystem modeler at Lehigh University in Bethlehem, Pennsylvania.

"More runoff could mean more contamination and flooding of our waterways," he said. "It could also mean fewer droughts than predicted for some areas and more water available for human consumption and farming. Either way, water resource managers need more accurate runoff estimates to plan better for the changes."

Felzer and colleagues from the Massachusetts Institute of Technology in Cambridge and the Marine Biology Laboratory in Woods Hole, Massachusetts, have published their findings in the "Journal of Geophysical Research – Biogeosciences."

The team used theoretical models to project various future scenarios for the amount of carbon dioxide in the atmosphere and what it would mean to the changing water cycle in forests east of the Mississippi River.

Swollen brook in Westhampton, Massachusetts (Photo by Chris McNulty)

They found that runoff would increase anywhere from three to six percent depending on location and the amount of the increase in CO2.

Felzer and colleagues also examined the role of two other variables - atmospheric ozone and soil-based nitrogen - in the changing water cycle.

Excess ground-level ozone harms the cells responsible for photosynthesis. Reductions in photosynthesis lead to less transpiration and cycling of water through leaves and more water added to runoff.

In most boreal and temperate forests, the rate of photosynthesis is also limited by the availability of nutrients such as nitrogen in the soil. The less nitrogen in the soil, the slower their rate of photosynthesis and transpiration.

"The increase in runoff is even larger when nitrogen is limited and environments are exposed to high ozone levels," said Felzer.

In fact, the team found an additional seven to 10 percent rise in runoff when nitrogen was limited and ozone exposure increased.

"Though this study focuses on Eastern U.S. forests, we know nitrogen and ozone effects are also important in South America and Europe," said co-author Adam Schlosser of the Center for Global Change Science at MIT.

"One region has seen a net increase and the other a net runoff reduction," he said. "Our environment and quality of life depend on less uncertainty on this front."

"Plant growth can have a considerable effect on the climate," says Wolfgang Buermann, a geographer at Boston University. But the issues raised by this relationship are complex.

"It's like sweating. When you sweat you cool the surface of your skin," says Buermann. Over a forest canopy or a vast expanse of grassland, large amounts of transpiration can markedly increase water vapor in the atmosphere, causing more precipitation and cloud cover in an area. The additional cloud cover often reinforces the cooling by blocking sunlight.

The amount of water that plants give up depends on how much carbon dioxide is present in the atmosphere. Studies have shown that despite a global drop in rainfall over land in the past 50 years, runoff has actually increased.

Other studies have shown that increasing carbon dioxide is changing how plant "pores," or stomata, discharge water.

With elevated CO2 levels, leaf pores contract and sometimes close to conserve internal water reserves. This "stomatal conductance" response increases water use efficiency and reduces the rate of transpiration.

Plants that release less water also take less of it from the environment. With less water being taken up by plants, more water is available for groundwater or runs off the land surface into lakes, streams, and rivers.

Along the way, it accumulates excess nutrients and pollutants before emptying into waterways, where it affects the health of fish, algae, and shellfish and contaminate drinking water and beaches. Excess runoff can also contribute to flooding.

Sometimes rising CO2 levels have the opposite effect, Felzer observed, promoting vegetation growth by increasing the rate of photosynthesis. More plant growth can lead to a thicker canopy of leaves with increased transpiration and less runoff, but this effect has been shown to be smaller than the effect of reduced stomatal conductance.

Copyright Environment News Service, ENS, 2009. All rights reserved.