Other studies have shown that increasing CO2 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 has the opposite effect, Felzer noted, 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. However, this effect has been shown to be smaller than the effect of reduced stomatal conductance.
Aware of these cycles, Felzer and colleagues 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. They found that runoff would increase anywhere from 3 to 6 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 leads 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 ava
|Contact: Sarah DeWitt|
NASA/Goddard Space Flight Center