Sustain Our Nation's Forests and Grasslands

Study reveals how trees are dealing with increase in available carbon dioxide

A tree in the forest
Tree in Chequamegon-Nicolet National Forest. USDA Forest Service photo.

MINNESOTA — Globally, forests remove about 30 percent of anthropogenic carbon dioxide emissions through photosynthesis. Forests also return almost 40 percent of precipitation resulting from rain or snow melt back to the atmosphere via transpiration. But that might be changing, according to new research by University of New Hampshire and USDA Forest Service scientists.

The study, “Disentangling the role of photosynthesis and stomatal conductance on rising forest water-use efficiency,” was published last week in the journal Proceedings of the National Academy of Sciences.

Carbon dioxide is vital for photosynthesis in all plants. However, there are tradeoffs for trees and other plants when they open stomata (the pores on leaves) to let carbon dioxide in. When stomata open, water is lost via transpiration. Northern Research Station scientists David Hollinger and Ken Clark were part of the team that found trees are dealing with a higher level of carbon dioxide in the atmosphere by being more efficient with water use, and that the reasons for this are more complex than originally thought.

Scientists found that with more carbon dioxide in the atmosphere, trees do not have to leave their stomata open as wide, allowing them to use water more efficiently. Because they are using water more efficiently, trees are taking up less water, leaving more to flow into streams and rivers. The study found that enhanced photosynthesis also was occurring in forests that were not limited by water availability. This indicates that other physiological mechanisms were involved in the observed increased tree growth that has occurred over the last 30 years.

The larger implications for forests have yet to be explored. One immediate takeaway for researchers modeling forest change is that elevated carbon dioxide is enhancing tree growth in a number of ways. The study offers equations that can be plugged into forest models to reflect the rising efficiency of water use with increasing carbon dioxide. This should allow researchers to better simulate the potential future interactions of carbon dioxide and water on forest growth.