Dry Air May Be More Stressful to Trees than Dry Soil

New research contributes to understanding of how drought affects forests

New findings show that in the future low relative humidity combined with high air temperature may play a significant role in tree stress in the temperate hardwood forests of the eastern U.S. Photo by ForestWander, courtesy of Wikimedia Commons.
New findings show that in the future low relative humidity combined with high air temperature may play a significant role in tree stress in the temperate hardwood forests of the eastern U.S. Photo by ForestWander, courtesy of Wikimedia Commons.

Scientists forecast that for many parts of the U.S., climate change will bring higher temperatures and more frequent and severe periods of drought. In parts of the West, forests are already changing as a result of drought, but all U.S. forests may be impacted, in turn affecting other important resources such as clean air and water.

Newly published findings based on data from Ameriflux sites across the U.S. shows that low relative humidity combined with high air temperature may play an increasingly significant role in tree stress in the future, especially in the temperate hardwood forests that dominate the eastern part of the U.S.

Published in the journal Nature Climate Change, with Indiana University assistant professor Kimberly Novick as lead author, the findings have important implications for the models currently used to predict the impacts of water stress on forest ecosystems under climate change and for future management activities.

 “When the air is warm and relative humidity is low, the atmosphere has a high demand for water, which can increase water use by plants,” said Chris Oishi, research ecologist at the Forest Service Coweeta Hydrologic Laboratory and co-author of the findings along with researchers from eight universities and the USDA Agricultural Research Service. “During drought, trees may try to limit water loss by closing stomata; however, this strategy makes them less effective at removing carbon from the atmosphere, reducing the ability of forests to offset climate change.”

The researchers analyzed evapotranspiration (the sum of water moving through plants and from Earth’s land surfaces into the atmosphere) and hourly weather data from flux towers at 38 Ameriflux sites that span a range of biomes, from dry environments to those with a moderate amount of moisture. They used previous observations and multiple climate models to separate the effects of changes in soil moisture and relative low humidity on evapotranspiration in relation to present and future drought conditions.

“Before this study, it’s been hard to tell how much of a ecosystem’s response to drought was due to low soil moisture and how much was due to low relative humidity,” said Oishi. “Drought conditions are characterized by low soil water availability and high atmospheric demand for water and typically persist for a period of weeks to months. However, throughout the growing season, we also observe shorter periods of time with equally high atmospheric demand, but non-limiting soil moisture. Using multiple years of hourly data allowed us to separate the effects on tree functions.”

The study found that soil moisture and relative humidity have different effects on tree physiology at different times of the day, and projected that humidity will become the dominant factor in the future as weather grows hotter and drier, especially in temperate forest ecosystems.

The findings provide important input for modeling the effects of climate change on forest ecosystems but also have implications for management approaches to alleviating future drought stress such as irrigation and thinning. The authors found that while these approaches – mostly used in agricultural settings – might improve soil moisture, they would have little effect on the local relative humidity that will become the major limiting factor for many areas of the U.S.

Read the full text of the article.

For more information, email Chris Oishi at acoishi@fs.fed.us.

Access the latest publications by SRS scientists.

Subscribe to our newsletter!

Receive weekly updates