How Drought Affects Forests and Streams

Understanding the interactions among drought, hydrology, and vegetation

Forested watersheds across the U.S. supply drinking water for nearby urban populations. Photo by U.S. Bureau of Land Management.
Forested watersheds across the U.S. supply drinking water for nearby urban populations. Photo by U.S. Bureau of Land Management.

Across the U.S., forested watersheds filter surface water that drains into the rivers that supply drinking water for many of the nation’s cities. Besides providing high quality water for humans needs, forest trees regulate streamflow, mitigate flooding, and help create and maintain the water conditions that support healthy aquatic ecosystems.

Drought affects the ability of forests to continue these important functions. In areas of the U.S. that experience seasonal drought on a regular basis, forest and stream ecosystems are often able to adapt to the hydrologic changes that take place, but severe or long lasting drought can push systems to the breaking point. In the western U.S., for example, more prolonged and frequent droughts have ramped up the spread and intensity of insect attacks that, in some cases, kill nearly all of the canopy trees in forest stands. Loss of forest canopy in turn affects the quality and quantity of water from those forests.

Western forests are not alone in their vulnerability to drought. The warmer temperatures and longer dry spells projected under climate change will probably lead to more extreme drought conditions for forests in all regions of the U.S. Forest vegetation can either mitigate or worsen the effects of drought on the ecosystems supported by forests, depending on the species composition of the forest and how those species make use of water and tolerate stress.

Understanding how drought, hydrologic processes, and vegetation interact in U.S. forests – the ecohydrological context – is the subject of a new article in the journal Forest Ecology and Management by U.S Forest Service scientists and collaborators. The review article is a modified version of a comprehensive synthesis on the effects of drought on U.S. forests and rangelands developed by some of the same authors to provide information for the sustained National Climate Assessment process.

“Understanding how drought impacts ecosystems requires knowledge of how it impacts the ecohydrological processes of individual tree species in relation to species composition and physical factors such as slope and soil,” said Jim Vose, lead author of the article and project leader of the Forest Service Center for Integrated Forest Science and Synthesis. “With this understanding we can start to develop management options that minimize the impacts of drought on water and water quality.”

For the article, the researchers examined how differences in forest species composition and structure impact evapotranspiration – the loss of water vapor through tree leaves – and how this in turn impacts the quantity and quality of water from forest watersheds. Sections include discussions of the forest harvesting research that has provided much of what is known about how changes in forest structure impact streamflow and water quality, as well as what is known about groundwater interactions with drought, and drought impacts on water quality and stream water temperature.

After noting the challenges to predicting the impacts of drought on hydrological processes, the authors review management options for minimizing impacts on water quantity and quality, including the idea of harvesting to increase water yield, which research has shown is not sustainable in the long run. Replanting cut forests with species that consume less water is another possibility, as are limiting streamwater withdrawals and wastewater discharge during periods of low streamflow, or restoring and maintaining riparian buffers to increase shade and reduce sediment loads.

“There’s still so much uncertainty about drought projections, so it is difficult to predict the ecohydrological interactions among trees, water, and ecosystems,” cautioned Vose. “This uncertainty means that there will be a continuing need for monitoring and adaptive forest management.”

Despite these challenges, new technologies coupled with data from established and emerging networks – such as Forest Service Experimental Forests and Rangelands and Forest Inventory and Analysis plots, the Ameriflux network, NEON, LTER sites, and USGS streamflow gauges — offer ways to measure and visualize ecohydrological processes at a range of spatial scales that may lead to new insights into how to respond to the inevitability of drought.

Read the full text of the article.

For more information, email Jim Vose at jvose@fs.fed.us.

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