Researchers from North Carolina State University discovered that during extreme droughts, higher elevation forests in the Blue Ridge Mountains are often maintaining, if not increasing, their water use.
The findings, which were published in the journal Landscape Ecology, show that upstream forests’ increased water use during drought could leave less water downstream for forests, towns, and wildlife.
“We’re expecting that droughts will become more severe and frequent, so it’s important to understand how that’s going to influence the amount of water we have available,” said the study’s lead author Katie McQuillan, a graduate student in the Center for Geospatial Analytics at NC State. “We found that these forests are using more water on average during droughts, and that leads to less water ending up downstream.”
Based on thermal infrared remote sensing data collected by satellites between 1984 and 2020, researchers looked at how trees utilize water and emit it as vapor. They used the information to better understand patterns in forest water usage in the Blue Ridge Mountains, which span Virginia, North Carolina, Tennessee, South Carolina, and Georgia.
“Forested mountain regions are super important for the quality and quantity of water that we have downstream,” McQuillan said.
“Mountain forests produce some of the cleanest and most stable sources of water, and have an impact on how much water is available downstream for people and aquatic species.”
Higher elevations are finding droughts more stressful, so they’re using more water every time there’s a drought. These high elevation and ridge forests are able to increase water use because they have first access to precipitation. With less runoff, that makes dry conditions for trees at lower topographic positions worse, leading to larger declines in forest water use in low elevation and valley forests.
Katherine L. Martin
During moderate, severe, and extreme droughts, they tracked whether forests consumed more or less water than normal for each pixel of mapped land averaged depending on elevation.
During extreme droughts, they discovered that the average water use of higher elevation forests remained constant or slightly rose. Those forests, which accounted for around 22% of the total forest area investigated, were mostly found above 3,280 feet. Meanwhile, they discovered that woods at lower elevations used less water on average.
“Higher elevations are finding droughts more stressful, so they’re using more water every time there’s a drought,” said the study’s senior author Katherine L. Martin, assistant professor of forestry and environmental resources at NC State.“
These high elevation and ridge forests are able to increase water use because they have first access to precipitation. With less runoff, that makes dry conditions for trees at lower topographic positions worse, leading to larger declines in forest water use in low elevation and valley forests.”
Lower-elevation woods may be less adaptable to drought due to their lower water demand.
“The processes of tree water use and growth are coupled, so when a tree reduces its water use it is also reducing growth,” McQuillan said. “More drought-adapted trees are typically able to continue using water, and growing, under greater water stress than less drought-adapted trees.”
Mountain forests drank more water than typical at all elevations during the drought’s climax, according to researchers. Researchers believe that rising temperatures as a result of climate change are a factor.
“We think that increasing temperatures are behind greater water use during droughts,” McQuillan said. “When it’s hotter, forests use more water to keep themselves cool.”
Researchers believe that changes in the sorts of trees found in forests as a result of fire suppression, as well as variations in precipitation or other trends, may have influenced their findings. Drought-tolerant trees like maples and tulip poplars, which were once only found at lower elevations, are now more widely distributed across the terrain.
“There are species that use a lot more water than others,” McQuillan said. “If those are in the high elevation or upslope areas, that exacerbates what we’re seeing.”
Changes in the way forests use water could exacerbate water shortages, according to the results.
