Thirsty mountain forests leave lower elevations with less water

Understanding how forests and other ecosystems respond to drought is critical to support water availability for all. A new study from North Carolina State University has found that higher elevation mountain forests, located above 3,280 feet, increase their water use during droughts. 

“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 study lead author Katie McQuillan. 

The study was focused on the Blue Ridge Mountains, a forested mountain region in the eastern United States. The researchers found that when upstream forests’ increase water consumption, less water is available for downstream ecosystems, wildlife, and even cities. 

Using satellite data from 1984 – 2020, the experts analyzed how trees use water and release it as vapor. The thermal infrared data helped identify trends in forest water use for more than 15,000 square miles of forest across 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,” said McQuillan. “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.”

The analysis revealed that during extreme drought, higher elevation forests’ water use was slightly increased. Meanwhile, lower elevation forests’ reduced water use during the same periods of drought. This is because higher elevations find droughts stressful, and end up using more water every time drought occurs – therefore adapting to droughts. 

“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.” said study senior author Katherine L. Martin.

Lower water consumption by lower elevation forests could mean that they are less adapted to drought.  Trees that are better adapted to drought are usually able to continue drinking water and can even grow under stressful conditions.

Across all elevations, the researchers saw that mountain forests drank more water at the droughts’ peak, likely due to climate change. “When it’s hotter, forests use more water to keep themselves cool,” said McQuillan.

The types of trees found in these forests have also changed, likely due to trends in fire suppression and changes in precipitation.  Drought-sensitive species like maples, historically found at lower elevations, are now more common throughout the landscape. These species drink more water than others.  

“If those are in the high elevation or upslope areas, that exacerbates what we’re seeing,” said McQuillan. 

The findings could mean that changes in the way forests use water would make water shortages more severe.

Given the large proportion of forests on the landscape distributed across high elevation, reductions in downslope water availability could be widespread – reducing runoff, impacting the health of downslope vegetation, and aquatic biodiversity.

The study is published in the journal Landscape Ecology.

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By Katherine Bucko, Earth.com Staff Writer