The planet is changing, and the hydrologic cycle will change along with it. Extreme droughts – as well as extremely wet weather – are expected to become more frequent and more intense. “These changes may interact with topography to affect species composition in unexpected ways,” says Chelcy Miniat.
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Miniat is a researcher and project leader at the U.S. Forest Service Coweeta Hydrologic Laboratory, in western North Carolina. She recently coauthored a study on the interactions between topography, drought, and tree growth.
The study was led by Sandra Hawthorne, who at the time was a postdoctoral associate at Coweeta. Hawthorne is from Australia, and long before she became a researcher at Coweeta, she stopped by as a tourist. “My husband and I were visiting the U.S.,” says Hawthorne. “I wanted to visit famous long-term experimental watersheds, and Coweeta was definitely on the list.”
Coweeta is one of the longest-running gauged watershed experiments in the world. It has been a living laboratory since 1933, and data of all kinds are constantly being collected.
Hawthorne visited Coweeta a number of times. “Every time she left, I would ask when she was coming back,” says Miniat. “She always said, ‘maybe in a few years,’ and every few years, she would visit again.” Eventually, Miniat collaborated with Forest Service International Programs to bring Hawthorne back to Coweeta as a postdoctoral research scientist in 2016.
Hawthorne and Miniat began collaborating, and recently published their study in the journal Ecohydrology.
There were two study sites, both in a single forested watershed in the Coweeta Basin. Both sites are steep – about a 50 percent slope, which is far steeper than any current roads in the U.S. Species like oak and hickory dominated the uphill site, and tulip poplar and other mesic species dominated the downhill cove site.
“We wanted to see how topography affected the amount of water available to trees in drought years and wetter years, as well as the amount of water they use,” says Miniat.
In the mountains, water runs downhill through the soil. This causes downhill areas to have wetter soils than uphill areas. Hawthorne and Miniat compared soil moisture and transpiration – the water plants lose from their leaves – at the two sites.
The scientists also evaluated the vegetation in the two plots and found that trees in the cove plot were 17 percent taller than those growing in uphill plots. Trees in cove plots also had 45 percent more sapwood, which is the part of the trunk that moves water and nutrients from the roots to the leaves.
The scientists also compared two years – a wet year (2004) and a dry year (2006). Because of the topography and the tree species present, the two sites responded differently to climate. In dry years, water stress was worse in the uphill site.
The soil in the cove forest stayed wetter, and helped buffer the effects of drought. When there was plenty of rainfall, trees on the uphill plot used as much water as trees in the cove plot. However, during dry years, trees on the uphill plot transpired about half as much water as their downhill neighbors, suggesting that the terrain worsened drought’s effects.
“However, not all trees in the cove forests benefited equally from the extra water,” says Miniat. When drought causes the humidity to drop, some trees conserve water by closing their stomata. This means they cannot exchange gases or synthesize carbohydrates.
Not all species conserve water in this way, but those that do – such as tulip poplar – could decline if droughts become more frequent. The interaction between drought, topography, and tree species’ response to drought could lead to complex shifts in species composition.
For more information, email Chelcy Miniat at firstname.lastname@example.org.