Shifting Rainfall Patterns May Change Southern Appalachian Forest Structure

Small summer storms make a big difference

Cove hardwood forest at Coweeta in western North Carolina. Photo by Paul Bolstad, courtesy of Bugwood.org.
Cove hardwood forest at Coweeta in western North Carolina. Photo by Paul Bolstad, courtesy of Bugwood.org.

A new research study by U.S. Forest Service scientists finds that changes in rainfall patterns in the southern Appalachians due to climate change could reduce growth in six hardwood tree species common to the region. The findings have implications for forest managers in the Southeast, where climate variability (more extreme events or changes in precipitation distribution) could cause major shifts in forest composition and structure.

The study, recently published online in the journal Global Change Biology, evaluated climate-driven patterns of growth for six dominant hardwood tree species in the southern Appalachians in relation to their topographic positions on slopes or in coves.

“We separated the species we studied into two functional groups based on how they regulate water,” says Katherine Elliott, lead author of the article and research ecologist at the Forest Service Southern Research Station Coweeta Hydrologic Laboratory. “Most of the hardwoods of the region fall into one of these groups, which we found to have different responses to rainfall patterns and drought in relation to their positions on dry or wet sites.”

One group of trees – maple, poplar, and birch – are diffuse-porous, meaning they form many vessels of similar size throughout the xylem (the water conducting tissue of the tree), and tightly regulate water loss during the day by closing their stomata, the tiny holes in leaves that release water into the air. The three species of oaks in the second group form large vessels in a distinct ring in the xylem (ring-porous), and control water loss through stomata less tightly.

The researchers sampled 465 trees to evaluate tree ring growth in relation to 70 years of on-site climatic data. They found that diffuse-porous species (maple, poplar, birch) growing on dry sites were the most sensitive to climate, while the ring-porous species (oak) were the least sensitive.

More importantly, they found that growth in all the species studied, regardless of whether they were positioned on dry or wet sites, was more sensitive to how precipitation was distributed rather than to the total amount of precipitation. Very small storms in the growing season were surprisingly important in maintaining forest growth.

“We found that years with low numbers of small storms and long periods between rainfall events resulted in significant reductions in tree growth, with potential declines in forest productivity of 29 percent on upslope sites and 25 percent on cove sites,” says Elliott.

“These are simplified estimates,” she adds. “It’s not likely that the only future change in rainfall patterns will be the number of small storms or lengthening the period between rainfall events, but these findings do highlight the importance of small storms and precipitation distribution in the southern Appalachian region.”

Read the full text of the article.

For more information, email Katherine Elliot at kelliott@fs.fed.us.

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