Newly published research from the U.S. Forest Service shows water yields from unmanaged forested watersheds in the southern Appalachian Mountains declining by up to 22 percent a year since the 1970s. Changes in water yield were largely related to changes in climate, but disturbance-related shifts in forest species composition and structure over time also played a role. The study findings have implications for managing the forest composition of watersheds to ensure water supply under future climate change.
Scientists analyzing 76 years of data (1938 through 2013) collected from six unmanaged, reference watersheds at the SRS Coweeta Hydrologic Laboratory located in the southern Appalachian Mountains in North Carolina, to determine whether annual water yield from those watersheds has changed over time, and if so, to determine causes for significant changes. They tied measurements of climate and streamflow to data collected in long-term vegetation plots and measurements of water use by individual tree species.
From 1938 to the mid-1970s, annual water yield increased by as much as 55 percent, but that was followed by decreases of up to 22 percent by 2013. Vegetation surveys showed increases in forest basal area since the mid-1970s and a shift from oak and hickory species to poplar and maple, which can use up to four times as much water as oaks and hickories of the same size. Changes in forest structure and species composition alone decreased water yield by as much as 18 percent in a given year since the 1970s after accounting for climate.
The forests in the Coweeta Basin reflect this trend in the region. It experienced early 20th century logging, drought, hurricanes, and insect and disease outbreaks and the extirpation of the American chestnut. Additionally, the arrival of hemlock woolly adelgid in the early 2000s has meant, at Coweeta, the almost total loss of a foundational riparian species, and an increase in the dominance of maple and poplar in the overstory and rhododendron in the understory. For more visit www.srs.fs.usda.gov/pubs/51055