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Goal: Sustain Our Nation’s Forests and Grasslands Water Yield Following Forest-Grass-Forest Transitions

Weir measuring streamflow

Since the 1930s, 32 weirs, or stream gauging stations, have been installed at Coweeta Hydrologic Laboratory. Today, there are 16 weirs in operation. Photo by USDA Forest Service.


Forested watersheds are important sources of drinking water. However, species identity affects water yield from deciduous forests. Through old-field succession, changes in species composition over time determine how much precipitation leaves the watershed as evapotranspiration versus water yield.


Forest Service scientists examined 80 years of streamflow and vegetation data in an experimental watershed that underwent forest to grass to forest conversion (i.e., old-field succession treatment). We hypothesized that changes in forest species composition and water use would largely explain long-term changes in water yield. Aboveground biomass was comparable among watersheds before the treatment (208.3 Mg ha-1) and again after 45 years of forest regeneration (217.9 Mg ha-1). However, management practices in the treatment watershed altered the resulting species composition compared to the reference watershed. Evapotranspiration and water yield in the treatment watershed recovered to pretreatment levels after nine years of abandonment, then water yield became less (averaging 5.4% less) than expected. In addition, evapotranspiration increased more (averaging 4.5% more) than expected. The changes became apparent after the tenth year and up to the present day.

We demonstrate that the decline in water yield and corresponding increase in evapotranspiration could be explained by the shift in major forest species. Before treatment, the site was dominated by oak (Quercus) and hickory (Carya). After treatment and old-field succession, poplar (Liriodendron) and maple (Acer) dominated. Furthermore, the annual change in water yield can be attributed to changes in seasonal water yield. The greatest treatment effect on monthly water yield occurred during the wettest growing-season months, when water yield was significantly lower than expected. In the dormant season, monthly water yield was higher than expected during the wettest months.

Even small shifts in species composition can affect water yield, and the effects may be especially noticeable in dry years. Droughts are expected to become more frequent and severe, so understanding how forests, tree species, and climate change interact to affect water resources will become increasingly important.