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Linking tree water use and soil moisture

people stand in the forest, next to a weir
Johnny Boggs (center) gives a tour of the study watershed. The study took place on the Hill Demonstration Forest in North Carolina. Photo by Martin Tsui, University of North Carolina Greensboro.

When more water is available, some tree species use much more of it. Loblolly pines growing near a stream used 65 percent more water than loblolly pines growing near the top of a hill, reports a new study led by USDA Forest Service researcher Johnny Boggs

White oaks near a stream only had 12 percent higher water use, as indicated by sap flux, than white oaks on the hill.

Boggs and his colleagues installed sap flux stations in three zones: a streamside area, halfway up a hill, and near the top of the hill. These zones have different amounts of water in the soil. These differences, along with forest composition, drive variations in water use at the tree and stand level.

Whenever a tree is photosynthesizing, it pulls water from the soil into the roots. This water moves up through the tree and eventually evaporates from its leaves. In this way, trees return large amounts of water to the atmosphere – an important component of evapotranspiration. Water loss through the leaves is the largest component of evapotranspiration.

Up to 75 percent of the rain and snow that falls in southern forests returns to the atmosphere through evapotranspiration.

Evapotranspiration is key to predicting how drought may affect forests – and the drinking water that forested watersheds provide to downstream reservoirs and to groundwater wells.

Results from the study will help scientists estimate evapotranspiration more accurately and understand the role of trees in watershed hydrology more thoroughly.

Read the study in Hydrological Processes. For more information, email Johnny Boggs at johnny.boggs@usda.gov.

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