Trees take up large quantities of water through their roots, but 99 percent or more of that water moves through the roots, trunks, and branches, ultimately evaporating from small openings called stomata, which are usually located on leaves. There are many ways to measure the movement of this water, although most methods measure the amount of water an entire stand of forest trees uses, rather than the amount used by individual tree species.
“We wanted to see how water use in five common tree species in the Piedmont of North Carolina – loblolly pine, oak, red maple, sweetgum, and tulip poplar – changed after a clearcut,” says U.S. Forest Service Southern Research Station (SRS) scientist Johnny Boggs.
In 2010, Boggs and his colleagues attached tiny probes called sap flow sensors to tree trunks. The sensors measured the velocity and density of sap (which is mostly composed of water) as it moved up the trunk of the tree. The trees were in the riparian buffer of two 30-acre watersheds on North Carolina State University’s Hill Demonstration Forest, and one of the sites had been clearcut except for a 50-foot riparian buffer along the stream edge. About a third of the trees in this buffer area were harvested.
“We found that after a clearcut, trees left behind in the riparian buffer use 43 percent more water than before,” says Boggs, a biological scientist at the SRS Eastern Forest Environmental Threat Assessment Center. Boggs led the study, which was recently published in the journal Hydrological Processes.
Few studies have measured the amount of water that riparian buffer trees use after a harvest, and until this study, no one knew whether changes in water use could affect the amount of water in streams. Usually, the amount of water flowing through streams increases after a harvest. Boggs and his colleagues found that after harvest, stream discharge increased by 150 percent during the growing season. The increase would have been slightly higher if remaining trees had not used more water after harvest.
Of the 5 species, tulip poplar and red maple used the most water, probably because of anatomical differences. The study suggests that these species, or others with similar anatomy, could be left in riparian buffers to help regulate stream discharge after clearcuts. Riparian buffers are already an important best management practice that protect water quality, but as this study suggests, trees in these buffer zones could also reduce total flow amounts to partly protect downstream areas from changes in soil water and fluctuations in nutrient concentrations after harvest.
For more information, email Johnny Boggs at firstname.lastname@example.org.