New Forest, New Water Yield

Water Yield after Forests become Fields, then Forests Again

weir
The study watersheds are outfitted with weirs, which allow scientists to precisely measure the amount of streamflow. Photo by Pete Caldwell, USFS.

Today, forests abound in the southern Appalachians. However, there was a time in the early 1900s when many forests were harvested or cleared so that the land could be used to grow crops or provide pasture. “The forests that have returned may use water differently,” says U.S. Forest Service research ecologist Katherine Elliott.

Elliott and her colleagues examined long-term data at three watersheds in the Forest Service Southern Research Station’s Coweeta Hydrologic Laboratory. Their results were published in Hydrology and Earth System Sciences.

Coweeta is located in the mountains of western North Carolina, and was founded in 1934, making it one of the oldest sites for continuous environmental research in North America. Because of the meticulous long-term records kept at Coweeta, Elliott and her colleagues had access to 80 years of data for streamflow and plant growth on three Coweeta watersheds.

The scientists paired two undisturbed watersheds with a similar watershed that had a long and well-documented history of disturbance. The disturbances closely mirrored the changes that occur when a forest is cleared, turned into a field, and then abandoned, eventually reverting to forest.

The disturbed site was first cut in 1941, and after additional logging, grass was planted in 1959. In 1967, an herbicide was used to kill the grass, and the watershed then began returning to forest. Elliott and her colleagues analyzed changes to plant communities, their water use, and stream water yield between 1934 and 2015. The scientists focused on the period of reforestation.

As the forest regrew, the tree species found there changed dramatically. Before the watershed was cut in 1941, it was dominated by oak and hickory. When the grass died and the new forest returned, tulip poplar and red maple became the dominant trees.

Tulip poplar
When tulip poplar dominates a stand, the water yield from the watershed is affected. Photo by Lindley Ashline, via Flickr.

Different tree species have unique water needs. Some require more water than others, which could explain long-term changes in stream water yield.

Grasses have different water needs than trees, and Elliott and her colleagues found that after the watershed was cut and became a field, the overall water yield increased significantly. When the grass died and the forest began to re-establish, the amount of water yield began to decline. Within nine years, water yield was about the same as before the trees were cut.

However, as the tulip poplar, red maple, and black locust trees in the disturbed watershed matured, they began using more water. “Tulip poplar and black locust are known to use significantly more water than oaks and hickories,” says Elliott. Ten years after the grass died, the disturbed watershed began yielding less water. Meanwhile, evapotranspiration – the water used by trees plus the water that evaporates from the leaf and soil surfaces – increased.

“The shift in tree species largely explains the trend of decreasing water yield,” says Elliott. “The changes appear to be relatively permanent – even after 45 years, the amount of water lost to evapotranspiration in the disturbed watershed is much greater than the undisturbed watersheds.”

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.

Read the full text of the article.

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

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