A stream meanders through a forest of hemlocks and mixed hardwoods. Credit: Nicholas A. Tonelli
Because of their dominance in riparian areas and constant year-round transpiration — the loss of water from needles or leaves — hemlocks have played a huge role in forests’ hydrologic cycle. As the hemlocks decline, they are largely being replaced by deciduous trees like sweet birch and red maple, which do not transpire year-round. The only evergreen which has been a contender to replace the hemlock has been the woody shrub rosebay rhododendron, which has a lower total leaf area and thus lower transpiration rate than hemlock. The researchers, led by Steven Brantley, found that as a result of the new species composition, annual transpiration rates fell by 22 percent from 2004 to 2011, while winter transpiration rates fell by 74 percent.
As Brantley says in the U.S. Forest Service publication CompassLive, “In the growing season, transpiration rates will likely rise, leading to lower streamflow in the summer. However, transpiration rates in the winter will be reduced, which could cause increased winter stream discharge.”
Not that streamflow is the only thing that would be affected by hemlock loss along riparian areas. The loss of shade is also causing increased water temperatures, threatening eastern brook trout and other species that live in the cold water. That was a problem in Fridley Gap in George Washington National Forest, Va., for example, where American Forests planted 240 trees in riparian areas to restore cooler water temperatures for the benefit of brook trout.
It remains to be seen what these forests will look like in the future, but by researching what to expect, we can hope to head off some of the negative effects to ecosystems.