The Potomac Highlands Watershed School
Oh Deer! Environmental Forum
Deer as a Watershed Problem
Executive Director, Cacapon Institute
Watershed: the area of land that drains to a river, lake, estuary, or ocean.
Cacapon Institute is a watershed-based science and education organization. We study how land use impacts water quality and quantity, seek innovative solutions to watershed problems, and offer related education programs. So, why are we running an environmental forum on deer overpopulation?
Because over abundant deer can make it difficult to improve water quality and, we believe, may cause water quantity problems as well.
The Chesapeake Bay Program is planting 10,000 miles of forested riparian (streamside) buffers in the Chesapeake Bay watershed to protect the health of streams and downstream waters like the Chesapeake Bay from pollution. These forested buffers capture pollutants before they can reach streams, and it is important that these forested buffer plantings actually grow into riverside forests. But, as we noted on the main page, excessive deer browsing can cause riparian plantings to fail. And that makes deer a watershed problem.
But over abundant deer are a much bigger watershed problem than that. Forests are the most beneficial land use for maintaining and ensuring clean water. Healthy forests store, clean and release most of the water that recharges groundwater levels and maintains stream flow. The health of a watershed depends on the amount of intact riparian forest, the overall amount of tree canopy coverage, and the health of the forested lands.
What do I mean by “healthy forest?”
Forests can be divided into three layers: the top layer (or canopy), the understory, and the forest floor. The canopy intercepts and slows rain, and provides habitat, protection, and shade for the animals and plants that live in the forest. The understory is the layer of smaller trees and shrubs below the canopy. In a healthy forest, the understory contains a diversity of tree species similar to those found in the canopy, of various size and ages, that enable the ecosystem to bounce back from disturbances and provide a variety of habitats. The young trees that grow here may eventually replace trees in the canopy when they die. The healthy forest floor is covered by a rich layer of decaying leaves and wood, populated by plants (such as grasses, herbs, mosses, vines, tree seedlings) and animals (microorganisms, worms, insects, bacteria, mammals). The litter on the forest floor is a storehouse of nutrients and organic material that also enriches and protects the underlying soil from erosion. For a forest to be healthy, all three of these layers must be intact and functioning.
When white tail deer are too abundant, both the understory and the forest floor layers become degraded, sometimes severely. The understory becomes thin, with few or no new seedlings given a chance to grow, and few leaves left below the deer browse height.
As WV Consulting Forester David Warner said in his Native Guide essay, the forest floor changes as well: "The dense vegetation layer, presence of tree seedlings, forbs, shrubs, and wildflowers, even the accumulation of fallen leaves that forms much of the litter layer on the forest floor, has largely disappeared (in this area) over the past 25 years." CI suspects that the cause of diminishing leaf litter is excessive browsing by deer that reduces woody stem density on the forest floor, which results in less of the “roughness” that holds leaves in place -- and allows leaves to blow away or wash off the hillsides.
Whatever the cause, the role that a thick bed of decaying leaves and good forest soil structure play in slowing runoff, storing moisture, and cycling nutrients may be diminished as a result. If true, this would lead to drier hillsides, more erosive runoff, and less infiltration which, in turn, would lead to more erosion, more sedimentation, less groundwater recharge, drought stressed forests, and flashier streams that dry up easily.
Thanks to a National Fish and Wildlife Foundation grant, CI has begun a long-term study to determine if exclusion of deer from sections of forest leads, over time, to an increase in leaf litter retention, restoration of a healthy forest soil structure, and an increase in retained moisture. The same project is assessing the effectiveness of a new electric fence design in protecting riparian forest buffer plantings and upland forests. This project will measure the relative success of fenced and unfenced sites in riparian and upland forest settings, and identify strategic approaches to restoration plantings in areas of high deer density – as in much of the Chesapeake Bay watershed.
You can read much more about the importance of healthy forests in the USDA Forest Service's The State of Chesapeake Forests here.