Severely over browsed forests loose their intermediate vegetation layers including shrubs, seedling and sapling trees, and forest floor plants including wild flowers, grasses, sedges, and other low-growing plants. In addition, the diversity of species declines in all forest layers.
Loss of seedling and sapling trees threatens the ability of forests to regenerate, trees that die or are cut are not replaced by new trees. Forest structure is reduced to a few species of canopy trees and a ground layer of plants that deer generally do not eat such as hay-scented fern, New York fern, and a few sedges and grasses. In some areas the ground is bare. Loss of understory, shrub, and forest floor plants reduces wildlife habitat.
A highly visible stage in over browsing is the creation of a browse line which results when deer feed non-selectively on everything they can reach. Development of a browse line, characterized by the lack of green leaves to a height of about 5 feet, is evidence that deer are exceeding the carrying capacity of the area. At lower numbers, deer feed selectively in the forest, seeking out the tastiest and most nutritious plants. While even this level of browsing can be a problem for some of the most highly preferred species, it does not alter forest structure.
Deer population levels, when the earliest European settlers arrived, have been estimated at 9-11 deer per forested square mile. Today levels of 30-80 deer per forested square mile are not unusual and in some urban/suburban sites they are even higher.
Simplification of forest structure and loss of biological diversity resulting from deer overabundance threatens the health and resilience of forest ecosystems. It also diminishes the ability of our forests to carry out important ecosystem services such as air and water purification, erosion control, soil building, mineral recycling, and preservation of biological diversity.
Highly vulnerable plant species are in danger of becoming so rare that they must be classified as endangered or threatened. At least one plant, bearberry (Arctostaphylos uva-ursi) has been lost from the state due to excessive deer browsing. Examples of native plants of Pennsylvania, many of them already classified as plants of special concern, that are being severely impacted by deer include: