Deer are herbivores, they eat plants.
Too many deer means too much browse pressure on trees, shrubs, and herbaceous species. Over browsing has been occurring since the 1930's in northern Pennsylvania, in other parts of the state, it is a somewhat more recent development. 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.