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How Can Deer Change the Forest?
White-tailed deer, through browsing, have the ability to change plant communities where they live. The changes they can impose on the forest occur over time and to the untrained-eye are subtle; however, to those trained in forest ecology and other fields, the changes can approach catastrophic.
Deer, as people, have food that they prefer to eat. Deer food preferences raise concerns about forest health, resilience, and sustainability. Deer are a natural part of the ecosystem and no one wants to eliminate them from the forest. Rather, forest managers want to ensure that deer numbers are in balance with their habitat so that they do not adversely affect natural plant community development.
Forests are dynamic – they continuously change. This change is natural and progresses through various stages. As a forest grows and develops naturally, plants compete for light, nutrients, water, and space. As they compete, individual plants and plant species have different strategies for gaining the advantage so that they can win the competition for resources.
Browsing can shift the competitive advantage among plant species and where deer numbers are or have been extremely high, the shift can be ecologically devastating as non-preferred browse species move to dominate the site. In some cases, those species that dominate the site (e.g., ferns, goldenrod, mountain laurel, striped maple) become stable plant communities that resist successional change, tending to perpetuate themselves and exclude other plant species.
In the hardwood forests of the northeast and north-central regions of the United States, the extent of fern cover is increasing. Single frond fern species (e.g., New York, hayscented, and bracken), which spread easily by extending their rhizomes to occupy new territory increasingly dominate forest understories. Deer do not eat any of these fern species.
As deer browse on some plants and ignore ferns, the lack of competition favors fern growth and expansion. Once ferns dominate the understory they capture light resources that other plant species may have used, and small mammal populations increase under the protection of the lush green fronds. The combination of less light and seed-eating rodents helps to ensure that deer preferred plants become increasingly rare.
When deer populations are in balance with their habitat, their selective browsing does not shift plant species composition to food species that deer do not eat. However, once the imbalance does occur, and preferred species become less common, relatively few deer can ensure dominance by species they do not eat. When preferred browse is abundant, and deer numbers are in balance, the community functions normally. When preferred browse is scarce, and deer numbers are too high, then the plant community-balance shifts rapidly and, perhaps, permanently.