The science used for managing deer and habitat on the Kinzua Quality Area was conducted by the USDA Forest Service Research Laboratory in Irvine, PA.  Beginning in 1988, forestry and wildlife scientists from this Forest Service research laboratory conducted a 10-year study of the impact of white-tailed deer impact on forest resources, evaluating the impacts of deer at densities of 10, 20, 40, and 64 deer per square mile.  Results of this research laboratory have been corroborated by scientists in other research locations in New York, West Virginia, Virginia, and Massachusetts.  The research represents the impact of deer on forest resources in areas completely forested with no surrounding or included agricultural lands (which would provide additional forage for deer).  The findings were condensed into an illustration (Fig. 1 below) that describes how the impact of deer on forest resources increases as deer density increases.

Figure 1.  Relationship between deer density and impact on forest resources.

    Deer begin to impact forest resources when their density exceeds 10 deer per square mile.  At this point abundance of songbird species that nest in habitats affected by deer (the shrub layer) declines, and wildflowers and shrubs preferred by deer begin to disappear.  When deer density exceeds 20 per square mile, abundance of seedlings preferred by deer (such as red maple, hemlock, sugar maple, and white ash) decline in abundance or disappear and plants not preferred by deer such as ferns and grasses begin to take over the understory, choking out seedlings and shrubs.  At densities exceeding 40 deer per square mile there is a distinct browse line, only tree species resistant to deer browsing (beech, striped maple) are left, and shrubs and wildflowers are basically gone.  When density exceeds 60 deer per square mile even resistant seedlings are heavily impacted and the forest understory is basically bare.  At this density deer starve to death.

    It is important to understand the relationship between deer density and fawn production, because the increase in the herd each year represented by fawns equals the number of deer hunters can harvest and keep the herd where it is, abundance-wise.  For example, if a herd of 1,000 deer produces 400 fawns each year, 400 adult deer can be harvested each year to utilize this annual increase.  Figure 1 indicates that the greatest number of fawns is produced when herd density is about 30 deer per square mile.  This point is called the optimum sustained yield because it is the deer density at which the greatest number of fawns is produced, allowing hunters to harvest the greatest number of deer.  Once deer density exceeds 30 deer per square mile, the number of fawns produced is actually less than when the herd is at 30 deer per square mile because there is not enough food for does to produce optimal numbers of fawns, and because there is not enough food to insure fawn survival over winter.

    For many decades, deer density exceeded 30 deer per square mile in many parts of the Kinzua Quality Area, and the understory was dominated by ferns and grasses with little seedling, shrub, or wildflower growth.  Deer were small, and antlers were small.  Foresters could not harvest trees without fencing harvest sites after logging because deer had eliminated most tree seedlings necessary to reforest harvested areas.  However, deer were plentiful, and hunters would see many deer (mostly does) on opening day, and during the rest of the season.  Hunters were able to sustain a fairly high harvest rate, because number of fawns produced each year was optimal, and it was this increase in herd number that hunters were able to harvest as deer older than fawns.

    Forest habitat and deer are healthiest when deer density is in the 10-15 deer per square mile range, and that is the goal density for the Kinzua Quality Area.  However, at deer densities in the 10-15 deer per square mile range, far fewer fawns are produced, meaning that the sustainable harvest of deer is approximately half what it was when deer density was 30 deer per square mile.  For the 113 square miles of the Kinzua Quality Area, hunters were able to harvest sustainably about 1,500 deer annually, and there were about 4,800 deer in the area during hunting season.  Now, at 12-14 deer per square mile, there are about 2,300 deer during hunting season and the sustainable harvest is about 750 deer.