Gifford Garden Notes
Impacts to forest ecosystems attributed to the current overabundance of white-tailed deer.
by Brad Roeller
Lets take a closer look at the impacts to forest ecosystems attributed to the current overabundance of white-tailed deer.
As mentioned to last week, biodiversity (or species richness) suffers from deer herbivory. When deer densities exceed 10 deer per square mile (remember that the average density here in the Hudson Valley is around 45 per square mile), species richness and abundance of plants in the herb/shrub layer shifts significantly. Studies have shown that when deer densities exceed 20 per sq. mi. two shrub species and one herbaceous species of plants disappear.
An eye-opening study, confirming deer impact on biodiversity, was done by forest ecologist, Gordon Whitney, published in 1984, and conducted in the Allegheny National Forest. Whitney duplicated a previous sampling study, done in 1929 by Lutz, to see if the species composition and distribution in 2 major tree associations had changed in 50 years. Whitney was able to locate Lutz's study plots and duplicated his sampling procedures. In 1929 Lutz found a rich mixture of herbaceous and woody plants on the forest floor. Remember, deer densities in this period were much lower than current levels. In 1929 Lutz found there were trees of 17 species in the "small sapling" height class. In 1979, Whitney found only 5 species in the association he was sampling (mountain holly, black and yellow birch, beech, and hemlock). Many important timber tree saplings were simply not present 50 years after the original survey. While Lutz had found at least 5,300 hemlocks per acre in this size class (small sapling) in 1929, Whitney found only 20! Many foresters are worried about the fate of hemlocks in our northeastern forests as this species can spend several years in the shrub layer and therefore be subjected to repeated browsing. Two other researchers, Rooney and Dress, resampled the ground layer vegetation in Lutz's 1929 study to document the extent of change in the herb and shrub community. They found that 33 species were missing from the beech-hemlock association and 16 species from the hemlock cosociation.
Not only were many tree and shrub species eliminated from northeastern forests due solely to deer browsing, but also many wildflower populations have steadily declined because of high deer densities. Species such as Britton Appalachian Jacob's ladder, small whorled pogonia, clintonia, northern monkshood, and broad-leaved twayblade are rare or threatened in our local forests due to deer herbivory.
Wildlife species, especially bird populations, are adversely affected by the current overabundance of white-tailed deer. Deer browsing significantly affects the vegetation that songbirds depend upon for foraging for food, for cover for escape and hiding from predators, and for nesting. Deer herbivory reduces the height of the woody vegetation in the 0-7.6 meter interval, thus impacting critical habitat that certain songbirds utilize for survival. Birds, which target this intermediate foliage layer, exhibited significant reductions in species richness and abundance when deer densities exceed 20 per square mile.
Deer also have a devastating impact on forest tree seedlings. As forests created by the turn of the century cutting in the northeast reached timber harvest size in the late 1960's, foresters had to alter prior management practices due forest seedling surveys, which indicated a poor regeneration of desirable timber tree species. Early surveys in the 1970's indicated regeneration failures occurring in up to two-thirds of the trials conducted in the northeast. A survey done in the early 1990's in Pennsylvania indicated that only 4%-20% of forested lands across Pennsylvania had advance regeneration sufficient to provide adequate renewal of diverse woody species.
It is now well documented that once white-tailed deer density exceeds that of pre-European settlement, sustainability of woody regeneration, songbirds and songbird habitat, and herbs and shrubs are adversely affected. In a nutshell: species richness and abundance of all these forest resources decline.
Next week: Can anything be done?
Gifford Garden Archives, January 2003 - present
Gifford Garden Archives, July 2001 - January 2003