Loss of Diversity
What factors lead to a loss in diversity?
There are several factors that can lead to the loss of diversity in a forest. Certain forestry practices, such as high-grading, selectively remove the largest and most valuable trees, generally diminishing species diversity. Herbicides that have spread to forested land can also lead to a loss of diversity, as herbicides have a different impact of each tree species, so only the most herbicide-tolerant species remain. Also, U.S. Forest researchers in northwestern Pennsylvania showed that the overpopulation of white-tailed deer have altered the composition of forests and reduced the size of trees that do grow.
The Forest Service study looked at tree growth and species in four 160-acre areas in northwestern Pennsylvania forests for 10 years. Each of the tracts was divided into quadrants where researchers controlled the deer population with fences. Researchers also thinned 30 percent and clear cut 10 percent of each area -- the ratio typical for managed forests in that area.
The study also found that when the deer population reached 64 per square mile, double the state's average density of 35 deer per square mile, the forests became dominated by black cherry, which deer apparently did not find as appetizing and which originally made up 19 to 31 percent of the forest. However, after a decade, black cherry trees and seedlings accounted for more than half of the trees. However, when the deer population was kept under 20 per square mile, the amount of black cherry held steady, regardless of whether the forest was cut.
The study showed that deer can change the makeup of forests with their eating preferences. They prefer certain saplings -- such as sugar maples, pin cherry, black and yellow birch -- leaving other species largely untouched. In eastern Pennsylvania, high deer populations have also shifted forests from oaks to black birch and red maple, Findlay said. Also, in open areas, deer grazing can sometimes allow ferns to spread, stealing necessary sunlight from some trees. The one strategy that most biologists feel confident can slow down the loss of diversity is to set aside large areas of natural forest and grassland that are no longer managed intensively for logging. These guarded wildlands can function as arks or safe havens for sustaining the whole complex of species that inhabit a region.