Ever-nibbling deer constantly changing landscape

Terry Devitt
UW Madison
Forest understory differences inside and outside of a fence designed to exclude deer at UW–Madison’s Kemp Research Station in Vilas County. The forest on the left is outside the exclosure and shows the dramatic effects of deer on forest understory.

MADISON - It is widely known that the white-tailed deer is a nonstop eater. Unless it is sleeping or fleeing from a predator, the keystone North American herbivore is nearly always nibbling.

Ecologically, deer herbivory is a fairly well understood phenomenon. The presence, abundance and reproductive success of many plant species are directly affected by deer, whose populations are orders of magnitude greater in some regions than they were before European settlement.

Now, scientists are looking beyond herbivory to better understand the indirect effects of deer on eastern North American forest landscapes. In particular, scientists are interested in how the animal's presence and behaviors affect the composition and overall health of the wild flowers and other herbs - what scientists call understory communities - that blanket the forest floor.

"Deer are affecting understory communities in many different ways," explains Autumn Sabo, a University of Wisconsin-Madison plant ecologist and the lead author of a new study that teases out some of the secondary impacts of white-tailed deer on forest ecosystems. "It is only in recent years that scientists have started to look at factors beyond herbivory."

A deer eating leaves from a snow covered tree may look cute, but it can alter facets of the forest environment, including light availability, soil compaction, and the thickness of a particular layer of soil.

Writing this week (Feb. 6, 2017) in the Journal of Ecology, Sabo and her colleagues detail how deer affect forest plant composition by altering facets of the forest environment, including light availability, soil compaction, and the thickness of a particular layer of soil.

The study focused on 17 deer exclosures, patches of forest ranging in size from a hundred square meters to eight hectares, where 2- to 3-meter high fences have been installed to keep deer out. The exclosures, some of which have been in place for decades, are located in the temperate hardwood forests of northern Wisconsin and Michigan's Upper Peninsula.

Archaeological evidence suggests that deer were once far less abundant in eastern North America, perhaps as few as two to four deer per square kilometer. Today, on average, there are about seven deer per square kilometer in the areas studied by Sabo and her colleagues.

"In northern Wisconsin and the Upper Peninsula of Michigan, we found little herbivory damage on forest herbs," says Sabo, who conducted the study with Katie Frerker of the U.S. Forest Service, and Don Waller and Eric Kruger of UW-Madison. "Deer seem to be eating primarily young trees."