MADISON, Wis. — New research from the University of Wisconsin-Madison suggests an overpopulation of deer is having a long-term impact on the state’s forests.

Biologists have long known that an overabundance of deer negatively affects the number and diversity of forest plants, Wisconsin Public Radio reported (http://bit.ly/2kPXAnQ ). But a new study headed by Autumn Sabo, a PhD candidate at the university, suggests it’s also changing the soil beneath the forest floor.

Sabo took samples from test plots that have been fenced off from deer for up to two decades and found less soil compaction and a thinner layer of depleted soil, which is called a leach zone.

“So, the thickness of this low-nutrient layer appears to influence the abundance of forest plants including species like lilies and violets,” Sabo said.

Sabo said she suspects the thinner leach zones are caused when deer eat hardwood tree saplings, but that more study is needed to find the exact correlation.

She said it’s estimated there were around eight deer per square mile in Wisconsin before Europeans settled in the U.S., and that there are twice as many in the area near the test plots she studied.

There are now as many as 72 deer per square mile in some counties, Sabo said. She said her findings show the impact on forests from an overpopulation of deer exceed what the animals eat.

“So, these changes in growing conditions might serve as a legacy of deer overabundance and that could influence how well the forest recovers even if we are able to reduce deer numbers,” Sabo said.

Sabo’s research began in 2011 and focused on test plots in the Ottawa National Forest in Michigan’s Upper Peninsula and the Peninsula State Park in Door County.