18-year study: deer inhibit trees, raise plant diversity

According to separate Cornell research, there are 1.7 million acres of abandoned agricultural land in New York, which could grow back into forest. State leaders have sought to incentivize reforestation as part of New York’s efforts to combat climate change. However, at very high densities, deer will likely prevent reforestation without active management to protect saplings, DiTommaso said. In the Ithaca area where these experiments were conducted, deer abundance has been estimated at 39 deer per square kilometer, roughly 10 times as abundant as before European colonization. 

While the researchers found that deer inhibit forest growth, they also found that the presence of deer increases the overall diversity of smaller plant species. The measure of diversity used by the researchers takes into account both the total number of different species found and their relative abundance – a habitat where several plants are present but one species dominates is considered less diverse than one where different plants are more equally represented, Agrawal said. They suspect deer improve this diversity by eating a lot of the plant species that would otherwise become dominant, he said. 

“Without predators like deer, there are often one or two plant species that dominate a system. And those also tend to be the plants that are most edible,” Agrawal said. 

Agrawal and DiTommaso emphasized that they are not in this paper advocating for any particular forest or deer management strategies, but providing information that private land owners or public forest managers can use to make decisions.

First author of the new paper is Sophie Westbrook, Ph.D. ‘24, formerly a graduate student in DiTommaso’s lab and now a research assistant professor at Kansas State University. Additional co-authors are: Aleah Butler-Jones, a graduate student at Cornell AgriTech; Scott Morris, a technician in DiTommaso’s lab; and Max Goldman ’24, a former undergraduate researcher in Agrawal’s lab. 

Initial research funding support to launch the project 20 years ago came from federal capacity funds to land-grant universities, which Cornell AES manages on behalf of New York State. The research plots are on Cornell AES-managed lands in Dryden, NY. 

“It would have been very difficult to do a long-term, decades-long project like this with private land, because ownership changes hands, or people may choose to do something different with their property,” Agrawal said. “Having these university land resources available has been crucial.”
 

Krisy Gashler is a freelance writer for the Cornell University Agricultural Experiment Station.