Cornell fenced deer out of 6 plots in New York and watched them for 18 years; every protected plot grew trees, while nearby open plots had almost none

Every night across the United States, from the Hudson Valley to the Blue Ridge Mountains, deer traipse through people’s backyards and eat their gardens. While this may seem to be a typical aspect of living in a suburban environment, the results of an 18-year study by a team at Cornell University show that the deer are having a large impact on the future of eastern forests.

The experiment nobody expected to last two decades

A team of ecologists led by Anurag Agrawal and Antonio DiTommaso set up a long-term experiment to assess how deer impact forests. They placed different plots of land into two groups. The first group was made up of six pieces of farmland around Ithaca, New York. These plots were enclosed by fences so that the deer couldn’t enter. The second group is also made up of six plots of farmland, however, with no fence, meaning the deer could enter. After 18 years, the results showed clearly that where there were deer, the amount of woody plants became half of what it was.

According to a 2025 study titled “Deer impact seedbanks and plant communities over 18 years of post-agricultural succession,” carried out by scientist Sophie Westbrook from Cornell University and published in the journal PLOS One, the difference between these two groups of plots became clear: deer decreased the number of woody plants by half, while only the fenced-in plots had a few trees left to be considered as actual trees after 16 years. Agrawal stated this more explicitly in Cornell’s own summary of the research results. Every single one of the six plots without deer has trees in it now, whereas those plots to which deer still have access have virtually no trees left. Eighteen years. Six plots. One fence. This is the sort of straightforward evidence that can’t be contested.

Not just about trees, it’s about who gets to grow

What makes this study interesting, however, is that the deer actually caused the plots to have a higher diversity of seeds by an average of 8%. This means that the deer are taking away the dominant plants so that the site becomes more diverse. Instead of the deer ruining nature, it shows a popularity contest between tree species. The deer are negatively impacting the number of trees. While they didn’t impact tree diversity and coverage, they did negatively impact the number of trees.

It is not just happening in New York

While it may seem like this study is purely about deer overpopulation in upstate New York, the impact of too many deer can be seen in most areas in the east of the United States, from Pennsylvania to the Carolinas. A 2016 U.S. Forest Service research paper titled “The legacy of deer overabundance: long-term delays in herbaceous understory recovery,” published in the Canadian Journal of Forest Research, shows that overpopulation of white-tailed deer has caused great impacts to a lot of forests in the Eastern United States.

They have created a very uniform landscape by reducing the understory of the forests. Recovery is also much faster than expected. Why are there so many more deer? Mostly because of humans! We have created huge areas of edge habitat for the deer to thrive in by creating vast, sprawling suburbs. We have also removed most of their natural predators, and hunting has decreased over the decades.

What does this mean for us?

This doesn’t mean we should be scared of Bambi; however, it is interesting to know that the next time you see a deer walking across your yard, it is actually playing a big part in deciding the future of entire forests. While the researchers at Cornell didn’t plan to create a villain of the deer, they allowed nature to take its course, and the forests are letting us know.