Deer change forests from the ground up

"It’s obvious that the deer are affecting the above-ground species, but it’s like an iceberg," says Antonio DiTommaso. "There are major effects below the soil surface. We are seeing a divergence of seeds contained within the soil from what should be there." (Credit: OnceAndFutureLaura/Flickr)

A growing deer population changes the progression of a forest’s natural future by creating environmental havoc in the soil and disrupting its natural seed banks.

“Deer are slowing down forest succession or natural establishment. In fact, the deer are preventing forests from establishing,” says Anurag Agrawal, professor of ecology and evolutionary biology at Cornell University.

Deer typically prefer to eat native, woody plants and rebuff invasive species. A new study shows that when deer consume native plants, the non-native species are left to flourish, dropping seed in the soil.

[related]

As forests normally mature, their grasses give way to herbs and shrubs, and then new trees eventually take root. Expanding deer populations in the Northeast, however, stall forest development, says Agrawal, and promote the growth of thorny thickets of buckthorn, viburnum, and multiflora rose bushes.

If deer leave the forests alone, such trees as cottonwood, locust, and sumac can sprout and grow unimpeded.

The impact of deer grazing on vegetation is severe and results in bare soil and reduced plant biomass, less recruitment of woody species and relatively fewer native species. Deer also decrease overall species richness and relatively more short-lived species of both annual and biennial plants.

For the study, published in PLOS ONE, researchers gathered soil cores—from both within and outside of fenced “deer exclosures”—and germinated the seed. They found the soil cores from outside of the exclosures contained many more seeds from non-native species.

Like an iceberg

Deer select forests for their trees but in doing so disrupt forest system growth trajectories, researchers say.

“It’s obvious that the deer are affecting the above-ground species, but it’s like an iceberg. There are major effects below the soil surface,” says co-author Antonio DiTommaso, associate professor of weed ecology and management.

“We are seeing a divergence of seeds contained within the soil from what should be there. We are not seeing the seeds of woody plants. Instead, we’re seeing an escalation of non-native seed and the virtual elimination of woody plant seeds.”

The multiyear study was conducted on Cornell land in Ithaca, New York where the deer density is about 39 animals per square kilometer—about 10 times greater than it was before European settlement in the late 1700s.

USDA Federal Formula Funds and the National Science Foundation supported this research.

Source: Cornell University