23 August 2023

The white-tailed deer population in the City of Pittsburgh has been so high for so long that most people think the browse line in our parks is normal, but the light-gap you see under the trees above is not normal in a balanced forest. It’s a sign of deer overpopulation. Here’s what a browse line is and how deer maintain it.

Browse line: A phenomenon that occurs when herbivores consume all of the vegetation in the woods between the ground and the level of their highest reach. A clearly visible line is formed between the leafed and the leafless areas.

— paraphrased from Blue Jay Barrens: Fallen Trees and Browse lines, May 2011

Is the browse line hard to recognize in the photo above? Here’s an extreme example.

This eye-level view of the Grande Allée of horse chestnut trees at the Tuileries in Paris is a man-made browse line in which gardeners trim the trees and clear the ground to maintain an opening beneath the trees at uniform height. Nothing is growing between the ground and the trimmed height.

An individual deer browsing the ground and lower branches of trees does not create the browse line. It’s the cumulative effect of too many deer eating at the same location over and over.

Last Friday I watched two 8-point bucks, antlers in velvet, maintain the browse line next to the Upper Trail at Schenley Park. The current browse line, seen in the video, is that clear view straight through the woods to the cars passing on the road beyond.

In the video the bucks eat herbaceous stems and leaves on the ground, then switch to twigs, leaves and stems of trees. About halfway in, the buck on the right stands on his hind legs to reach the lowest branches. The buck on the left wrestles with a tree to yank off the branches. Deer only have lower teeth so they can’t sharply bite off a branch like a beaver would.

In late August, when forage should be quite plentiful, these bucks are forced to eat their own cover and what little remains of the edible plants.

p.s. Here’s what the forest would look like if there was no browse line.

(photos by Kate St. John, taken with my cellphone)