Ask a forester or a deer biologist in Pennsylvania what density a forest can handle, and you'll often hear the same answer: about 20 deer per square mile. It's worth knowing where that number comes from, and what it does and doesn't mean for your land.
Where 20 comes from
The figure traces largely to Pennsylvania research from the 1980s. Nancy Tilghman's 1989 study in northwestern Pennsylvania stocked fenced forest areas with deer at different densities and tracked what happened to tree seedlings. As density climbed above roughly 20 deer per square mile, regeneration and the variety of young trees declined. That threshold has been the reference point for forest-health goals ever since.
It's a forest benchmark. Goals for reducing crashes, protecting gardens, or keeping a preserve's wildflowers can be lower or higher, and a township's plan should say which goal it is chasing.
What local counts have found
Published counts around Montgomery County are few and mostly old, but they show how far above the benchmark suburban herds can run:
- Valley Forge National Historical Park: a 2009 plan set out to cut density from 241 to 31–35 deer per square mile; the park estimated 49 in 2016.
- Lower Merion Township: 58 per square mile in 2008, 40.5 in 2012, against a goal of 10.
- Upper Dublin Township: about 53 per square mile in a 2016–17 estimate.
- Solebury Township, Bucks County: USDA surveys from 2006 to 2019 ranged from 42 to 197 per square mile, with 189 in 2019.
Most of these figures are a decade old or more. The herd you're managing today may look very different.
Density depends on the area you count
Density is deer divided by area, so the boundary matters. Count a 40-acre woodlot where deer bed in winter and you may get a very high number; count the whole township and the same deer spread thinner. Good surveys state exactly what area they covered and use the same boundary every year.
Finding your number
A useful count is taken in winter, when leaves are down and thermal contrast is strongest. It reports the deer it saw, the area it covered, and a range that accounts for the deer it likely missed. Repeated on the same lines each year, it tells you whether your herd is moving toward your goal.
See how our winter census works, or estimate the cost for your acreage.
Sources
- Tilghman, N. G. (1989). Impacts of white-tailed deer on forest regeneration in northwestern Pennsylvania. Journal of Wildlife Management 53(3): 524–532.
- National Park Service: Valley Forge deer management update (2016)
- Philadelphia Inquirer: Lower Merion deer (2013)
- Cornell Deer Advisor: Upper Dublin Township case
- USDA Wildlife Services fact sheet, Solebury Township