Case Studies in Deer Management

White-tailed deer are a key species in healthy eastern forest ecosystems.  In appropriate numbers , they help improve understory biodiversity. For many park visitors, they can also be a joy to observe. But with abundant habitat, the absence of their usual predators (namely wolves and mountain lions), and development that has reduced hunting opportunities, deer populations easily grow out of control. In their search for food, too many deer can empty the forest of most native plants aside from mature trees.

Such unsustainable deer browsing reduces habitat for birds and other wildlife. And with tree seedlings and saplings on the menu, it impedes forests’ ability to regenerate after mature trees succumb to storms or other disturbances.

Inventory & Monitoring Program scientists studying  eastern forest health  have found that forests in a majority of eastern parks are facing probable or imminent regeneration failure linked to deer overpopulation.

Recognizing this crisis, several parks started managing their deer populations over the last two decades. They have learned many lessons along the way that can make deer management in other parks more effective and efficient from the outset.

But in the 1970s and 80s, park staff began noting a lack of young trees, indicating a worrisome lack of forest regeneration. At the same time, numbers of deer-vehicle collisions were reaching new highs on local roads, and huge deer herds could be spotted in the park’s agricultural areas, eating yields of historical crops. Staff suspected the deer were also browsing on seedlings and saplings in the woodlots to the extent that very few could become established. They began long-term vegetation and deer monitoring projects in the late 1980s to test that hunch.

Gettysburg researchers and their partners at Pennsylvania State University found growing deer densities that exceeded 100 per square mile by 1992. To put that into perspective, researchers have estimated that around 8-11 deer per square mile browsed North American forests prior to European settlement. Vegetation monitoring, meanwhile, revealed that tree seedling and sapling densities were declining. Comparisons of fenced [deer exclusion] and unfenced plots linked the declines specifically to deer browsing.

Scientists estimated that, in Gettysburg, the deer density would need to decline to 25 deer per forested square mile to allow enough seedlings and saplings to survive and sustain the woodlots. So the park began an environmental impact statement (EIS) process to identify the best deer management pathway. Gettysburg’s  final deer management plan EIS , specifying lethal removal of deer to achieve the recommended density, was  published in 1995 . The first removals began later that year.

Unfortunately, Gettysburg’s deer management success is not without caveats. A substantial number of tree seedlings and saplings in the park have been ash. Due to the spread of an invasive insect pest, the emerald ash borer beetle, young ash trees can no longer survive long enough to regenerate the forest canopy. That has changed the math around sustainable regeneration at Gettysburg, shifting it back to a “probable regeneration failure” designation because other species’ seedlings are not numerous enough to regenerate the forest on their own. In addition, forest thinning to promote regeneration in the park also created openings for invasive plants to take hold. Invasives are now more prevalent in Gettysburg compared to elsewhere in the Mid-Atlantic Network.

• Healthy levels of forest regeneration may take 15+ years to attain after deer management begins. A sustained commitment is necessary for success.  
• Invasive plant management is a must, in conjunction with deer management, when creating canopy gaps to promote forest regeneration through increasing light on the forest floor.
• Tracking seedling and sapling species diversity in addition to density, and excluding historic canopy species that are no longer viable, leads to more accurate sustainable regeneration goals.

The local deer population grew as well, from hardly any, to estimated pre-colonial numbers, to ever higher record highs. In the 1980s, park staff began to worry about the impacts that the deer herds were having on the native forest ecosystem. Young trees, non-woody forest plants, and understory wildlife all appeared to be suffering. Clusters of the state-threatened greater purple-fringed orchid, for example, needed deer exclusion fencing to persist.

Thus, after years of studies confirming the adverse effects of too many deer on Catoctin vegetation, wildlife, and cultural landscapes, the park finalized a  deer management plan and environmental impact statement (EIS ) in 2008-2009. It calls for strategic use of fencing and repellents, continued deer and vegetation monitoring, and lethal removal of deer to maintain a winter population of 15–20 deer/square mile. Around the time the EIS was signed in 2009, there were over 120 deer / square mile in the park. Deer management began in February 2010, and has been ongoing every year since.

Though deer management is unquestionably benefitting Catoctin’s forests, achieving sustainable forest regeneration could still be far off. It can take decades for seedlings to grow into saplings and young trees. In addition, seedling diversity is not yet where it needs to be to eventually regenerate the forest canopy. Similar to Gettysburg, nearly  75% of initial seedlings at Catoctin  were from a single species—white ash. Even as their seedlings are surging,  mature ash trees are dying in staggering numbers  from invasive emerald ash borer beetles, likely rendering the species an unviable canopy regeneration candidate.

In response, the park has been using tree tubes, prescribed burning, and invasive shrub control to boost regeneration potential for other species. And though there is a long way to go, seedling diversity may be on the upswing.  From 2017–2020, ash accounted for a smaller share (~62%) of seedlings  at Catoctin, as other tree species’ seedling counts rose.

Additionally, park staff have ceased monitoring vegetation plots annually and now rely on  National Capital Inventory & Monitoring Network  plot data.  The park uses the time saved to instead monitor the sensitive plant population spread in order to learn about them and protect them from disturbance and poachers.

• Deer management can help non-woody understory plants quickly, but tree regeneration takes time. It’s never too soon to start.
•  Don’t delay deer management because of invasive Japanese stiltgrass . It won’t impede seedling regeneration.     
• Attention to appropriate seedling species composition is an important piece of eventual, successful forest regeneration, especially in light of emerging threats to historic canopy species like  ash  and  beech .
• The availability of Inventory & Monitoring Program forest vegetation data means parks may not need to do their own vegetation surveys.

After steep declines in deer numbers following European settlement, deer began returning to what was then Valley Forge State Park in the 1930s. By the time the park became a unit of the National Park Service in 1976, deer populations were robust across Pennsylvania, but the park’s forests were still thought to be in good shape. From 1983-1985, researchers from Pennsylvania State University pegged the park’s deer density at around 31-35 per square mile. Continued monitoring showed, however, that deer density in the park was still on the rise. It eventually peaked at an astounding 310 deer per square mile in 2008.

Meanwhile, pairs of fenced and unfenced plots established in park forests in 1992 starkly demonstrated the impact of so many deer on the park’s once-diverse understory. The park decided to take action. They began an Environmental Impact Statement process, proposing to manage deer and use  long-term forest monitoring  data collected by the Mid-Atlantic Inventory & Monitoring Network (MIDN) starting in 2007 to guide their efforts.

In 2009, Valley Forge completed its  Deer Management Plan/Environmental Impact Statement  with goals including restoring native plant communities and promoting tree regeneration. To get there, the plan set an initial target deer density of 31-35 deer per square mile, in line with the densities recorded in the park in the 1980s, but higher than what  some research  suggests may be necessary for comparable forests. They began deer removals in 2010.

After a few years of deer management, Valley Forge was reaching its initial deer density targets. Then, after several years of consistently meeting those targets, park staff had two big questions:

1. Were the latest MIDN vegetation surveys, which only cover ¼ of the park’s forest plots each year (and thus all plots every four years), representative enough to guide decision-making?

2. Were Valley Forge’s forests recovering at the initial target deer density?  

Yes and no.

To arrive at those answers, Valley Forge worked with MIDN staff to plan two years of surveys of all park forest plots in 2017 and 2018. MIDN then compared forest regeneration results from those single-year, all-plot surveys to those from their most recent four-year survey cycle. The results lined up. Data collected from all plots over four years closely tracked the data collected from all plots in one of the years. That was great news, as it’s far easier to survey fewer plots each year. Unfortunately, all those results also showed that park forests were not regenerating. In fact, despite increases in the smallest class of seedlings, measures of forest regeneration were still trending slightly downward.

As a result, Valley Forge staff set a new, lower target of 25-30 deer per square mile. If the most recent rounds of vegetation monitoring show that their forest recovery goals remain out of reach, they’ll lower the park’s deer density targets again.

• Huge deer density reductions don’t help if target densities remain too high. At Valley Forge, forest regeneration metrics continued to decline even following a substantial, 10-fold deer density reduction.
• 31-35 deer per square mile has been too high of a target at Valley Forge. Start lower.  Previous research  has indicated that different landscapes can sustain different deer densities, but that in many cases, 20 deer per square mile or fewer is necessary to lower browsing pressure to the point where forest regeneration can occur.
• Inventory & Monitoring Program forest monitoring data is a reliable tool for high stakes deer management decision-making, even where only a subset of plots are surveyed each year.