Forest Habitat Conditions
DCNR has long advocated balancing white-tailed deer populations with forest habitat conditions. Recent efforts to manage the state’s deer herd in this manner, as well as new tools for landowners—such as the Deer Management Assistance Program—have yielded significant improvements in habitat conditions in some areas of the state. Other areas, however, continue to suffer from a lack of new forest growth, and habitat conditions and overall forest health remains poor.
Many DCNR stakeholders, from hunters to wildlife watchers to wildflower enthusiasts to private forest landowners, often ask about “forest recovery” in light of more recently balanced deer populations. While forest habitat recovery is complex process, this article will attempt to explain some of the many factors and variables involved.
Pennsylvania forests have sustained deer population densities high enough to cause impacts to forest vegetation since the 1920s. Researchers have studied the effects of deer on forest ecosystems for many years, and the impacts of deer on the forest are well established in decades of scientific literature. How do deer impact the forest? Through selective browsing of native plants, shrubs and trees, they influence the vegetation that grows in the forest. For example, deer like to feed on oak seedlings. By preferring oaks, they can effectively diminish the presence of the species in the forest while allowing less preferred species, such as striped maple, to thrive. In the most severe cases, deer can completely prevent the capacity of the forest to renew itself.
In some areas of the state, deer overbrowsing has eliminated the understory – the lower vegetation layer that includes young trees, shrubs and other plants. Deer feed extensively on many herbaceous plants such as wildflowers and other low-growing plants and even fungi. Some forest plants that deer prefer include large white trillium, bluebead lily, Canada mayflower, and numerous orchids. Because they never outgrow the reach of deer, plants that grow on the forest floor are continually vulnerable to deer impacts.
Forest Habitat Conditions
Pennsylvania’s forests have benefited from recent efforts to balance deer populations with forest habitat conditions. Deer fencing on state forest land over the last two years (2009-2010) have been less than half of what they were five years ago. For the first time, fence dismantling equaled fence installation.
Despite recent gains in forest habitat health, in general, the forests of Pennsylvania are still in poor condition. According to USDA Forest Service Inventory and Analysis (FIA) data across all ownerships, approximately 54 percent of Pennsylvania’s forests are adequately stocked with regeneration—the young trees and plants that will make up the future forest. (Only inventory plots with sufficient sunlight to establish regeneration are counted in this inventory.) Only 41 percent of the sample plots in the north-central region were adequately stocked with tree seedling and sapling regeneration. These numbers indicate that only about half of Pennsylvania’s forests would regenerate following an overstory disturbance such as a wind event, insect outbreak or timber harvest.
Excessive deer impacts can alter the course of forest vegetation development with long-lasting consequences. Often referred to as “legacy impacts,” they can hamper forest recovery and habitat health for a long time. In some cases, such as the northern tier of Pennsylvania, legacy impacts can affect forest regeneration for decades.
One type of legacy impact is the increase of “competing vegetation.” When deer selectively browse on vegetation, they not only reduce the occurrence of preferred species in the forest understory, but they also create conditions for unpalatable or resilient species to become so plentiful that they may suppress other plants by producing dense shade on the forest floor. Examples of competing vegetation in Pennsylvania include hay-scented fern, New York fern, striped maple, American beech and striped maple. In many areas of the state, the forest understory is completely dominated by these species—both an unnatural and unhealthy condition. FIA data indicates that nearly one-third of forest understory communities are dominated by this type of competing vegetation.
Competing vegetation not only provides poor habitat for deer and other forest wildlife, it also prevents the establishment of more desirable trees and plants. Mats of fern and stands of shade-producing striped maple create poor conditions for many trees and plants to grow. Competing vegetation is a significant problem across Pennsylvania that continues to impede forest recovery. Even in areas where deer populations are relatively low, competing vegetation can prevent or significantly delay the establishment of regeneration.
Reduced Seed Banks, Production, and Slow Growth Rates
A major impediment to the recovery of the forest is the lack of seeds and other means of reproducing where browsing has been a factor for long periods of time. In these areas there may be few local seed sources remaining. Also, most forest herbs do not have long-distance dispersal abilities. In addition to limited seed dispersal mechanisms, rates of seed production are often affected in deer-impacted forests. This occurs because deer often selectively remove the flowering or fruiting stem even though they do not consume the entire plant.
The combinations of these factors, as well as the abundance of non-preferred and browse resilient species in many areas, adds to a scarce seed bank. These issues will affect vegetation dynamics long into the future.
When managing forests and deer from an ecosystem perspective it is important to consider the myriad factors that may be affecting forest regeneration and other processes. Accounting for these factors is necessary in making sound predictions about recovery times following a reduction of deer browsing impacts.
Insect infestations are just one of the many factors reducing the number of seedlings that become established. However, they are occasionally severe enough to prevent the regeneration of tree species, especially when insect infestations occur in conjunction with other stressors.
One example of an insect that has wreaked havoc on Pennsylvania forests is the gypsy moth. Gypsy moths were accidentally introduced into Massachusetts from Europe in the late 1860s and slowly spread to Pennsylvania where it began to cause heavy defoliations in 1969. When these insects move into an area they can kill large numbers of trees, especially hitting white oak and chestnut oak.
Gypsy moth outbreaks can have several effects on the natural regeneration of forests, particularly oak-mixed hardwood stands. One impact is the decrease in acorn production due to affected oak trees aborting undeveloped seeds and reducing subsequent crops. Acorn production is also reduced due to direct mortality of oak trees of seed-bearing size. These dynamics affect the establishment of desirable forest regeneration even when deer impacts are low.
Acid deposition occurs when acid-forming substances are transferred from the atmosphere to the surface of the earth. The deposited materials include ions, gases, and particles typically resulting from power generation and heavy manufacturing.
Research has shown that acid deposition can cause slower growth, injury, or death of trees, particularly sugar maple and red spruce. It has been implicated in forest and soil degradation in many areas of the eastern U.S., particularly in high elevation forests. Acid deposition generally causes stress to trees by interfering with calcium and magnesium nutrition and the physiological processes that depend on these elements. Acid deposition does not usually kill trees directly. Instead, it is more likely to weaken trees by damaging their leaves, limiting the nutrients available to them, or exposing them to toxic substances slowly released from the soil.
Quite often, injury or death of trees is a result of these effects of acid rain in combination with one or more additional threats. Other factors contribute to the overall stress of these areas, including air pollutants, insects, disease, overabundant deer, drought, or very cold weather. In most cases, in fact, the impacts of acid rain on trees are due to the combined effects of acid rain and these other environmental stressors.
The USDA Forest Service has been conducting a long-term research (23 years) on the effects of broad-scale lime application on Susquehannock State Forest to mitigate impacts from acid deposition and improve regeneration In this study, started in 1985, researchers treated stands with all possible combinations of liming, fencing and herbicide. The results indicate that fencing is by far the strongest predictor of good regeneration. Some less common species are positively affected by lime application or by the combination of lime and one of the other factors, but these effects appeared to be much subtler than the effect of controlling deer through fencing.
The Bureau of Forestry recognizes that there are many serious impediments to regeneration, but are supportive of studies to determine if liming should be more broadly used to mitigate the impacts of acid deposition.
Forest recovery in Pennsylvania is affected by a variety of factors. Some of these additional factors include invasive plant species, fire suppression in oak-dominated forests, diseases, and a host of other issues. However, the consensus among biologists and natural resource managers remains that the most important factor affecting forest regeneration is deer browsing and legacy impacts.
Deer management on state forest lands focuses on the ecosystems in which deer are a part. Deer densities in Pennsylvania need to be maintained at levels that will allow the restoration of full forest structure, diversity, ecological processes, and ecosystem function.
Forest Recovery: How Long Will it Take?
The forests of Pennsylvania will most likely not return to the same conditions that existed prior to heavy browsing impacts of the twentieth century, even with reduced deer browsing pressure. There have been too many changes in the forest ecosystem due to introduced tree diseases, insect infestations, and invasive plants. Nevertheless, the return of a diverse native understory can be expected; however, this recovery will have varying time depending on local and regional conditions.
Forest recovery begins with the restoration of quick-responding understory species such as blackberries and raspberries. The abundance and height of woody and herbaceous species often preferred by deer will also increase. These signs of recovery can be reached relatively quickly in stands where deer browsing impacts occurred only for a relatively short time. In these areas, when deer densities are brought back into balance with the habitat, and there is enough light to reach the forest floor, partial recovery can be achieved within 10 years.
Full forest restoration occurs when a full suite of shrubs and herbaceous plants become reestablished in the understory. This kind of recovery can take more than 50 years in more severe cases. How long it takes for full recovery depends on the severity of deer impacts, legacy impacts such as competing vegetation, and other factors such as the site’s logging history, soil chemistry, length of the local growing season, presence of introduced species, and seed sources.
State Forest Habitat Management Efforts
To sustain the forest and help improve forest habitat conditions, the Bureau of Forestry strives to accelerate forest recovery. The bureau harvests approximately 15,000 acres of state forest land each year, increasing sunlight to the forest floor and spurring new growth and creating early-successional habitat.
When timber harvests occur in areas where deer impacts are high on state forest land, fences are installed to increase the probably of successful regeneration. Once deer browsing pressure is reduced, herbicide is often used to remove ferns and other competing vegetation that hinder seedling establishment and growth.. In addition to herbiciding, the bureau implements many practices designed to establish healthy new forests. In 2010 the Bureau of Forestry brush mowed 380 acres to control woody vegetation, planted trees to augment natural seedlings on 1,043 acres, scarified the seedbed to expose mineral soil on 411 acres, and implemented prescribed fire on 89 acres to control competing vegetation.
Overall, these practices are successful in establishing regeneration. However, because of their cost, they are usually only viable solutions on a small scale. Areas not as intensively managed will take much longer to establish healthy habitat conditions and recover for deer impacts.
A healthy forest will consist of young trees, shrubs, and a variety of wildflowers. The presence or absence of some wildflowers such as trillium, Canada mayflower, and Indian cucumber root can be key indicators of the level of deer impact on a given forest site. Another reliable indicator of a healthy forest is an understory, the young trees and plants that grow in the lower layer of the forest. A well developed understory is an essential habitat element for many plants and animals, but it also indicates the forest’s capacity to renew itself. For more information visit the Healthy Forests-Healthy Deer Brochure.