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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

Deer feed heavily on every edible twig within reach. © Shane Rucker
Deer feed heavily on every edible twig within reach.

© Shane Rucker

October 2007

Appetite for trouble

How browsing deer change the landscape in forests, croplands and homesteads.

Robert J. Manwell


Deer replace cattle as primary woodland grazers
Deer controls are costly
Agriculture and transportation
Addressing the problem
Deer hunting helps, but often is not enough

No other wildlife animal in Wisconsin generates as much enthusiasm, economic clout or discussion as the white-tailed deer. Given so much interest among so many, how could there be too many whitetails? The answer depends on who you talk to once you understand how the whitetail fits into its environment.

Aldo Leopold, whose text Game Management was a foundation of modern wildlife management, warned as far back as the 1930s and 40s of the threat overabundant deer populations pose to forests. At the time, sighting a deer in many areas of the state was something of a novelty. By contrast, seeing deer today is commonplace – sometimes too common.

For nearly two decades, deer populations have been above community-established goals. A long succession of mild winters has increased deer productivity and survival. The popularity of food plots, baiting and feeding has added to deer herd growth. Meanwhile, land ownership changes have reduces hunter access. All of this has confounded proper deer herd control.

This fall, wildlife managers estimate Wisconsin's white-tailed deer population will top a million animals. Deer find Wisconsin's landscape mosaic of farm, forest and suburbs much to their liking, and their numbers and browsing habits are changing the forests and woodlots they inhabit.

Browsing, or deer "herbivory," as it is referred to by foresters and biologists, is measurably altering the make-up and diversity of the forest floor. Returning to pioneering research conducted by ecologist John T. Curtis, a study team of University of Wisconsin researchers (Rooney, Wiegmann, Rogers and Waller) found that over the last 50 years the number of native species had declined by an average of 18 percent on 62 of Curtis' study sites that had been browsed by deer.

Deer are a "keystone species." As their numbers increase, the plants they prefer for food become less abundant or are lost. This in turn takes a toll on small mammals, birds and insects that rely on ground-level herbs and shrubs for food and cover when breeding, nesting, foraging and escaping predators. Further, these plants are often replaced by less desirable species.

Deer browsing also bites into the forest economy by reducing landowners' ability to establish new tree plantations or regenerate harvested areas. On numerous sites, regeneration of valuable forest species is at a standstill. Valuable seedlings are nipped off repeatedly, never getting a chance to grow more than a couple of feet in height, if that.

Deer replace cattle as primary woodland grazers

"Twenty years ago, when I started my forestry career, one of my first jobs was working with landowners to fence cattle out of the wooded portions of their pastures," said Scott Fischer, a DNR forester in Bowler.

"The cows were selective grazers and happened to prefer the more commercially valuable maple, ash and oak seedlings found under the canopy. After a while, these woodlots looked like parks with few or no tree seedlings. In their place, the understory filled in with plants the cattle didn't like such as ironwood, prickly ash and hickory.

"DNR foresters would decline assisting landowners who were planning harvests in those grazed woodlots because if cattle grazing continued, there would not be any natural tree regeneration and that forest could not sustain itself. Now, we are faced with a similar situation only it is deer that are retarding or preventing regeneration in harvested areas."

In a 2005 survey, 81 percent of DNR foresters identified browsing deer as a significant barrier to reforestation, natural regeneration and successful tree planting. And, the damage attributed to hungry deer goes far beyond forest trees.

When populations of browsing animals attain high densities, the plant composition of the forest floor can also be expected to change," says University of Washington biologist Robert T. Paine. He first coined the term "trophic cascade" in 1980 to describe chains of events resulting from changes in one component of a natural system.

The consequences of a large deer herd have concerned biologists for years. Local effects vary considerably depending on the number of deer, the part of the state and other factors like how close the deer population is to the carrying capacity. Small fenced areas (deer exclosures) around the state visibly show how high deer populations or local deer concentrations can greatly reduce the variety and abundance of forest plants.

Staving off starvation in cold winters, deer leave a distinct browse line by the end of the winter season. © DNR Photo
Staving off starvation in cold winters, deer leave a distinct browse line by the end of the winter season.

© DNR Photo

Even small numbers of deer can have dramatic consequences where the land can't support a larger herd. At a deer density of 12 to 15 animals per square mile of range, herbaceous plants like trillium, Indian cucumber, showy lady's slipper and white fringed orchid decline.

When deer densities reach 20 to 25 animals per square, species like pines, white cedar, hemlock, oaks and Canada yew can stop regenerating and small mammals like red-backed voles, an important prey species, starve out without the forest floor vegetation they need.

At 25 to 35 animals per square mile of range, birds like hooded warblers decline from lack of needed ground, shrub and tree layers.

"To an untrained eye the effects of deer browsing can be difficult to see," says Joe Kovach, a DNR field research forester. "To many folks, a clear understory or one that is full of ferns topped by mature trees looks like a park, an inviting place to walk, picnic or hang out. In contrast, visibility is more limited in a forest with healthy regeneration in the understory. It can be thick with plants that are very difficult to walk through."

Deer have definite food preferences and species like hemlock, white cedar, maple, basswood, birch, oak and aspen are the first to disappear, while less palatable species like spruce, beech and red pine remain longer. The whitetails' eating habits can drastically alter forest composition and the plans of landowners and foresters who advise them.

"In the northern hardwood forests, we'd prefer to have a whole range of trees from seedlings to big mature specimens. These stands of uneven-aged trees are most desirable from an economic, aesthetic and biological perspective," says Kovach. "But, such stands depend on natural regeneration after selected trees are harvested and sunlight reaches the forest floor. This harvesting technique is the most natural-looking. It closely mimics natural processes and it provides a steady supply of forest products.

"However, if deer prevent the forest from regenerating after a selective cutting, we have to consider other management schemes such as even-age stands where we remove more timber from larger areas. This method creates long intervals between times when the landowner can earn some income, but may be successful because the deer are simply overwhelmed by the huge numbers of trees all at the same growth stage. Some trees survive the browsing simply because of sheer numbers."

Deer controls are costly

No research in Wisconsin has quantified the economic impacts of deer herbivory or the estimated loss of forest productivity caused by deer, but landowners who have planted trees know the costs.

At a site in western Shawano County, forester Fischer points out stunted ash seedlings struggling to rise above the grass around them.

"Given the quality of this site and the weed control the landowner has undertaken, these white ash should be about four feet tall," says Fischer.

Leaning over an oddly-shaped seedling, he observes "this tree shows evidence of being nipped seven times already this season.

"Summer browsing brings the worst kind of damage for hardwoods. Extensive leaf damage prevents the tree from manufacturing food and weakens it, making it susceptible to other plant competition, disease and insect attack.

"It only goes to show that the high deer populations are forcing deer to feed on less desirable species in order to get enough to eat."

"Imagine the heartbreak experienced by this landowner," adds Kovach. "He is reaching the point where the entire project may be lost. He's replanted twice already, adding 1,000 additional seedlings in each of the last two years. He stands to lose about $7,000 in his investment."

Agriculture and transportation

High deer populations lead to other costs as well. Statewide deer damage to corn crops alone was estimated to be as high as $15 million a year in 1993. Actual damage payments to farmers due mainly to deer browsing are running about $1.5 million per year according to wildlife officials. The number of deer-vehicle collisions is in the range of 40,000-50,000 per year and combined property damage and personal injury from deer-vehicle accidents is estimated to be over $100 million per year.

The Wisconsin Council on Forestry also recognizes the problems landowners and foresters face. In a letter to Governor Jim Doyle, the council states "Deer herbivory is a serious problem that if not addressed will affect the sustainability of forestry in Wisconsin."

The County Forest Administrators Association submitted a resolution last year calling for deer herd control.

This alarm has also been sounded by the Wisconsin chapter of the Society of American Foresters which adds that "deer populations should be reduced to allow for the efficient and desired regeneration of forests, and to sustain a diverse array of plant and animal communities."

Addressing the problem

Many techniques have mixed results in reducing the consequences of deer browsing on forest regeneration and plantations. Fencing, tree shelters, bud caps and repellents can all deter deer browsing around plantations and high value crops like fruit trees. All are expensive and take much more labor after tree harvesting and planting. To recoup costs, producers have to pass on some of these costs to wood and pulp buyers.

"A tall fence is probably the best deterrent but it is obviously a very expensive undertaking," says Kovach. "Tree shelters – two- to three-foot plastic tubes wrapped around the base of young trees – can help, but they are labor intensive to install, provide shade and moisture for some disease and insect damage, and in some cases will harbor rodents that damage the tree."

Repellants work best when deer densities are low, but they often have to be reapplied after each rainfall and a variety of products need to be applied to keep discouraging deer.

Deer hunting helps, but often is not enough

Wisconsin's long history of deer management dates back into the 1800s when seasons, bag limits, game wardens and licenses were first introduced. Then, in 1962, the Legislature approved setting population goals for each deer management unit.

Deer population goals balance the "social" carrying capacity in farm country – the crop damage, reduced forest regeneration and vehicle collisions that people will tolerate, and the "biological" carrying capacity in forested regions – the maximum number of deer that can survive on the land under average habitat and weather conditions.

The primary tool that aims to keep the deer herd at these population goals is recreational hunting. Though the harvest has been impressive during the last decade, it doesn't hold a candle to the deer herd's ability to grow given food and shelter.

Over the last 10 years, Wisconsin hunters have harvested on average more than 460,000 deer per year. Population goals for deer management units vary from 10 to 35 deer per square mile of deer range. Yet, just 17 of Wisconsin's 130 DMUs currently are close or at their goals.

In the unit the author toured with foresters Fischer and Kovach, the population goal is 25 to 30 deer per square mile. The actual population estimate in fall of 2006 was 95, more than three times the desired limit.

"Deer population goals must consider many things," says Kovach, and the state attempts to balance these public interests with sound biology. "There is the desire for hunters and wildlife watchers to see deer. There are the farmers' concerns over crop damage. There are the ecological impacts to consider. From a forest regeneration and diversity perspective, our established over-winter density of about 20 deer per square mile is reasonable – the problem is, we are way above those goals in most deer management units."

Deer impacts on their environment add to the challenge of practicing sustainable forestry. Many private forest landowners have enrolled their forested acres in the Managed Forest Law (MFL) program. A condition of enrollment is that the landowner file a management plan drawn up by a certified professional forester. Plans usually include periodic timber or pulpwood harvests and often prescribe converting cleared agricultural fields back into forest. Plans are individually tailored to each landowner's objectives and use widely accepted forestry practices. By enrolling in the program, landowners can defer much of their annual property taxes until the time that they harvest timber. Taxes are then paid from the proceeds of the timber sale.

Knowing what forest practices to apply in the face of heavy deer browsing can be challenging.

"We see results on managed forest lands," says Fischer. "We don't know how much longer this level of browsing can be sustained before we need a radical change in the management plan, such as going from a shelter wood or selective cut management plan to perhaps a clearcut. And it may not be possible to grow new forests where deer numbers are too high regardless of the system we attempt."

"Browsing has nearly stopped forest regeneration in many areas. If as foresters we can't depend on the forest's natural ability to regenerate itself following a timber harvest, we have to ask ourselves if we really are practicing sustainable forestry."

Not all landowners have the same objectives for their property. One of the most hotly debated topics among landowners is hunting deer on their properties or allowing hunting access.

"Landowners have strong control over the makeup of their woodlot through timber harvest and planting," says Fischer. "But control over deer populations are another matter. Individual landowners can have only a limited effect on deer populations. It takes neighbors with similar management objectives to address high deer populations. If one neighbor creates a refuge where deer are inaccessible to hunters or if the neighbor allows only limited hunting, deer can hide in protected areas during daylight and emerge at night to browse neighboring woodlands."

In addition to the ecological cost of deer browsing, inability of the forest to regenerate itself threatens the $28 billion forestry industry in Wisconsin and the 96,000 jobs it supports. A steady supply of quality wood is the lifeblood of this economic machine and affects tens of thousands of small forest landowners. Much of the wood arriving at factories comes from privately owned woodlots. Fifty-two percent of Wisconsin's 34.7 million acres of forest land is privately owned. Many of Wisconsin's most valuable forest trees take 50 to 100 years to mature. Many landowners will only see one or perhaps two harvests on their property in their lifetime.

Large numbers of deer can clearly affect valuable trees, shrubs and flowers of forest owners and homeowners. A few industrial forest owners have even considered selling their land and buying other lands where herds take less of a bite out of their bottom line. Some Christmas tree farmers have resorted to high-priced electric fencing to protect their crops. Homeowners in both rural and suburban settings also complain about deer eating their prized landscaping plants as well as their gardens. Deer will browse trees and shrubs planted for windbreaks, screens between neighbors, backyard wildlife habitat and scenic beauty when they are hungry.

Clearly, it takes a mix of strategies to address all these issues, but just as clearly deer will not naturally resolve these issues in a manner that will be acceptable to people or the environment. The herd keeps growing and its appetite for vegetation is shaping both the physical and cultural landscape.

Robert J. Manwell is DNR's Public Affairs Manager for the DNR Lands Division, including the wildlife management, forestry, parks and endangered resources programs.