Effects of Clearcutting on Forest Vegetation

Studies of vegetation response to clearcutting have found that the intensity of harvesting, site preparation, and the type of regeneration affects floral composition. We believe that wildlife in general respond positively to the increase in availability of nutrients and plant species richness associated with these disturbances.

Burning undertory for forest managementNet primary productivity may decline initially following clearcutting. For example, components of hardwood forest regeneration in a Southern Appalachian watershed were assessed the first year following clearcutting by Boring et al. (1981). First-year net primary production in the clearcut stand was 1,740 lbs/ac, only 22% as high as the net primary production of a nearby undisturbed hardwood forest. However, large amounts of biomass and nutrients rapidly develop in ground-level vegetation. Following forest disturbance, fast growing woody and herbaceous species flourished and conserved substantial pools of nutrients in their biomass. The rapid elemental uptake was due in large part to the composition of the biomass, which included a high proportion of foliage to woody tissue. Similar findings were reported in a northern hardwood forest by Siccama and Borman (1970) at Hubbard Brook in New Hampshire and by Van Lear et al. (1983) following harvest and natural regeneration of loblolly pine on a Piedmont site. These studies help explain why deer and other herbivores, including small mammals, find clearcuts so inviting.

Long-term changes in floral composition resulting from clearcutting have not been fully explored. Some studies have recently suggested that stands harvested in the Southern Appalachians regain their original vegetative composition slowly or not at all. Duffy and Meier (1992) examined herbaceous cover and species richness in the understories of 9 old-growth forests and 9 comparable even-aged secondary forests ranging in age from 45 to 87 years in the Southern Appalachians. Neither cover nor plant species richness increased with age in the secondary forests, suggesting to the authors that: 1) recovery of plant diversity was so slow that 87 years was an insufficient time to detect recovery; 2) the forests will never recover to match remnant old-growth forests due to the fact that climatic conditions are different today; or 3) the recovery of certain herbaceous plants must await the growth, death, and onsite decomposition of trees in secondary forests before the forest takes on truly old-growth characteristics. They concluded that "Whatever the mechanism, herbaceous understory communities in the mixed mesophytic forest of the Appalachians appear unlikely to recover within the present planned logging cycles of 40 to 150 years, suggesting a loss of diversity of understory herbaceous plants.”

Reviews of their study results by Johnson et al. (1993), Elliott and Loftis (1993) and Stein-beck (1993) pointed out: (1) certain methodological errors; (2) that interpretation of the history of the study areas is of doubtful validity; and (3) that the authors presented mainly selected data that would support their own philosophies. Also, comparing second-growth and old-growth could be like comparing apples and oranges. It is impossible to recreate conditions in which present-day old- growth developed. Climatic conditions and disturbance regimes which shaped the structure and composition of existing old-growth stands are different now, so comparisons may not be of much value.

In some ecosystems, selected plant species important to wildlife may decrease in abundance following clearcutting. Indications that clearcutting in cove hardwoods in the Southern Appalachians changes the woody stand composition were reported in a study by Beck and Hooper (1986). A mixed stand of 53% oak, 33% yellow-popular, and 14% other species was clearcut in 1963. Twenty years later, a developing even-aged stand, predominantly of sprout origin, was dominated by yellow-poplar, black locust, red maple, and sweet birch. Oaks were a minor and decreasing component. Similar results have been reported by Elliot et al. (In Press).

It is possible that additional management measures, such as prescribed fire (Van Lear and Watt 1992, Johnson 1992, Brose and Van Lear 1996, Keyser et al. 1996) or use of herbicides (Loftis 1990), may be required to regenerate oaks in the Southern Appalachians. Planting seedlings or direct seeding of the acorns may be a viable method to increase oaks in bottomland hardwood sites (Johnson and Krinard 1987). Fortunately, since acorn crops are notoriously undependable, species of wildlife that utilize these fruits have adapted to periods of scarcity and none is entirely dependent on them. The persistence of such a species is proof that they can survive on other foods.

Because abundances of individual plant species can be changed by clearcutting, community measures such as diversity also may change. A 40-ac watershed in western North Carolina was clearcut in 1939 and again in 1962. Vegetation was inventoried in 1934 and at about 7-year intervals to 1991 (Elliott and Swank 1994). After the first clearcut, tree diversity remained high until after the second cut. Diversity based on density and basal area decreased significantly 14 years after the second clearcut and remained low through 1991. Diversity was highest in the early establishment stage of development, then declined at the intermediate stage with canopy closure. Evenness declined somewhat because basal area of yellow-poplar increased substantially from 1977 to 1991. Tree diversity was negatively related to basal area growth and dominance.

A multi-disciplinary study of the environmental and ecological effects of clearcut harvest, site preparation, and planting of pine in flatwoods forests under 2 distinct management regimes in Florida was described by Swindell et al. (1983). Floral composition was dramatically altered under both management regimes; woody species diminished in abundance while herbaceous species increased. The less intensive combinations of practices increased floral species richness while the more intensive had little effect on species richness. We would expect an increase in the use of the areas by deer, game birds, small mammals and many songbirds, but a temporary decline in use by some amphibians. Hebb (1971) studied the effects of site preparation (double chopping) and slash pine planting on game food plants in sandhill sites of northwest Florida over a 12 year period. Oaks were nearly eliminated on the chopped sites while gopher apple decreased but maintained a steady presence and blackberry became a prominent and abundant plant. The legumes milkpeas, prairie clover, partridge pea, and (Rynchosia spp.) were equal or greater in percent and number on the chopped areas compared to the untreated areas. Wiregrass greatly decreased while broom-sedge and panicum increased.

Changes in vegetative composition can enhance habitat for many wildlife species. Prior to clearcutting in a loblolly pine-hardwood forest in eastern Texas, Stransky et al. (1986) reported that the forest contained few herbaceous species and a low net community production in the herb-shrub stratum (vegetation below 5 ft in height). Removal of overstory trees by clearcutting increased the number of herb species and the net community production of herbs and woody plants. Again, we would expect all herbivores, especially deer, and many early successional birds to benefit from these changes in vegetative composition. Of 35 plant species or groups important as food for songbirds in east Texas, 28 were more abundant on a clearcut area 15 months after a timber harvest than on an adjacent uncut pine forest (Stransky et al. 1976).

In terms of plant species richness, the seedling-sapling-shrub stages usually rate higher than the poletimber stands (Shaw 1969, Mathisen 1988). Although young stands provide an abundance of wildlife habitat, the common mechanical site preparation practices for reducing weed competition temporarily limit plant species diversity and thus may diminish habitat for some species of wildlife. Harris et al. (1979) suggest that the negative effects of mechanical site preparation are proportional to its intensity. Long-term rotations are especially valuable to wildlife associated with older age classes. However, not every stand in the landscape has to be mature. Selected stands in the landscape, e.g., SMZs or other reserved stands, can be managed to contain these features.

Because of concerns about site productivity, use of the site preparation technique called “windrowing” is diminishing. With this practice, woody debris is bulldozed into long, parallel strips 10-30 ft wide and perhaps 200 ft apart, which are then burned. We found no studies reflecting the effect of this technique on wildlife. When topsoil and nutrients are concentrated in these strips they often become strips of hardwoods, a feature that potentially adds to the diversity of the area.

  Next page...Summary

Table of Contents  |  Literature Cited  |  Definitions  |  Common and Scientific Names