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| Home > Final Report > TERRA-4 |
Uneven-aged management has been used successfully in several southern forest types. In this type of management, trees of several age classes are present in the stand at all times. Stands are usually regulated by volume and managed to maintain a specific diameter distribution, with many smaller trees and fewer large trees. Since most commercially desirable tree species in the South are relatively shade intolerant, the upper canopy must be reduced such that younger trees are able to grow into the overstory.
This type of management has many benefits for wildlife, especially birds. Due to high levels of canopy stratification, many bird species are able to utilize these stands (Dickson and others 1995). Different bird species rely on different portions of the canopy. Wood thrushes require dense understory growth, while cerulean warblers utilize emergents, which are individual trees that are taller than the main canopy. With respect to emergents, it has been demonstrated that canopy height is not as important as relative height. In most uneven-aged stands, larger trees act as emergents due to their size relative to their immediate neighbors.
Uneven-aged management of both pines and hardwoods requires frequent entry into the stand, increasing risks of disturbing wildlife and rutting or compacting the soil. More access roads are also generally required for this type of management, and they must constantly be open. In uneven-aged pine management in particular, increased herbicide use is often required to release pines from more shade-tolerant hardwood competition (Dickson and others 1993).
Area regulation in uneven-aged management has become an accepted method for managing both pines and hardwoods, especially when wildlife enhancement is the primary objective. Area regulation differs from volume regulation in that equal areas of land within a stand are harvested at each entry, rather than cutting the stand to a specific diameter distribution. Area regulation has been used with great success in longleaf pine and bottomland hardwoods, where large, homogeneous stands exist. In bottomland hardwoods, waterfowl habitat is enhanced, particularly in areas where foraging and pair bonding occur.
Even-aged management is very common in the South. It lends itself well to southern ecosystems mainly because most of the commercially desirable tree species are shade intolerant. In even-aged management, only one or two age classes of trees are present in a stand.
A clearcut is the most basic technique for initiating an even-aged stand. In the following paragraphs, clearcutting with natural regeneration is addressed. Artificial regeneration will be discussed in the narrative on plantations. In clearcutting, the entire stand is removed in one harvesting operation, and a new stand of trees takes its place. Clearcut areas may be regenerated naturally from sprout reproduction, from seeds from surrounding stands, or from seeds that were in place before mature trees were removed. Hardwood stands often are regenerated with advance reproduction, which was in place before the initial harvest (Baker 1997, Hodges 1997).
From a wildlife management perspective, clearcuts have the benefit of providing maximum amounts of light reaching the ground, which improves growth of herbaceous plants (Pietz and others 1999). Many wildlife species thrive in early successional communities created by clearcutting (Wigley and others 2000). The possibility of erosion may discourage clearcutting on sites with steep slopes. In wet areas, clearcutting may raise the water table excessively because transpiration is greatly reduced by removing most plants. If the water table rises to the soil surface, establishment of a new stand may be impeded.
Seed trees were often used for regeneration in the South until about 15 years ago. This approach is losing favor to clearcutting and planting, which allows introduction of genetically improved stock. In the seed-tree method, four to eight mature trees per acre are left to provide seeds for regeneration. After the stand is regenerated, the seed trees are removed. From a wildlife management perspective, this technique provides the benefits of large amounts of light reaching the ground, while some structural elements are retained for several years after harvest (Dickson and others 1995). In some cases, seed trees are left on the site, rather than being removed.
Regeneration by the shelterwood method is common with tree species that regenerate best in partial shade. Heavy-seeded species are generally not regenerated with either seed-tree or shelterwood techniques. Shelterwood cuts are attractive to neotropical migratory bird species that are associated with either early- or late-successional stages (Dickson and others 1995). Shelterwood cuts in overcup oak stands in green-tree reservoirs have also been successful. Overcup oak acorns are disseminated widely by water, and the reduction in canopy density attracts macroinvertebrates, which are important food items for waterfowl.
In both seed-tree and shelterwood regeneration techniques, a second and sometimes third entry is made into the stand to remove remaining trees. In shelterwoods, entry is usually essential to release reproduction. Irregular shelterwoods may retain "leave trees," which are mature trees left in the stand to provide structural diversity, wildlife habitat, or seed sources. Management of two-aged stands is becoming popular on public lands and initial evidence is that with respect to forest birds, this may be an acceptable option to clearcutting (Duguay and others 2001).
Thinning is a common silvicultural technique used to concentrate growth on fewer trees. Stands are commonly thinned during the stem exclusion stage and are sometimes thinned again later in the rotation. Thinning temporarily reduces canopy coverage and allows light to reach the forest floor, promoting growth of understory plants. Thinning may also temporarily create canopy complexity, which is positive for many bird species (Dickson and others 1995, Wigley and others 2000).
Timber stand improvement (TSI) cuts are used to remove trees that are less desirable because of their species, form, or health. Although these cuts allow sunlight into the stand, in many cases they remove individual trees that are beneficial to wildlife due to their form or the presence of cavities.
Herbicide use has become extremely common in forest management. Historically, prescribed fire was used to remove unwanted vegetation. Herbicide treatments have taken the place of prescribed fire in many areas. Herbicides may be sprayed from the air or from the ground, injected into unwanted stems, or squirted onto wounds hacked through the bark. Such treatments are very effective in reducing competition and promoting crop-tree growth. Most herbicides labeled for forestry use today have extremely low vertebrate toxicity and are not immediately detrimental to wildlife. Negative impacts of herbicides usually are associated with decreases in plant diversity, but herbicides can be positive for wildlife under specific circumstances and especially where prescribed fire is no longer a viable management option (Wigley and others 2002).
Other than reducing competition, herbicides are also used to change stand structure. Individual stems in hardwood stands are commonly treated to reduce shade-tolerant species and allow space for advance reproduction (Hodges 1997). Trees treated with herbicides create snags and downed wood, which are beneficial to some wildlife species. Overstory trees are sometimes treated chemically to allow sunlight penetration, creating large upper canopy snags. Although they are beneficial to a variety of wildlife species, canopy snags usually remain standing for only a few years in the South (Dickson and others 1995).
Fertilizer application is increasingly common in southern forests. Both pine and hardwood stands are treated to increase crop-tree growth, but the practice is almost totally restricted to pine plantations. Productivity of forest sites is increased by applying nitrogen, often in combination with phosphorous (Lauer and Zutter 2001). Fertilizers are generally applied at the time of establishment and again at midrotation.
Fertilization produces several wildlife benefits. Most plant species on the site respond to increased nutrient levels, creating more browse and more fruit production. These effects, however, are usually short-lived, because stands generally reach canopy closure sooner with fertilizers than without. Responses usually last only two to three growing seasons (Dickson and others 1995).
Forest plantations are not all created equal. They take many forms, depending on intensity of management, species being managed, and site. Like any other plant community, a plantation is affected by hydrology, topography, and climate. Plantations range from loblolly pine plantings on old fields to hardwood fiber farms that are irrigated and fertilized. Well-managed plantations on good sites often produce vastly greater yields than natural stands. Operations in these stands are straightforward and relatively easy. Although plantations produce wood rapidly, the ecological consequences, described below, can be very large.
Plantations established on clearcuts retain biological legacies from the old stand in the form of seed left in place and rootstocks that have the potential to sprout. Plantations established on old pastures or agricultural fields tend to contain mainly pioneer species. Ecologically, plantations established after timber harvests tend to mimic secondary succession, while those established on old fields are more similar to primary succession. In the Mississippi Alluvial Valley, stands originating on abandoned agricultural fields contain plant communities similar to those originating from primary succession on river bars (Baker 1997). Similarly, cottonwood plantations tend to have species composition similar to natural cottonwood stands of river front origin.
Natural primary succession tends to establish stands of a single species. In a landscape mosaic, these stands provide many positive values for wildlife. These stands are usually short-lived and provide structure, cover, and food for a variety of wildlife species. On heavy clay sites that are frequently flooded, pure black willow stands provide many benefits for waterfowl. Invertebrate production is great, cover is dense, nest cavity formation is high, and temperature fluctuations are moderated. Investigators have demonstrated that ambient winter temperature is higher in black willow stands than elsewhere. As these stands break up naturally, longer lived species take their place, providing structural components that are favorable for many migratory bird species. These stands grow rapidly during stand initiation, providing vertical structure sufficient for bird use within 2 to 5 years. Birds are a major dispersal mechanism for oaks (Hodges 1997), and as bird use increases in new stands due to increased vertical structure, oaks seed dispersal is increased.
In the South, primary succession takes place when new land is formed by river movement. In other parts of the world, it may take place after volcanic or glacial activity. Primary succession does not generally occur on sites where pine plantation establishment is the main objective. Although forest monotypes occur naturally in the South, they are restricted to hardwood species along river and stream corridors. On upland sites, where these situations exist, they must be artificially created and maintained. Even in instances where severe fires have taken place in the uplands, biological legacies still exist and no new lands have formed.
Wildlife species that thrive in early successional habitats use plantations heavily during the first few years after planting (Wigley and others 2000). Browse is abundant and species such as white-tailed deer, eastern cottontails, and black bears frequent young plantations. Small mammals also use these areas heavily; consequently, raptor use is high. Several neotropical migratory bird species use plantations early on, when insects and seeds are abundant. After canopy closure takes place, plant diversity decreases and wildlife use declines.
When plantations are first established on previously forested sites, water, nutrients, and sunlight are plentiful, supporting diverse and abundant plant communities. Even though sites are mechanically prepared and competing vegetation is usually controlled with herbicides, other plant species are still able to survive. Many wildlife species benefit from the grasses and forbs that are present on these sites during stand initiation. As the planted crop trees mature, they shade out competing vegetation, reducing plant diversity and structural complexity. As a result, soft mast, browse, and cover are reduced. Subsequently, fewer wildlife species find these sites suitable after canopy closure.
Loblolly pine is the most common plantation-grown species in the South. Its wood has desirable properties, it grows rapidly, and it is easy to establish. That is why it is the species of choice over much of the Southeastern United States. Slash pine is also a common species for plantation management. It is similar to loblolly pine in most characteristics, and cultural practices are also similar.
Plantations may be established in a variety of ways, but they all begin with some form of site preparation. Site preparation may be as simple as removing the old stand from the site, or as intensive as chopping, windrowing, burning, ripping, bedding, and fertilizing. Site-preparation treatments are designed to give the crop trees a competitive advantage over competing vegetation. On the Coastal Plains and Piedmont, ripping and bedding are common practices, despite high costs. Seedling survival is enhanced with these practices, as is rapid early growth of planted stock. Herbicides are commonly used when sites are ripped and bedded, and are effective in reducing competition.
In managed pine plantations, positive aspects for some wildlife species are gains in vertical structure in a short time period and rapid provision of cover. Negative aspects are reductions in time until canopy closure and subsequent shading of competing vegetation (Dickson and others 1995, Wigley and others 2000). In plantings on clearcut sites, downed wood is usually abundant and in some cases snags are left. Snags left standing may present a danger to loggers, but they provide great benefits to cavity-nesting wildlife species.
Pruning is common in the West Gulf region, where production of high-quality products like poles or lumber is the goal. Many plantations are pruned to produce clear, knot-free wood on the bole in a shorter period of time than without pruning. Pruning is usually done after a thinning and has the potential to positively impact many wildlife species. It has the potential to increase canopy complexity and increase understory vegetative growth. It also increases amounts of dead wood on the forest floor, providing habitat for small mammals and increasing organic carbon levels in the soil. Use of these stands by some hawks and owls may be increased due to greater visibility and increased numbers of small mammals.
Bird use in young plantations is generally high until the canopy closes about 10 to 12 years after establishment. Use declines because there is no canopy stratification and understory vegetation decreases. Leaving mature trees in the stand creates a structural element that has the potential to greatly increase bird use, but the residuals slow the growth of crop trees where shading occurs. Structural diversity is created in the stand on two levels (Dickson and others 1995).
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content: Jim Baker and Charles Hunter |
created: 4-OCT-2002 |