Supporting article W: Understanding the importance of forest buffers next to streams to Plant and Animal Communities
Julia C. Klapproth, Faculty Assistant-Natural Resources, Maryland Cooperative Extension; James E. Johnson, Extension Forestry Specialist, College of Natural Resources, Virginia Tech
The riparian area is that area of land located immediately adjacent to streams, lakes, or other surface waters. Some would describe it as the floodplain. The boundary of the riparian area and the adjoining uplands is gradual and not always well defined. However, riparian areas differ from the uplands because of their high levels of soil moisture, frequent flooding, and unique assemblage of plant and animal communities. Through the interaction of their soils, hydrology, and biotic communities, riparian forests maintain many important physical, biological, and ecological functions and important social benefits.
Riparian areas in the eastern United States are among the most productive biological systems in the world (Dickson and Warren 1994). Rich soils, abundant moisture, and regular inputs of nutrients and biological materials result in a complex natural community. The loss of native riparian vegetation can result in a loss of habitat for many species of animals, both on land and in the stream itself.
Riparian Plant Communities
Riparian areas support some of the most diverse and productive of all plant communities. This is primarily a result of the rich soils and abundant moisture. Readily available water and productive soils support a greater plant biomass than is usually found in upland areas, resulting in forests with a wide variety of species and complex vertical structures (LaRue and others 1995).
The diverse plant community is also a function of regular disturbance. Disturbances, both chronic (predictable, occurring monthly to yearly) and episodic (not predictable, usually occurring decades or centuries apart), are common to the riparian area, due to floods, fire, wind, and pests (Gregory and others 1991, Bohlen and King 1996). These disturbances can produce large-scale changes in the plant community, or smaller patchy clearings scattered about the floodplain. As a result, young growth and mature vegetation are often found growing close together.
Disturbances contribute to the highly variable topography, hydrology, and soils found in riparian areas. It is not uncommon to find deep deposits of soil and gravel, rock outcrops, spring seeps, and wet, mucky areas all within the same floodplain. This range of physical habitats each supports distinct vegetation.
Landscape position also adds to the complexity of the riparian plant communities. Because of their location between the uplands and the stream, there is regular movement of nutrients, sediment, organic matter, and living organisms across the three environments (Gregory and others 1991, Nilsson and others 1994). Their landscape position also means that riparian areas support large amounts of “edge” habitat, both along the stream and at their borders with adjacent uplands.
Recent studies have documented the vegetative diversity of mid-Atlantic riparian areas. Hedman and Van Lear (1995) examined the vegetative characteristics of Southern Appalachian riparian forests in different stages of succession. They found that the species composition of riparian forests underwent change through time in relation to shade tolerance and the adaptability of the species to disturbance. Dominant overstory species in early- and mid-successional stands were tulip poplar, birch, white basswood, and black cherry. Late-successional and old growth stands were dominated by hemlock, white pine, and oak. They identified increasing abundance of rhododendron in the understory as a problem for the regeneration of other species. Along the Susquehanna River Valley of southeastern Pennsylvania and northeastern Maryland, Bratton and others (1994) documented the importance of riparian areas in maintaining populations of mesic forest floor herbs. They recommend the preservation of a “key number of locations with key microhabitats,” particularly “the mouths and banks of the larger creeks, minor tributaries on high base rock, and the more extensive areas of the floodplain.”
Riparian Animal Communities
Riparian areas provide critical habitat for many types of wildlife, because of their diverse and productive plant communities, complex structure, and close proximity to water. The wildlife may be permanent residents of the riparian area or occasional visitors that use the area for food, water, or temporary shelter.
Food. Food availability varies with the type of vegetation in the riparian area, but includes fruits, seeds, foliage, twigs, buds, and insects and other invertebrates. Trees and shrubs produce a variety of foods that are eaten by many animals, and may be especially important sources of nutrition during the winter months. Grasses and herbaceous vegetation provide seeds and forage both within the riparian area and along the forest border.
Water. The stream environment provides moving waters for many animals to drink, feed, swim, and reproduce. Water is also available on the moist vegetation and in the shallow wetland pools and backwaters common to many riparian areas. These areas, both permanent and temporary, are especially important for amphibians and macroinvertebrates (Clark 1978).
Cover. Riparian areas provide a sheltered environment for many species of animals to feed, rest, and reproduce. Animals use these areas to seek shelter from extremes of weather and to escape predators and human activity (Compton and others 1988, Johnson and Beck 1988). Riparian areas may also provide important travel corridors for some species, and are frequently used as stopover points for migratory birds.
Factors That Influence Wildlife Use of Riparian Buffers
Although riparian areas can support many types of wildlife, the importance of a particular riparian area will depend on the surrounding land uses, the vegetation actually present, and species.
Landscape setting. In areas of intensive agriculture, forested riparian areas can provide important “islands” of wildlife habitat. Here, species that depend on trees and forests for their survival can live and reproduce. Areas adjacent to riparian forests offer supplementary habitat by providing additional foods, nesting and roosting sites, and cover. Riparian forests are also important habitats in urban areas where they are among the only remaining natural areas for wildlife. However, the animals are not always riparian-dependent species; rather, they may be making use of habitat that is otherwise lacking.
Riparian buffers can also be important to wildlife in forested settings. Today, most forest harvests incorporate the use of Best Management Practices (BMPs) and maintain Streamside Management Zones (SMZs) during forest harvest operations. Many studies have shown the importance of SMZs to wildlife, particularly when associated with clearcuts, and indicate that they are important reserves for maintaining wildlife (Darveau 1996).
Activities within the immediate vicinity, as well as upstream and downstream from the buffer can detract from wildlife use. These activities include the presence of industrial operations, urban development, pollution, recreational activities, roads, and other uses.
Vegetative characteristics of the buffer. The type of vegetation growing in the buffer affects its usefulness to wildlife through the availability of food, foraging and nesting sites, and other habitat needs (Johnson and Beck 1988). The more diverse the habitat, the greater its utility to many species of animals. For example, in Iowa, biologists found a greater abundance and greater number (50) of species of birds in woodland edges around cornfields than in grass/herbaceous edges (23 species) (Best and others 1990).
In Charlotte County Virginia, wildlife biologists studied bird communities along channelized streams where all the woody vegetation had been removed from the streambank (Ferguson and others 1975). They found an increase in bird species diversity and density and more breeding birds as shrubs and trees begin to regenerate the site.
Complex habitats can support more animals because animals partition their habitat very precisely, feeding and nesting only in certain sites within the environment. For example, in riparian areas of the Pacific Northwest, the presence of small mammals correlated with very specific habitat features: certain species occurred on sites where there was low to moderate cover, while others preferred overgrown thickets; some mammals preferred deciduous cover, some evergreen cover; and some mammals were only found where there were abundant snags and decayed logs (Doyle 1990). Burrowing mammals preferred areas with high organic soils. Other small mammals were associated more with the presence of a particular prey species than with specific vegetation. Likewise, researchers have documented distinct habitat preferences among reptiles and amphibians in Kentucky (Pais and others 1988).
Animals respond to the particular species of vegetation present, as well as the conditions created by the vegetation: moderated temperatures; high humidity; moist, loose soils; diversity of canopy layers; and availability of nesting and denning sites. Therefore, animals find riparian areas useful not only because of the presence of water, but also due to the variety of available habitats.
Wildlife species of concern. An animal’s specific requirements for food, water, cover, and territory will ultimately determine its use of a particular riparian area. Many studies have been made on wildlife use of riparian forests.
A brief summary follows:
Large mammals. Mature riparian forests can provide refuge for large animals, particularly when large tracts of forest are otherwise lacking. In south Florida, for example, the cougar is occasionally found in remnant bottomland stands (Dickson and Warren 1994). Black bears may also be found in riparian areas, particularly where there is brushy cover for hiding and mature hardwoods for denning and mast production (Oli and others 1997). White-tailed deer make use of the areas for forage and cover (Compton and others 1988, Dickson and Warren 1994). Other mammals commonly associated with riparian forests are beaver, mink, muskrat, river otter, and raccoon.
However, in order for the riparian area to be useful to animals with large territorial requirements, it must be large or connect to other large tracts of contiguous forest. Mammals that use the riparian area for only part of their needs (such as white-tailed deer) and animals with smaller space requirements can make use of smaller riparian areas.
Small mammals. Doyle (1990) examined the use of riparian areas by small mammals in the forests of the Pacific Northwest, and found that riparian areas had more small mammals (shrews, mice, voles, chipmunks, northern flying squirrels, and ermine) than adjacent uplands, and that many species from the riparian area weighed more and included a greater number of adults in breeding condition. She suggested that riparian areas provide superior habitat for small animals because of greater availability of water, forage, and invertebrates; loose, friable soils which facilitate burrowing; and more stable temperatures. In the Southeast, researchers found that hardwood SMZs were important components of gray squirrel habitats in pine and mixed pine-hardwood stands (Fischer and Holler 1991). The availability of mast, cavities for nesting, diversity of trees, tree canopy development, and distance to water and cultivated crops were also important habitat features. However, in agricultural areas of Iowa, researchers found that small mammals (mice, shrews, voles, eastern chipmunks, and ground squirrels) preferred grazed, grassy riparian areas to deciduous floodplain forests or upland forests because of the greater variety of food and cover in the grazed areas (Geier and Best 1980).
Studies that have attempted to determine optimal riparian buffer widths for small mammals have produced conflicting results. Dickson and Williamson (1988) assessed the use of hardwood SMZs by small mammals in forest clearcuts and found that there were significantly more small mammals in narrow SMZs (less than 82 feet) than in wider SMZs. They attributed this to dense, brushy vegetation, abundant seeds and forage, and dense logging slash found in the narrow zones. Tappe and others (1994), however, found that the width of hardwood SMZs had little effect on small mammal abundance, richness, or diversity in managed pine stands of the Ouachita mountains of Arkansas. Rather, it was the structure of adjacent pine stands which determined the presence of small mammals – SMZs along young pine plantations had the greatest abundance of small mammals, while SMZs in closed canopy plantations had the lowest number of individuals.
Reptiles and amphibians. Rudolph and Dickson (1990) evaluated SMZs of various widths in eastern Texas and found a wide variety of reptiles and amphibians in SMZs greater than 98 feet wide but few in SMZs less than 82 feet wide. However, there were significant differences in the vegetative structure of the SMZs. Narrow SMZs had dense shrub and herbaceous vegetation, while wider SMZs had a well developed overstory and midstory canopy, sparse understory vegetation, and abundant leaf litter.
In New England, researchers reported a greater abundance of reptiles and amphibians in streamside forests than in upland forests in three different forest cover types: red maple, balsam fir, and northern hardwoods (DeGraaf and Rudis 1990). However, the greatest differences in species abundance and diversity occurred between the coniferous and deciduous forest types, with both hardwood types supporting more species.
Birds. A number of studies have examined how birds use riparian areas, in both agricultural and forested settings.
Studies of bird communities in areas of intensive agriculture suggest that riparian areas, shelterbelts, and small woodlots are very important habitats for birds. For example, Croonquist and Brooks (1991) evaluated bird use of riparian areas in central Pennsylvania and observed that even very narrow riparian strips (7 feet) significantly increased the number of birds in the area. However, “area-sensitive species” were not found unless there was a corridor of at least 82 feet on both sides of the stream. They recommended a 410-foot buffer of natural vegetation to “support the full complement of bird communities in the area,” although they suggested that protecting at least 82 feet of riparian habitat would provide dispersal and breeding opportunities for many birds. Keller and others (1993) evaluated bird use of riparian forest buffers in agricultural areas of the coastal plain of Maryland and Delaware. They found “the presence of even a narrow riparian forest dramatically enhances an area’s ability to support songbirds compared to a stand surrounded only by agricultural fields or herbaceous riparian vegetation.” Riparian forests less than 328 feet wide were dominated by short-distance migrants, while forest buffers wider than 328 feet had more neotropical migrant species, and these continued to increase in numbers but much more gradually in forests wider than 656 feet. The number of resident species was not related to the width of the riparian forest. Stauffer and Best (1980) studied bird use of riparian forest buffers surrounded by row crops and hay fields in Iowa, and found that riparian woodlands supported higher densities of birds than either upland woodland or herbaceous buffers. Bird species richness increased with increasing buffer width. However, species gains were also associated with other habitat features, such as snag size, number of canopy layers, sapling/tree size, and diversity of vegetation.
Studies conducted in Virginia, Kentucky, Georgia, Arkansas, Texas, and Canada on bird use of SMZs in managed forests all support the practice of leaving hardwood corridors along streams during forest harvest operations (Holbrook and others 1987, Triquet and others 1990, Tappe and others 1994, Darveau and others 1995, Dickson and others 1995, Hodges and Krementz 1996). However, their recommendations for SMZ width ranged from 98 to 328 feet, and depended largely on the species of interest. For example, narrow buffers (less than 82 feet) are used primarily by “edge” species and those associated with young, brushy, or open stands, such as yellow-breasted chat, indigo bunting, orchard oriole, eastern kingbird, common yellowthroat, blue grosbeak, and prairie warbler. Wider buffers (164 feet or more) attract birds that commonly breed in mature forests, such as yellow-billed cuckoo, Acadian flycatcher, tufted titmouse, Carolina wren, red-eyed vireo, and others. Recommendations for buffers 300 feet or larger were targeted to “area-sensitive” forest interior dwelling birds.
Recently, two studies examined bird use of riparian areas in forested settings in the mid-Atlantic region (Croonquist and Brooks 1993, Murray and Stauffer 1995). They found no significant difference in species diversity or abundance of birds between riparian areas and adjoining upland forests. However, Murray and Stauffer (1995) found that in southwest Virginia, two species, the Acadian flycatcher and the Louisiana waterthrush, showed a strong association with streams and may be considered riparian-dependent species.
Two Issues of Controversy: the Desirability of Edge Habitat and the Importance of Wildlife
Due to their long, linear nature, riparian areas create abundant edge habitat, an area considered to be highly productive for many wildlife species. However, not all wildlife is suited to edge habitat, and the deliberate construction of large amounts of edge has been contested by some wildlife biologists. They are concerned that an abundance of edge may cause reproductive failure, restriction of range, loss of genetic variability, and mortality for species that have very specific habitat requirements (Harris 1988, Wigley and Roberts 1997). In urban areas, edge effects may be more pronounced (Adams and Dove 1989). Buffers surrounded by commercial, residential, and industrial development are frequently occupied by a large number of avian predators, such as brown-headed cowbirds, raccoons, and domestic animals, as well as exotic plant species.
Riparian forest buffers have also been promoted because they can serve as corridors for wildlife movement. The presence of corridors is believed to be especially important to reptiles, amphibians, less mobile birds and small mammals, and for the young as they establish new territory (Clark 1978, Machtans and others 1996). However, some scientists suggest that corridors may hinder native wildlife populations because they can enhance the spread of contagious diseases, fires, predators, and exotic species, and may promote the movement of generalist species at the expense of area-sensitive species. There is also debate about whether the corridor is necessary to maintain genetic diversity within a population (Wigley and Roberts 1997).