Poster presented at BioEnergy '98: Expanding Bioenergy Partnerships, Madison, Wisconsin, October 4-8, 1998. Sponsored by the Biofuels Systems Division, U.S. Department of Energy, under contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corporation

ABSTRACT

Biomass crops offer considerable potential as habitat for wildlife while providing the feedstocks for energy. Studies of small (approx. 4 ha) clonal trials of hybrid poplar in the upper Midwest began first in 1992 and later on larger plantings to quantify the potential of short-rotation woody crops (SRWCs) as habitat for wildlife. These surveys have shown that both breeding birds and small mammals use the plantings more extensively than row crops but that both the numbers of species and individuals using the plantings were less than those using forested areas. Habitat sensitive birds and birds of management concern were noticeably absent in these plantings, and several species found in adjacent natural forest/shrub habitat adjacent to the trial plots were absent in the tree crop stands. The Midwestern studies provided the basis for additional surveys of breeding bird use of hardwood tree crop plantings across an array of planting ages (5-20 years) and sizes (10-100+ ha) that were conducted in 1996-1998 in the southeastern coastal plain. Breeding bird use of tree crops (sweetgum and sycamore) and other land uses (i.e., naturally regenerating hardwoods, pine plantations, and agricultural fields) was compared. These surveys show that the numbers and species of breeding birds using the two tree crops were neither substantially different from each other, nor from the naturally regenerating forests. The age of the stands was the primary determinant of breeding bird assemblages. Young plots were dominated by early successional species which were replaced by forest-edge and forest-interior species in the older stands. Stand area and tree crop species had only minor influences on breeding bird assemblages. Species of management concern were detected in both young and old plots, and a majority of bird detections consisted of neotropical migrants. These surveys show that short-rotation woody crops can play a role in providing habitat for wildlife, particularly if these crops replace land uses that provide less diverse habitat and thus are of poorer quality than natural systems (i.e., agricultural lands).

Keywords: environment, diversity, birds, wildlife, biomass crops, short-rotation woody crops

INTRODUCTION

Short-rotation woody crops (SRWCs) continue to show promise as feedstocks for energy and fiber production. Hardwood tree crops are being developed by the United States Department of Energy’s (DOE’s) Bioenergy Feedstock Development Program (BFDP) as feedstocks to reduce America’s use of fossil fuels (Wright et al., 1993). An analysis of the potential land for biomass crops has identified 158.6 million hectares capable of supporting these crops nation-wide (Graham, 1994). Over 90% of this potential land is currently being used for agriculture or related land uses. Large scale conversion from agricultural crops to intensively managed biomass plantings could have dramatic local and regional implications for the environment, including wildlife (Tolbert and Schiller, 1996; Cook et al., 1991). Concerns involving wildlife use of biomass crops need to be addressed prior to the incorporation of these crops into the landscape in order to ensure acceptance by producers and consumers. The BFDP continues to develop an information base regarding how wildlife could be impacted by large-scale conversion of lands to energy feedstock production.

Prior to 1992, little information was available in the open literature regarding wildlife use of short-rotation woody crops (but see Verch, 1986). The BFDP initiated studies on young hybrid poplar (Populus sp.) stands in the upper Midwest in 1992 to quantify the potential of short-rotation woody crops to provide wildlife habitat. Limited site availability in the upper Midwest prohibited comparisons to be made between 1) wildlife use of hardwood plantings of greatly different sizes, 2) between different tree crop species in the same region, and 3) between various habitats available to landowners in a different geographical region. The Midwest studies were therefore supplemented by additional studies conducted in the southeastern coastal plain region in 1996-1998.

The research in the Southeast was completed on lands owned by Union Camp Corporation that are used for production of short-rotation trees utilized in their fiber operations. Two tree species grown by Union Camp-- sweetgum (Liquidambar styraciflue) and sycamore (Platanus occidentalis)-- are also potential feedstocks for energy in the Southeast. The broad array of planting sizes and acreages present on the Union Camp lands allowed us to examine breeding bird use of single-species stands across a broader and more representative spectrum of stand charateristics than was possible in the Midwest study. A complete life-cycle analysis of short-rotation crops was conducted, and breeding bird use of two tree crops (sweetgum and sycamore) of varying ages and sizes were compared with alternate land uses (e.g., agricultural fields, naturally regenerating hardwood forest, and planted pine) of analogous age and spatial characteristics. Information from these studies are being incorporated into a model intended for use by land managers. When completed, the model will graphically display responses of birds to land-use decisions across a virtual landscape, and will provide a tool that can be used to optimize landscape-level management so that wildlife benefits of biomass crops can be maximized.

METHODS

Abundance and species composition of small mammals and breeding birds that utilized Populus stands in the upper Midwest were surveyed in 1992 and 1993 by Christian et al. (1997). Study plots were established across Minnesota, Wisconsin, and South Dakota on eight Populus stands of approximately 5 years of age and 3-5 ha, as well as on adjacent or nearby habitats. Small mammals and birds were sampled by snap trapping and a line-transect technique, respectively. Additional sites were added to the upper Midwest study in 1994 after establishment of the scale-up plantings near Alexandria, MN (Kroll and Downing, 1995; Tolbert and Downing, 1995). These plantings were established in either agricultral or mixed forest/agricultural landscapes. Relationships between bird assemblages and landscape context as well as plantation habitat were analyzed.

In 1996-1998, study sites were established in the southeastern U.S. within a 50 mile radius of Franklin,Virginia. Bird use was measured by conducting bounded point counts on intensively managed sweetgum and sycamore plantings. Sites of various ages (5-21 years) and sizes (7-100+ ha) were selected in order to encompass conditions predicted to occur on future large-scale biomass plantations. Vegetational characteristics were also measured at each sampling point in order to link bird use with habitat qualities.

Planted pine and naturally regenerating hardwood forests with the same approximate age and areal extents, and agricultural fields (soybeans) were also selected and surveyed for comparisons to tree crops. These cover types are prevalent in the area, and are the main alternative land uses to growing tree crops in the region. We extended the age comparisons of natural forests beyond the age at which tree crops are expected to be harvested (generally about 20 years) to include older natural forests (39-43 years of age). These “mature” natural stands were included as a means of comparing bird use of tree crops to the most pristine analogous habitat in the area.

This paper summarizes the results from the earlier studies in Minnesota (Christian et al., 1997; Christian et al., 1998; and Hanowski et al., 1997), and provides a more in-depth report of our results of breeding bird use of tree crops in the Southeast within the framework of other studies (Verch, 1986; Hoffman et al. 1993, 1995). These are the only known studies to date in the U.S. that have focused on wildlife use of intensively grown and managed woody crops. By investigating bird-habitat relationships within woody crops, we hope to provide information to land managers that will allow them to manage short-rotation stands in a way that maximizes benefits to birds and other wildlife while meeting feedstock needs for energy and/or fiber.

RESULTS AND DISCUSSION

Wildlife Use of Tree Crops in the Midwest: A Summary of Previous Findings

Christian et al. (1997) found that mammal species richness and abundance were lower on Populus plantings than on adjacent wooded and non-wooded wildlands, but higher than on lands covered by rowcrops and small grains. The prairie deer mouse (Peromyscus maniculatus bairdii) and the white-footed mouse (Peromyscus leucopus) were captured at higher than expected frequencies on tree crop plantations, but all other species exhibited depressed capture rates in relation to their representation in the overall study. The presence of ground cover was determined to be the most important factor for small mammals utilizing Populus sites. Areas that contained vegetative or debris cover supported small mammal assemblages that were more similar to that in the overall landscape than to sites lacking cover.

Higher numbers and species of birds were detected in Populus plantations than in rowcrop/small grain fields and pasture/hayfield cover types, but lower than in wooded wildlands (Hanowski et al., 1997; Christian et al., 1998). Species composition of birds in the plantations was controlled by habitat changes in the tree stand as they develop from open-field habitat in the first year to a shrub-like/successional habitat in years 2-4, and finally reaches a tree-like habitat from year 5 following canopy closure until the stand harvest after 7-9 years. Bird species detected on tree crop plantings in the Midwest were primarily nonspecialist birds that were common throughout the landscape. Several grassland and forest bird species present in nearby natural settings were absent in Populus stands. Bird communities using the poplar plantings were dominated by short-distant migrants (Hanowski et al., 1997). Long-distance migrants and area-sensitive birds were not common in plantings, and were detected less frequently than in adjacent natural lands.

Bird Use of Tree Crop Stands in the Southeast

In the Midwest, the lack of habitat-sensitive birds such as neotropical migrants and species that are generally associated with forest interior conditions within Populus stands was possibly attributable to the stands’ relatively young age and small size (Christian et al., 1997 and 1998). Hoffman et al. (1993) surveyed harvest-age plots of Populus stands in the Pacific Northwest and found that these older plantings supported forest-interior neotropical migrants. These sites were small-scale (< 5 ha) plots surrounded by extensive forests, however, and not truly representative of large-scale biomass plantations. The study conducted in the Southeast was intended to overcome limitations of site characteristics present in the earlier studies, and to develop a better understanding of bird use of treecrops across the full spectrum of habitat conditions present during a tree planting life-cycle.

AGE, PLANTING SIZE, AND TREE SPECIES COMPARISONS. Breeding birds were found to use tree crops extensively in the Southeast. Forty-nine species of birds were detected in the sweetgum and sycamore plantings. The age of a site determined the primary species composition of the bird assemblages. Younger tree crop plantings were strongly dominated by successional forest bird species (Figure 1) such as Common Yellowthroat (Gelthlypis trichas), Yellow-breasted Chat (Icteria virens), and Indigo Bunting (Passerina cyanea). The older plantings supported a dramatically different assemblage of birds than the younger plantings; species associated with forest-edge and forest-interior conditions such as Acadian Flycatcher (Empidonax virescens), Ovenbird (Seiurus aurocapillus), and Red-eyed Vireo (Vireo olivaceus) were dominant in the stands that were >11 years old (Figure 1).

Figure 1. Distribution of birds detected in cover types by habitat pPreference. Cover types of similar ages are placed next to each other.

In addition to determining the dominant species of a given tree crop stand, age also controlled other aspects of bird use. Multiple regression analyses revealed that age and related habitat qualities were positively correlated age-with bird density and diversity. The greater three-dimensional structure of a mature SRWC stand could be responsible for the heightened bird use of older stands as a function of the structure becoming more complex throughout a tree crop’s life-cycle. The greater structural complexity would increase the niches available for a greater number and variety of organisms.

The species of tree crop generally did not influence bird use of short-rotation stands; species overlap between sweetgum and sycamore stands was very high (83%). Bird communities on large vs. small stands were remarkably similar, and no effects relating to the size of the stand were detected in statistical analysis when analysis was limited to tree crop surveys. This finding may be partly attributable to the high degree of forest cover nearby; tree stands that are isolated from other canopy cover tend to exhibit more pronounced size-effects than stands that are embedded in a forested matrix because of inter-stand immigration and dispersal (Freemark and Collins 1992).

SENSITIVE BIRD SPECIES. In contrast to the findings in the Midwest by Christian et al. (1997 and 1998), many birds that are particularly sensitive to habitat quality or are of management concern were detected in habitat provided by Southeastern tree crop stands. Neotropical migrants are of particular interest due to their rapid decline in numbers in recent decades (Terborgh, 1989). They are thought to be particularly susceptible to human-wrought changes in habitat quality, particularly fragmentation and absolute loss of breeding territories (Terborgh, 1989; Whitcomb, 1981). Christian et al. (1997) noted the lack of these birds in their plots in the upper Midwest. In the Southeastern study, however, neotropical migrants were dominant in all cover type categories (Figure 2).

Figure 2. Proportions of bird detections in each land use that are permanent residents, short-distance migrants, or noetropical.

Birds that were detected within tree crop plantings were not generalist species that are ubiquitous across the landscape. Rather, they tended to be either specialists of early-successional forests or mature forests depending on the age of the tree crop stand (Figure 1). Species that have suffered some of the greatest decreases in numbers and distribution breed in the interior of forests or in early successional habitats. These two habitats have been reduced in recent decades as a result of human activity (Terborgh, 1989; Thompson and Dessecker, 1997). Tree crop plantations, however, provide a spatially compressed source of both early successional and mature forest conditions that continuously cycle and replenish themselves as individual plots mature from saplings to closed-canopy stands that are harvested and then replanted.

BREEDING BIRD USE OF TREE CROPS VS. OTHER LAND USES. We found that agricultural lands are virtual deserts for breeding birds (Figure 3); surveys conducted on this cover type were excluded from additional analysis. Tree crops, as identified above, provided habitat for surprisingly high numbers and species of birds. Natural hardwood forests supported slightly greater densities of individuals and species than sweetgum and sycamore plantings (Figure 3), but overall bird use did not differ substantially from intensively managed short-rotation crops. Interestingly, when we compared populations of habitat-sensitive species (e.g., forest-edge and forest-interior species) in harvest-age (approx. 20 years) sweetgum and sycamore plantings with assemblages detected in the older, mature (approx. 40 years) natural hardwood stands, we found no significant difference between the two habitats.

Figure 3. Comparison of abundance and richness densities across cover types.

Breeding bird use of tree crops was similar, overall, to that of natural hardwood forests. Differences in bird use were determined to be a function of habitat variables at the sites themselves, which are not necessarily related to the cover type. Rather, regression analyses revealed that the age of the stand and age-related habitat factors such as canopy development and amount of vegetation cover on the forest floor were the governing factors that controlled bird use (Table 1). Older sites that were characterized by a well-developed canopy, the presence of snag/cavity trees, and high coverage in the herbaceous layer supported more diverse bird communities. Bird abundance was positively correlated with natural hardwood forest and greater vegetation cover on the forest floor. Also, the proportions of detections that were forest-interior specialists increased with larger-sized plantings when all cover types were pooled.

Because data for the full range of planting age and acreage for pine stands were just recently completed, analysis of bird use of planted pine stands is still in progress. Early indications are that birds use pine stands extensively, but not as much as naturally regenerating hardwood forests or tree crop stands. Some species such as Pine Warbler (Dendroica pinus) and Scarlet Tanager (Piranga olivacea) were detected at higher frequencies in planted pine than in hardwood cover types, and substantial pine components in the landscape matrix may be necessary for these species to reach their peak abundances.

MANAGEMENT IMPLICATIONS AND CONCLUSIONS

Bird communities were similar between hardwood cover types regardless of whether they were natural stands or managed tree crops. A substantial shift from birds associated with early successional/shrub habitat to birds associated with forest habitat occurred between younger (approx. 5 years) and older (approx. 20 years) stands. Because higher proportions of forest birds and forest interior birds were not detected in mature (approx. 40 years) hardwood forests than in older tree crop stands, very little change appears to take place in communities following canopy closure. Therefore, the amount of time that a tree crop remains within a landscape following canopy closure (at 7-10 years) can influence its value as habitat for forest dwelling birds. Land use management that maintains a variety of plantings with closed canopy can provide substantially greater value for breeding birds within a forested landscape than a series of younger-aged tree crops that are harvested immediately following canopy closure. Furthermore, staggering the location and timing of harvesting and planting cycles of tree crop stands within production areas would help ensure that plantings of an assortment of ages optimal for a wide variety of birds and other wildlife are continuously available to replace nesting sites/habitat that were harvested in the previous years.

Spatial organization in a landscape can help ensure that conditions are optimal for birds of special management concern. Because greater proportions of forest interior specialists were found to use larger plots when hardwood cover types were pooled, tree crop plantings should be established in large blocks that maximize interior conditions or established adjacent to remnant patches of natural forests to increase patch size. Establishment of short-rotation crops in large blocks could provide more extensive habitat for birds restricted to interior forest environments. Such a planting strategy also reduces habitat fragmentation and lowers the proportion of area that is exposed to edge conditions which are widely thought to be responsible for the decline of bird populations in the United States (Wilcove, 1985; Terborgh, 1989).

The decline of forest bird species coupled with the future potential for conversion of agricultural lands to short-rotation woody crops represents a unique opportunity to create or improve/supplement wildlife habitat in conjunction with the production of these crops as an alternative, renewable fuel and fiber supply. Even though single-species tree crops of an even age may lack the vegetational diversity of natural forests, our research shows that they can support a diverse assemblage of bird species.

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