Biodiversity1, or biological diversity, can be defined as the number and variety of plant and animal species, genes, ecosystems, and ecosystem functions in a specific area. Habitat and habitat diversity are particularly important concepts for forest managers and owners to understand to maintain biological diversity in bioenergy production areas. Habitat1 is defined as the natural environment of a plant or animal and is characterized by the quality and quantity of food, water, shelter, cover and space available to a particular species. Habitat diversity refers to the complexity, distribution, and abundance of different plant and animal habitats in a given area.
Forest managers who manage for bioenergy production systems will have a direct effect on habitat and habitat diversity. This is primarily due to manipulation of the vegetation, soil, water, and temperature of a given site. Bioenergy-related activities have the potential to decrease the quantity and quality of habitat available at both stand and landscape levels. Activities of concern include harvesting of trees and deadwood for energy feedstock, increasing road networks, more frequent interventions into the forest (especially during times of the year when wildlife are most vulnerable, e.g. breeding, nesting and calving seasons), land use changes, and creation of protected areas. Foresters and landowners must be aware of the effects of bioenergy production on biodiversity, and ways any negative impacts can be mitigated or minimized. Table 1 provides a concise summary of the potential effects of bioenergy production on forest biodiversity.
Ensuring adequate habitat area and connectivity throughout forested landscapes is one way to mitigate against extinction and cultivate biological diversity. Species requirements for habitat vary in terms of scale and their specific demand for food, shelter and reproduction over time. Managers must consider the relative value of a large number of individual habitat areas versus a single large habitat area. Both may be necessary to conserve biodiversity across a number of species. However small the habitat requirements of a species, managers must understand that small habitat areas can lead to problems such as inbreeding depression, overpopulation, and overuse of the habitat area. These same problems can be created if the distance between areas is too large and the terrain is too rough. Species must be able to freely move from one habitat area to another. Vegetation quality and water quality are also important for biodiversity. Habitat area, connectivity and forest cover type, along with other factors that affect the amount of food and shelter available, affect the quality of habitat available for species.
Several conservation techniques and perspectives can maintain biodiversity across forests managed for bioenergy. These include the landscape perspective, the umbrella species concept, and manipulation of structural complexity and spatial configuration. Using these techniques, forests can be managed for both biomass production and wildlife attributes.
Using the landscape perspective1, forest managers get a complete picture of how a small stand fits into the larger landscape. This approach allows managers to evaluate the amount of habitat area over a larger area, rather than just one stand. This is extremely important for species requiring large habitat areas.
The umbrella species concept1 allows forest managers to manage for one species, yet maintain habitat for several species. To use this technique, managers identify one species that has particular habitat requirements, and whose presence and population health is highly correlated with other species. By managing the habitat area for the target “umbrella species,” managers can successfully maintain habitat for several species by association.
Biodiversity can also be maintained in managed forests by manipulating stand structure and spatial configuration2. There are three ways to manage stands for biodiversity: 1) structural retention at the time of regeneration harvest; 2) management for creation and/or maintenance of structural complexity; and 3) management for long rotation periods.
Table 1. Potential Environmental Impacts of Bioenergy Harvesting on Forest Biodiversity
When harvesting biomass for bioenergy, retain suitable habitat areas such as large trees, snags, standing deadwood and downed woody debris (DWD) (Image 1)2. While some of this material may be attractive for bioenergy production, if your management goals include wildlife conservation, evaluate large, dead woody material for its value as habitat before harvest1. Generally, large trees and deadwood are important wildlife habitat and should be maintained. They are also the precursors of downed woody debris, a component of forest ecosystems necessary for the survival of many species of fungi, insects and small mammals. Retaining suitable amounts of these materials have additional benefits for soils and hydrology (see relevant fact sheets). However, evaluate each stand with landowner management objectives in mind.
Image 1. Deadwood is important for maintaining habitat. source: C. T. Smith, IEA Bioenergy Task 31
Managing stands for structural complexity and biodiversity
There are several ways to manage stands while ensuring biodiversity. The first method is through thinning and harvesting to produce stands with a more complex structure, including mixed ages of trees, which favor biodiversity2. Small-diameter trees can be removed during biomass harvesting, decreasing competition for remaining trees and opening small gaps on the forest floor in which undergrowth can thrive. This can provide suitable habitat for birds and mammals that prefer low brush cover over standing timber. The second method is to create suitable habitat, such as installing nest boxes (Image 2) for red-cockaded woodpeckers. You can also create and maintain suitable habitat through the use of buffer zones3. Buffer zones along streamsides (streamside management zones) protect aquatic species while also maintaining water quality. Buffer zones can also be located along forest edge to protect terrestrial species. These zones are areas that are not harvested and generally placed near or between harvested areas. Native species, both plant and animal, can also be introduced into a stand in order to ensure biodiversity.
Image 2. Install sest boxes to create habitat. source: U. S. Fish and Wildlife Service
Long rotations are another technique that can be used to create structurally complex stands2. Long rotations allow for the growth of old, large trees that are important species habitat. Rotation length can be extended by 50 to 300 percent. While this is not an attractive option to many managers and landowners with goals of timber production or possibly biomass production, it can be useful in buffer zones not intended for harvesting. For individuals with specific management goals for wildlife conservation, long rotations may be of particular value.
Careful consideration of habitat conservation will ensure that species are maintained for future generations while providing raw materials to produce bioenergy and other bio-based products. In stands managed for bioenergy feedstock, habitat preservation may require special attention as biomass utilization has the potential to remove essential habitat. For example, in more mature forests, it is important to retain old, large, and even dead trees for habitat. Also, during harvest one may not want to remove all residues but leave some on site for bird and small mammal habitat. While habitat preservation is important, remember that habitat can also be created by careful planning of biomass harvests. For example, thinning small diameter trees can open the forest canopy and allow for the growth of low, herbaceous cover favored by turkey and quail in the South. Assessing stand structure, biomass availability and species requirements in forest management plans and applying Adaptive Forest Management procedures can help forest managers to protect biodiversity while achieving other objectives, such as the provision of bioenergy feedstock, timber, watershed protection and recreation.
1 Angelstam, P.; Mikusinski, G.; and Breuss, M. 2002. Biodiversity and forest habitats. In: Richardson, J.; Bjorheden, R.; Hakkila, P.; Lowe, A.T.; and Smith, C.T., eds. Bioenergy from Sustainable Forestry: Guiding Principles and Practice. Dordrecht, The Netherlands: Kluwer Academic Publishers: 216-243.
2 Lindenmayer, D.B.; Franklin, J.F. 2002. Conserving forest diversity: A comprehensive multiscaled approach. Washington: Island Press: 351 p.
3 Russell, K.R.; Wigley, T.B.; Baughman, W.M.; Hanlin, H.G.; and Ford, W.M. 2004. Responses of Southeastern amphibians and reptiles to forest management: A review. In: Rauscher, H.M. and Johnsen, K., ed. Southern Forest Science: Past, Present, and Future. GTR-SRS-75. Asheville, NC: USDA Forest Service Southern Research Station: 319-334.