Plant Species Diversity and Management of Temperate Forage and Grazing Land Ecosystems

sion illustrates the difficulty that current researchers face in obtaining consensus from this generality. More than a century since Charles Darwin stated that diverse Species diversity is a hot topic in grassland ecology regrasslands produce more herbage than monocultures, scientists still search because the reported benefits of biodiversity apdebate the relationship between species diversity and ecosystem function. Postulated benefits of diversity in experimental grasslands inpear to contradict the high productivity obtained from clude greater and more stable primary production along with more relatively few species (through high inputs of fertilizers efficient nutrient use. These benefits have been extrapolated to forage and chemicals) and from an increasingly narrow genetic and grazing land systems with little supporting objective data. Most diversity seen in current agriculture. Some research reinformation on the potential benefits of increased plant diversity sults indicate that increased plant species diversity incomes from studies of synthesized grasslands that have not included creases primary production in grasslands and benefits domestic grazing animals. We explore this debate relative to the manother ecosystem functions such as nutrient retention agement of temperate forage and grazing lands. Plant species diversity and resistance to weed invasions. refers to the number of species (richness) and their relative abundance These results and concepts have spilled over into other (evenness) within a defined area. Plant relations influence biodiversity areas (Brummer, 1998), such as forage and grazing land responses through positive (e.g., facilitation, N2 fixation, hydraulic lift) and negative interactions (e.g., competitive exclusion, allelopathy). research, and are beginning to influence management Early 20th century research on complex mixtures of forage species recommendations. For example, Tilman et al. (1999) (limited to grasses and legumes) for pasture indicated equivocal results suggested exploring the concept of high diversity grazing regarding benefits of species-rich mixtures and typically recommended lands for livestock production. Similarly, Minns et al. using the best adapted species in simple grass–legume mixtures. Re(2001) extrapolated results from the large BIODEPTH cent research indicates potential herbage yield benefits from species(BIODiversity and Ecosystem Processes in Terrestrial rich mixtures for pastures. Limited animal productivity research on Herbaceous Ecosystems) experiment in Europe to agspecies-rich mixtures indicates variable responses and much more ricultural management. It is not clear, however, how research is needed. Grazing land productivity is a primary focus for these results and concepts relate to managed forage biodiversity benefits because of the direct economic relevance to proand grazing lands. Most grassland studies reporting the ducers. However, taking a broader view of the multifunctionality of grazing lands to include environmental and aesthetic benefits to effects of plant species diversity on ecosystem functionhumans reveals a great scope for using biodiversity in grazing land ing have used cutting, fire, or in some cases no form of management. herbage removal, rather than grazing. Results under grazing could be very different because of the capacity of grazers to affect plant community diversity and structure C Darwin (1872) boldly stated that “It has (Bullock and Marriott, 2000; Rook and Tallowin, 2003). been experimentally proved, that if a plot of Early research on pasture management seemed to ground be sown with one species of grass, and a similar advocate relatively complex mixtures of grasses and leplot sown with several distinct genera of grasses, a gumes (Foster, 1988). During the 1950s, however, the greater number and greater weight of dry herbage can emphasis shifted to monocultures of grasses maintained be raised in the latter than in the former case.“ Unfortuby N fertilizers or simple mixtures of grasses and legumes nately, Darwin omitted details of the soils, climate, graz(e.g., one of each) and management of those mixtures ing pressure, and spatial scale that give the context for to maintain the legume component (Blaser et al., 1952; his statement on grassland species diversity. This omisDonald, 1963). Blaser et al. (1952) stated that complex mixtures of grasses and legumes were destined to fail because of seedling competition. Thus, forage and pasM.A. Sanderson and R.H. Skinner, USDA-ARS Pasture Systems and ture management research since that time focused on Watershed Management Research Unit, Building 3702 Curtin Road, University Park, PA 16802; D.J. Barker, Department of Horticulture monocultures or simple forage mixtures. and Crop Science, The Ohio State University, 202 Kottman Hall, 2021 Grazing lands are an important component of global Coffey Road, Columbus OH 43210; G.R. Edwards, Animal Sciences land use. Roughly 50% of the earth’s terrestrial surface Section, Department of Agricultural Science, Imperial College Lonis grazed by large herbivores (Frank et al., 1998). Forage don, Wye Campus, Kent, U.K. TN25 5AH; B.F. Tracy, Department of Crop Science, University of Illinois, 1102 S. Goodwin Avenue, and grazing lands form the backbone of profitable forUrbana, IL 61801; D.A. Wedin, School of Natural Resources, Univerage–livestock systems and contribute substantially to sity of Nebraska, Lincoln, NE 68583-0814. This paper resulted from the agricultural economy. Interest in the management the symposium “Plant Diversity in Forage and Grazing Lands” sponof pastures in temperate regions is moving beyond tradisored by Division C6 at the Annual Meetings of the Crop Science tional concerns, such as management to optimize the Society of America, Indianapolis, IN, 14 Nov. 2002. Received 9 June 2003. *Corresponding author ([email protected]). quality and quantity of herbage for animal production, to encompass a new set of issues. These issues cross traPublished in Crop Sci. 44:1132–1144 (2004). ditional boundaries between farming, agronomy, con Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA servation, ecology, and landscape management to include