Grassland Ecosystems

G Grassland Ecosystems Osvaldo E Sala, Arizona State University, Tempe, AZ, USA Lucıá Vivanco, Universidad de Buenos Aires, Buenos Aires, Argentina Pedro Flombaum, CONICET and Universidad de Buenos Aires, Buenos Aires, Argentina r 2013 Elsevier Inc. All rights reserved. Glossary commissioned by United Nations Environment Convention on biological diversity The Convention was Programme. first enacted in June 1992, has been signed by many Niche complementarity Refers to how the ecological countries, and its objectives are the conservation of niches of species may not fully overlap and complement biological diversity, the sustainable use of its components each other. Consequently, an increase in the number of and the fair and equitable sharing of the benefits arising out species that complement each other may result in a larger of the utilization of genetic resources, including by volume of total resources utilized and in higher rate of appropriate access to genetic resources and by appropriate ecosystem processes. transfer of relevant technologies, taking into account all Sampling effect Refers to the phenomenon where rights over those resources and to technologies, and by increases in the number of species increase the probability appropriate funding. of including in the community a species with a strong Functional type A group of species that share ecosystem effect (Huston, 1997). This phenomenon yields morphological and physiological characteristics that result an increase in ecosystem processes with increases in in a common ecological role. diversity without invoking niche complementarity. See Global biodiversity assessment (GBA) The Global Functional Diversity chapter (00061) in this work for ways Biodiversity Assessment is an independent peer-reviewed of distinguishing between niche complementarity and analysis of the biological and social aspects of biodiversity sampling effect. Extent of Grasslands the latter controls the loss of water from the ecosystem, since as temperature increases so does the evaporative demand. Grasslands are one of the major vegetation types in the world Increases in temperature result in increases in soil evaporation accounting for almost 40% of terrestrial surface (Shantz, and plant transpiration; consequently, for a similar precipi- 1954). They exist in all continents and cover a vast area of tation regime, the water balance becomes more negative as 49 Â106 km2. In North America, grassland is the potential temperature increases. In contrast with most biological phe- natural vegetation of most of the Great Plains and it reaches nomena, primary production in grasslands decreases with from the Chihuahuan desert in the south to the deciduous increasing temperature, highlighting the importance of the forests of Canada in the north, and from the Rocky Mountains indirect mechanism of the temperature control on the distri- in the west to the deciduous forest of the eastern United States bution of grasslands (Epstein et al., 1996). (Figure 1). In South America, grassland is the potential Grassland ecosystems occur in areas of the world that have vegetation of the vast pampas and most of the Patagonian an annual precipitation between 150 and 1200 mm and mean steppe. Finally, in Asia, grassland ecosystems cover a huge area annual temperature between 0 and 25 1C(Whittaker, 1975). from Ukraine to China. Along precipitation gradients grasslands are located between forests and deserts. In North America, South America and Asia, clear E–W precipitation gradients exist. In North and South America there are very small changes in elevation along those Determinants of Grasslands precipitation gradients and vegetation changes are mostly ac- counted for by precipitation. At the eastern and wettest end, Grasslands are water-limited ecosystems, and water availability Tall-grass Prairie is the dominant vegetation, which is replaced defines their distribution in space. The amount of water by Mixed-grass Prairie and by Shortgrass Steppe at the driest available for plants primarily depends on precipitation end of the gradient. A similar pattern occurs in Asia and South amount and temperature. The former is the input of water and America (Figure 1). Encyclopedia of Biodiversity, Volume 4 http://dx.doi.org/10.1016/B978-0-12-384719-5.00259-8 1 2 Grassland Ecosystems Figure 1 Map of the global distribution of temperate grasslands. Adapted from Figure 7.2 in Bailey RG (1998) Ecoregions: The Ecosystems Geography of the Oceans and Continents, 87 pp. New York: Springer. Although temperature and precipitation are the major determinants of the distribution of grasslands, fire also may play High an important role. Fire becomes particularly important in the grass–forest ecotones where the dominance of grasses or Advantage grasses woody plants in many cases is determined by the frequency and intensity of fires. For example, in North American Tall- grass Prairie, the area covered by woody plants has increased dramatically in the past 100 years and the human intervention in reducing fire frequency is largely responsible for the change (Briggs et al., 1998). Similarly, data from pollen profiles, tree PPT-TEMP overlap Advantage woody veg ring analysis, and photographic sources documented a shift in the grassland–forest ecotone in northern Patagonia with woody vegetation invading grasslands (Veblen and Markgraf, Low 1988). Again, fire control implemented by land managers was Fine Coarse responsible for the forest expansion. Soil texture Soil texture also modulates the distribution of grasslands Figure 2 A conceptual model explaining the relative abundance of by modifying the soil water-holding capacity and the location grasses and woody vegetation as a function seasonality of of water in the profile. First, water penetrates deeper into the temperature and precipitation (congruence of warm and wet seasons) soil profile in coarse-textured soils than in fine-textured soils and soil texture. The gray shaded area represents conditions that are because soil water-holding-capacity depends on soil texture likely to favor the persistence of grasses, while the speckled area and is lower in the former. Therefore, the same rainfall event represents conditions that favor woody vegetation. The intersection of would penetrate deeper in a coarse than in a fine textured soil. the two areas represents points where biotic influences are likely to Second, grasses and woody vegetation have contrasting root- be most pronounced. Adapted from Sala OE, Lauenroth WK, and ing patterns with grasses having predominantly shallow roots Golluscio RA (1997) Plant functional types in temperate semi-arid regions. In: Smith TM, Shugart HH, and Woodward FI (eds.) Plant and shrubs and woody plants having deep roots (Jackson Functional Types, pp. 217–233. Cambridge: Cambridge University et al., 1996). Consequently, for a given amount of precipi- Press. tation, grasses dominate in areas with predominantly fine- textured soils (Sala et al., 1997)(Figure 2). Finally, seasonality of precipitation and the synchrony between the wet and warm of the distribution of grasslands with their distribution seasons also affect the dominance of grasses and woody modified at the edges by fire, soil and seasonality of vegetation. Locations where precipitation occurs during the precipitation. cold season, when evapotranspiration is low, result in a deeper This article focuses exclusively on climatically determined distribution of water in the soil profile and these locations grasslands, in contrast with grasslands resulting from human would benefit woody vegetation over grasses (Figure 2). In intervention. Anthropogenic grasslands are located in areas synthesis, precipitation and temperature are the major drivers where potential natural vegetation is forest. Humans, in an Grassland Ecosystems 3 attempt to produce forage for domestic animals, have logged southward in North America and northward in South America forests and have maintained these plots as grasslands by and C4 species show the opposite pattern (Paruelo et al., mowing them periodically. 1998). Similarly, the abundance of C3 species increases whereas that of C4 decreases along an altitudinal gradient (Cavagnaro, 1988). These biogeographical analyses correlate Biodiversity with ecophysiological studies showing that C4 species have photosynthesis optima at higher temperature, have higher Biodiversity can be examined in many different ways, and water use efficiency, and are better adapted to low water multiple definitions exist for what constitutes ‘‘biodiversity.’’ availability conditions (Kemp and Williams, 1980). Nevertheless, the authors of the Global Biodiversity Assess- ment, using definitions originally proposed by the Convention on Biological Diversity, defined biological diversity as ‘‘vari- ability among living organisms from all sources’’ (Heywood Animal Species Diversity and Baste, 1995). Here, we focus on biodiversity in terms of All major taxonomic groups are represented in grasslands, but taxonomically defined species, and the vast majority of studies despite their large areal extent (40% of the area of terrestrial quantifying ecosystem variation have used this measure. ecosystems), overall faunal diversity is lower than in many However, genetic biodiversity (genetic variation within a single other biomes. The number of bird and mammalian species species) and ecological diversity (including landscape diversity that are found primarily in grasslands are estimated to be and functional group diversity) are also important components 477 and

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