Abiotic Controls on Primary Productivity and Nutrient Cycles in North American Grasslands

Abstract

At simple and general levels, the occurrence and physiognomy of grasslands and savannas are easily related to the spatial and temporal patterns of such abiotic factors as growing season, precipitation, and temperature (Borchert 1950; Curtis 1959; Lauenroth 1979; Singh et al. 1983; Transeau 1935). Across the Great Plains from the North American Rocky Mountains to the tallgrass prairie and the savannas of the eastern prairie-forest border, precipitation increases and the frequency of droughts decreases (Risser et al. 1981). From the southern plains of central United States to south central Canada, the growing season becomes shorter, average temperatures decrease, and greater proportions of annual precipitation fall as snow. Across these broad gradients from west to east, shortgrass steppes are replaced by mixed grass and tallgrass prairies and, eventually, by savannas. Similarly, species composition changes from south to north, especially increasing the proportion of cool season species. These gradients, however, are confounded by more immediate weather and climate conditions, by topography and soil characteristics, and by management practices (Schimel et al. 1985a; Sims et al. 1978; Weaver 1954; Weaver and Albertson 1956; Weaver and Bruner 1954).