INTERACTIONS OF WATER AND NITROGEN ON PRIMARY PRODUCTIVITY ACROSS SPATIAL AND TEMPORAL SCALES IN GRASSLAND AND SHRUBLAND ECOSYSTEMS

Soil moisture, and hence precipitation, exerts a dominant control on net primary productivity (NPP; the rate of carbon accumulation by autotrophs) in semiarid to subhumid ecosystems, such as grasslands and shrublands. At the global scale, mean annual precipitation has been shown to account for >50% of the variance in aboveground net primary production (ANPP) in grassland ecosystems (Lauenroth 1979; Le Houerou et al. 1988). Within specific regions, numerous investigators have demonstrated strong positive relationships between mean annual precipitation (MAP) and primary productivity (or some surrogate variable) for various ecosystems globally, including the Great Plains grasslands (Sala et al. 1988; Knapp et al. 2001; Epstein et al. 2002), the Patagonian steppe (Austin and Sala 2002; Jobbagy et al. 2002), African grasslands and savannas (Breman and de Wit 1983; McNaughton et al. 1993; Scanlon et al. 2002), and grasslands and shrublands of Inner Mongolia and northeastern China (Gao and Yu 1998; Yu et al. 2004).