Grasslands and savannas share a number of biotic and abiotic features that differentiate them from other vegetation types. Their major characteristic is the dominance of a herbaceous layer largely composed of grasses and sedges (Walter, 1977). In grasslands, the herbaceous component forms a unique vegetation layer, whereas in savannas there is an additional woody layer which can range from sporadic, isolated shrubs or trees to relatively dense woodlands. Climatically, grasslands and savannas extend over a broad range of mean annual precipitation (200–1300mm) and temperature (0–30ºC)(Lauenroth, 1979). However, all of them have a negative water balance and undergo marked dry seasons (Lauenroth, 1979; McNaughton et al., 1982). Grasslands and savannas are fuzzy segments of a climatic gradient from deserts through closed forests. In many regions of the world, deserts turn gradually into grasslands, as grasses become more important in response to an increase in mean precipitation (Paruelo and Lauenroth, 1996; Paruelo et al., 1998). A southeast–northwest gradient in eastern Europe from Artemisia desert to Stipa steppe (Walter, 1977), an east–west gradient in southern South America, from Nassauvia glomerulosa semi-desert to the western steppe with Festuca pallescens (Leon and Facelli, 1981), and a west–east gradient in North America from the Chihuahuan Desert with Larrea tridentata to shortgrass steppe with Bouteloua gracilis (Gosz, 1993) are all examples of this transition. At the other end of the gradient, dense grasslands and savannas are replaced by forests. In some regions, this transition is sharp enough for no intermediate regions with savannas to be distinguishable at the scale of world or continental maps. For example, in Patagonia, Festuca pallescens grassland turns into a Nothofagus forest (Soriano, 1983; Schulze et al., 1996). In other regions, particularly in the tropics, there is a gradual increase in the tree/grass ratio as precipitation increases: grasslands are progressively replaced by savannas whose woody component becomes more important as precipitation increases (Sinclair, 1979; Walker and Noy-Meir, 1982; McNaughton, 1983a; Belsky, 1990). The broad range of mean annual precipitation of the grassland/savanna biome is one of the most important causes of its structural and functional diversity. Plant cover, plant biomass, leaf area, and canopy height of the herbaceous layer predictably increase along gradients of increasing precipitation. For example, the heterogeneity of the grassland region of North America has been repeatedly described on the basis of a gradient from the relatively dry, shortgrass steppe in the West through the more humid, tallgrass prairie in the East (Coupland, 1992). Similar gradients are observed in southern South America (Oesterheld et al., 1992) and East Africa (McNaughton, 1983a). Parallel to these changes in structural features, ecosystem function also changes dramatically and predictably along the precipitation gradient. In particular, more than 75% of the variation in above-ground net primary production of the herbaceous layer of grasslands and savannas can be accounted for by their mean annual precipitation (McNaughton, 1985; Sala et al., 1988a; McNaughton et al., 1993). Precipitation also explains a substantial proportion of the seasonal variability of carbon gains (Paruelo and Lauenroth, 1996, 1998). Taken together, these observations suggest that mean annual precipitation causes a great deal of variation among grasslands and savannas, but, for this same reason, it becomes a highly integrative variable to be