Introduction

Oak savanna in the southwestern United States and northern Mexico represents a diverse, widely distributed vegetation type. The northern Sierra Madre of Mexico has 41 species of oak (Quercus L.) (Nixon 1993), and in southeastern Arizona, plant species richness is greater in open oak woodland than in any other plant community (Whittaker and Niering 1975). In this chapter we focus on the southwestern United States, because there is little information about oak savanna in Mexico.

Current and Past Geographic Distribution

Oak savanna generally is considered a subset of the more extensive evergreen oak woodland, or encinal (D. E. Brown 1982; Rzedowski 1983). The savanna is characterized by tree canopy cover between 1% and 30% and is restricted to relatively low (1,100–2,200 m) and dry elevations, where encinal grades into desert grassland or mattoral (shrubland) (White 1948; Gentry 1957; Rzedowski 1983; McPherson 1992; McClaran 1995) (Figure 17.1). Savanna tends to be located at lower elevations as the latitude increases. In Arizona, New Mexico, and northern Sonora, the savanna generally has a discontinuous distribution in the foothills of isolated mountain ranges that are separated by lowlands covered by desert grassland and shrubland. In the northern part of the Sierra Madre Occidental, in Chihuahua, Durango, Sinaloa, and central-southern Sonora, the savanna forms a somewhat more continuous distribution in large areas of rolling topography above desert grasslands or mattoral (Rzedowski 1983) (Figure 17.2).