Pinyon-juniper woodlands involve vegetation dominated by about seven species of Pinus and 17 species of Juniperus scattered over more than 75 million acres of the Southwestern United States and Mexico. The junipers are more widespread latitudinally, longitudinally, and elevationally than the pinyons. The understory is much more diverse and reflects largely local climatic patterns. Grasslands and shrub steppes have successionally preceded pinyon-juniper savanna to woodland on sites with gentle slopes and fine soil textures. Excessive livestock grazing and direct fire control are the major factors which have led to present tree dominance. Tree dominance can be regarded as a sign of ecosystem degradation on sites formerly occupied by native herbs and shrubs. On many sites, trees will be eventually replaced by introduced herbs following fire storms unless proactive management is undertaken.