Encroachment and Secondary Succession

There are estimations suggesting that approximately 60 million ha of grasslands in the American southwest have been encroached by various woody species and converted to savannas, shrublands, or woodlands (Humphrey 1958; Grover and Musick 1990). Interestingly, this is more than the total estimated area of these grasslands (Laurenroth 1979). Thus, the composition, cover, density, productivity, and structure of most if not all of these grasslands have changed. In addition, it has been suggested that 220–330 million ha of all North American grasslands have been encroached by woody species (Knapp et al. 2008). Not all of this woody plant encroachment is by woody legumes, but a considerable amount is (Van Auken 2009). Prosopis, Senegalia, and Vachellia are genera with major encroaching species in southwestern North American grasslands, but they are not the only species.