Does rangeland degradation have implications for global streamflow?

Received 27 June 2007 Accepted 29 June 2007 Rangelands are the most extensive land area of the earth and most occur in dry climates. Collectively, they include the dryland biomes classified as savannas, grasslands, shrublands, and deserts, which constitute about 51% of the earth’s total land area (Asner et al., 2004) and support almost 40% of the global population (Reynolds et al., 2007). This much is generally well appreciated. Less appreciated, however, is the degree to which these landscapes have changed—mostly for the worse—in the last 150 years. I use the term ‘rangeland degradation’ to describe the vegetation changes that have taken place as a result of three interrelated phenomena: desertification, woody plant encroachment, and invasion of non-native species(Wilcox and Thurow, 2006a, b). These three phenomena have altered most rangelands having arid, semi-arid, and sub-humid climates and together are a major component of global change that has important implications for the water cycle. Reynolds et al. (2007) estimate that some 10–20% of drylands have been severely degraded, meaning that soils are mostly exposed and severe erosion has occurred or is continuing to occur. Less severe, but nevertheless significant, degradation and loss of cover are seen in most rangelands in drier climates, largely as a result of overgrazing (Hess and Holechek, 1995; Middleton and Thomas, 1997; Dregne, 2000; Asner et al., 2004). Another change observable on rangelands is woody plant encroachment, a global phenomenon whereby grasslands and savannas are becoming woodlands (Archer, 1994; Huxman et al., 2005). In drier climates, this change is part of the desertification process, as it leads to the exposure of more and more bare soil and surface connectivity (Schlesinger et al., 1990), which in turn increases erosion (Grover and Musick, 1990); but in more humid locations, woody plant encroachment would not be part of a desertification process (Archer, 1989). The third important driver of change is invasion by non-native species. The following are two examples from among many: (1) the transformation of riparian areas in the American Southwest by salt cedar (Tamarix chinensis, T. ramosissima Tamarix) (Newman et al., 2006; Wilcox et al., 2006b) and (2) the alteration of more than 10 million hectares of rangelands in South Africa by alien invasive plants (Binns et al., 2001). For many areas, these changes have occurred quite recently, certainly within the last 130 years. In some areas, such as the Mediterranean region, the transformation began centuries ago (Brandt and Thornes, 1996). But the degradation continues in many locations, mainly as a result of overgrazing; and if anything, the pace of change is quickening (Hess and Holechek, 1995; Middleton and Thomas, 1997; Dregne, 2000; Asner et al., 2004). The effects are significant enough that Asner et al. (2004) argue that rangeland degradation is a major, yet often overlooked, component of global change. The main point of this commentary is, first, to emphasize that rangeland degradation is an important type of land-use change that potentially affects the water cycle on the global as well as the regional scale.