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Although grasslands have been altered by humans for thousands of years (Wedel 1961; Bond et al. 2003), the loss of grassland as a result of anthropogenic activities has increased dramatically over the past 150 years. When Europeans first settled the Midwest and Great Plains, the greatest threat to native grasslands was the conversion of the most highly productive of these ecosystems to row-crop agriculture (Samson and Knopf 1994). Later, with improvements in soil moisture management and irrigation technology, even low-productivity grasslands were plowed. Today, those remnants of the most productive grasslands that escaped the plow are threatened, as are most of Earth’s ecosystems, by a variety of global change phenomena (Vitousek et al. 1997), with the invasion and expansion of woody species into grasslands one of the greatest of these threats. The replacement of grasslands by shrubland, woodland, and forest is a concern not only in the United States but worldwide (Archer et al. 1988; Van Auken 2000; Roques et al. 2001; Silva et al. 2001). Species of woody plants that invade grasslands may include both native plants which previously existed as more minor components of the ecosystem as well as alien species (Bragg and Hulbert 1976; Harcombe et al. 1993). In the mesic grasslands of the central United States, the last remaining extensive tracts of tallgrass prairie in Kansas, Oklahoma, and Texas are at risk from both shrub expansion from within and forest encroachment from the edges. Alteration in land management (grazing regimens) and the loss of an essential natural driver in the system (fire) associated with human population growth are two causal mechanisms often cited for the increase in shrubs and trees in northeast Kansas tallgrass prairies (Figure 1;Knight et al. 1994; Hoch et al. 2002; Briggs et al. 2002; Heisler et al. 2003). Shrub patches or “islands” that rapidly expand with reduced fire frequencies may be key focal sites for the establishment of many forest species in this and similar grasslands (Petranka and McPherson 1979; Gehring and Bragg 1992; Lett and Knapp 2005). Ultimately, however, closed-canopy Juniperus virginiana forest can completely displace the native grassland ecosystem in many sites (Hoch et al. 2002). A similar replacement of grassland by this and other Juniperus species has occurred in Texas and Oklahoma (see other chapters in this volume). |
