Ecological Impacts of Ashe Juniper on Subtropical Savanna Parklands and Woodlands

Savannas are inherently patchy systems where the distribution and abundance of the two dominant plant growth forms (woody plants and grasses) varies greatly over space and time in response to grazing, browsing, fire, drought, and topographic/edaphic factors (Huntley and Walker 1982; Scholes and Archer 1997). In recent times, the balance between woody plants and grasses has been disrupted in many savannas around the world as woody plants have increased in abundance at the expense of herbaceous cover and production (Van Auken 2000; Archer 2005). This woody plant proliferation is thought to be driven primarily by intensification of livestock grazing and reduced fire frequency (Archer et al. 1995), although other factors (e.g., elevated atmospheric CO 2 concentration and increased atmospheric nitrogen deposition) may also contribute (Polley et al. 1996; Kochy and Wilson 2001). Regardless of the cause, this vegetation change has the potential to alter a number of fundamental ecosystem properties, including productivity, biological diversity, biogeochemistry, and energy exchange, which can influence ecosystem processes at regional (Archer et al. 2001; Jackson et al. 2002; Huxman et al. 2005) and global scales (Schlesinger et al. 1990; Goodale and Davidson 2002). Although woody plant encroachment is a general phenomenon of many contemporary savannas, not all woody species present in savannas show this increase and, indeed, some appear to be stable or declining in abundance (Allen-Diaz et al. 1999; Van de Vijver et al. 1999; Weltzin and McPherson 1999). A case in point is the Edwards Plateau of Texas, USA. The Edwards Plateau is a major natural region that covers about 93,000 km in the west-central part of Texas (Figure 1, inset). At present, much of the vegetation of this region is classified as live oak–Ashe juniper savanna (parkland + woodland; McMahan et al. 1984). Plateau live oak (Quercus virginiana P. Miller var. fusiformis (J.K. Small) C. Sargent) is a sprouting, fireresistant species (Muller 1951) that is thought to have been common in upland savanna parklands of the Edwards Plateau for centuries (Olmstead 1857; Buechner 1944). Populations of Plateau live oak appear to have remained static or declined in recent times (Russell and Fowler 1999), and in some cases may be displaced by Ashe juniper (Owens 1996; Smeins and Fuhlendorf 1997; Wu et al. 2001).