The Combined Influence of Grazing, Fire, and Herbaceous Productivity on Tree–Grass Interactions

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Although Juniperus communities are native to most regions of North America, they have proliferated in many areas of the Great Basin and Great Plains that historically supported grasslands, shrublands, and savannas. Explanations for the observed increases in Juniperus dominance, as well as other woody plant communities, are the subject of ongoing debate. The balance between herbaceous and woody vegetation is regulated by complex interactions between climate (e.g., amount and seasonality of rainfall), soils (e.g., soil texture and depth), and disturbance regimes (e.g., fire, gazing, browsing) (Walker 1987; Scholes and Archer 1997; Higgins et al. 2000). Changes in one or more of these factors can potentially elicit a change in the ratio of woody to herbaceous plants. Accordingly, climate change, intensification of grazing, elimination of fire and browsing (Hastings and Turner 1965; Grover and Musick 1990; Archer 1994; Fuhlendorf et al. 1996), atmospheric CO2 enrichment (Idso 1992; Johnson et al. 1993), and nitrogen deposition (Köchy and Wilson 2001) have all been invoked as potential reasons for woody plant proliferation over the past century (see reviews by Archer 1994; Van Auken 2000). However, because these factors are correlative and interact across multiple spatiotemporal scales, it is neither feasible nor realistic to assess their relative importance using traditional, short-term factorial experiments. Field studies based on space-for-time substitutions and comparisons of landscapes with differing management histories have been used to assess long-term changes, but results from such studies are difficult to replicate, interpolate, or extrapolate and do not explicitly test causality. As a result, there is still considerable debate as to the relative importance of grazing, climate, and fire influences on community dynamics in drylands (O’Connor 1995; Fernandez-Gimenez and Allen-Diaz 1999; Illius and O’Connor 1999; Fuhlendorf et al. 2001). Dynamic simulation modeling is an underutilized tool that can be used to evaluate how climate or climate–disturbance interactions potentially affect tree– grass ratios and to test competing hypotheses attempting to account for woody plant increases over the past century.