The effects of prescribed fire on grasslands of the southern Great Plains




Behr, Whitney L.

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Grasslands can support high levels of biodiversity and provide numerous ecosystem services, but they have been widely degraded, often via loss of natural disturbance regimes. North American grasslands were once created and maintained by fire. In some cases, fire has been more important than climate in determining the distribution and extent of grasslands. Conservation of the biodiversity harbored by grasslands relies, in part, on ecological restoration of these habitats and the fire regimes that historically maintained them. In this dissertation, I examined the effects of prescribed fire on grassland plant species and plant communities of the southern Great Plains in the short-term (up to two years after fire) and longer-term (twelve years after fire). Cool-season prescribed burns (those conducted in January – March) were not sufficient to shift overall plant community composition (e.g., increase richness of native plant species or reduce cover of the invasive grass Bothriochloa ischaemum) in 10 sites distributed from central Texas to southern Oklahoma (Chapter 2). However, these fires did have measurable effects on eight individual forb species in the same sites (Chapter 3). In general, the eight forb species studied individually responded to the winter fire individualistically, but all three annual species increased their floral displays (flowers/m²) in the burned plots in the short term. Forb species that increased their floral display in burned areas did so via increased plant biomass (grams of dry aboveground biomass) or plant density (plants/m²). We found little evidence that these forb species shifted their resource allocation towards reproduction. In a separate study (Chapter 4), a prescribed fire conducted in July was sufficient to shift plant community composition in the short term, mostly by reducing the cover of the invasive grass B. ischaemum and increasing native species richness; the latter effect was likely the result of reducing B. ischaemum. In the same study, only the increases in native grass cover and richness were still detectable twelve years after the fire. Perhaps due to two additional cool-season fires across the entire site, B. ischaemum cover remained low twelve years after the fire in burned plots but unexpectedly had also decreased in unburned plots. The results from all three chapters supported our expectation that summer fires would be more effective than cool-season fires in changing the plant community composition, including controlling the invasive grass B. ischaemum. Interestingly, forb species were highly individualistic, from differences in their abundances among sites in the multi-site study (Chapter 3) to differences in their responses to fire (Chapter 4). These findings support conducting prescribed fires in summer months to control invasive grasses and to increase native plant species richness, and consequently conserve the biodiversity supported by grasslands in this region.



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