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Submitted to: Biological Invasions
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 7/6/2016 Publication Date: 10/12/2016 Citation: Porensky, L.M., Blumenthal, D.M. 2016. Historical wildfires do not promote cheatgrass invasion in a western Great Plains steppe. Biological Invasions. 18:3333-3349. Interpretive Summary: Plant invasion and wildfire are often tightly linked. In western North America, positive feedbacks between wildfire and cheatgrass (Bromus tectorum) invasion have led to plant community conversion across millions of hectares of land. Impacts of this conversion include reduced biodiversity, wildlife habitat, and livestock weight gains, as well as increased costs associated with fighting fires and rehabilitating invaded areas. It is unclear whether the fire-invasion feedback cycles observed in the Great Basin operate similarly in the western Great Plains. We asked how 28 wildfires influenced cheatgrass and a related species, Japanese brome (B. arvensis), in northeastern Wyoming. We found that instead of promoting invasion, wildfires had no effect on the occurrence of two invasive bromes. In sites where B. tectorum occurred, fire tended to reduce its abundance. Aside from fire, we found that soil texture, cover of other plant species, and microclimate (slope and aspect) were key drivers of annual brome abundance. Cheatgrass was more likely to occur on coarser-textured soils, while Japanese brome was more likely to occur on finer-textured soils. Both brome species were positively associated with southeasterly slopes (warmer, wetter microsites). In the western Great Plains, where annual bromes coexist with native species that are well-adapted to frequent fire and grazing, wildfire does not appear to promote cheatgrass invasion, although climate change might. Technical Abstract: Plant invasion and wildfire are often tightly linked. In western North America, positive feedbacks between wildfire and Bromus tectorum (cheatgrass) invasion have contributed to plant community conversion across millions of hectares of land. Impacts of this conversion include reduced biodiversity, wildlife habitat, and livestock weight gains, as well as increased costs associated with fighting fires and rehabilitating invaded areas. While B. tectorum has been studied intensively in the Intermountain West, much less is known about its invasion east of the Rocky Mountains. It is unclear whether the fire-invasion feedback cycles observed in the Great Basin operate similarly in the western Great Plains. We asked how wildfire has influenced B. tectorum and its congener, B. arvensis, in a shrub-grass ecotone in northeastern Wyoming. We also asked whether the effect of wildfire on B. tectorum varies based on soil type, topography, vegetation, or disturbance. We found that instead of promoting invasion, wildfires in northeast Wyoming had no effect on the occurrence of two invasive brome species. In sites where B. tectorum occurred, fire tended to reduce its abundance. Aside from fire, we found that soil texture, cover of other plant species, and microclimate (slope and aspect) were key drivers of annual brome occurrence and abundance. Bromus tectorum was more likely to occur on coarser-textured soils, while B. arvensis was more likely to occur on finer-textured soils. For B. tectorum, higher competitor cover was associated with increased likelihood of occurrence but reduced abundance where present. Both brome species were positively associated with warmer, wetter microsites. In the western Great Plains, where annual bromes coexist with native species that are well-adapted to frequent fire and grazing, wildfire does not appear to promote B. tectorum invasion. Our findings stress that relationships between fire and plant invasion are governed not by invader identity alone but by ecosystem-specific interactions among invaders, fire regimes, and resident species. |
