Fire is unlikely to be a simple, deterministic process that will result in increased rates of invasion by nonnative plants. Relative dominance of nonnative plants at any postfire site likely results from a combination of factors, including propagule pressure (aboveground and belowground), time since invasion, interspecific competition, disturbance history, rainfall patterns, soil nutrients, environmental heterogeneity, land use, plus the actual fire dynamics. Consequently, the role different factors play in nonnative plant invasion is still relatively unexplained, and the possibility that wildfire may promote nonnative plant species remains a concern. Our aim was to evaluate the state of knowledge concerning how nonnative plant species establish, survive, and spread following wildfire in wildland areas for the main vegetation types of the Intermountain West. Although the particular invading species varied with vegetation type, the most frequently reported species were annual grasses (e.g., downy brome [Bromus tectorum], Japanese brome [Bromus japonicus]) and long-distance, dispersing forbs (e.g., Canada thistle [Cirsium arvense], common dandelion [Taraxacum officinale], and prickly lettuce [Lactuca serriola]). Overall, many of the reviewed studies found an increase, followed by a long-term decline, in nonnative species after wildfire, particularly when other disturbances were minimal. Regardless of this finding, many people consider rapid detection and control of new, nonnative plant populations after wildfire one of the most effective management activities. That axiom may be correct, if the nonnative propagule pressure is high and the anthropogenic disturbances are frequent, but if this is not the case the question to answer is whether immediate management, including reseeding, will benefit the system or whether off-target effects could make that approach undesirable. Continued investigation and increased quantitative analysis of the complex factors that likely contribute to changes in nonnative plants dynamics following wildfire should improve our understanding of the issue and assist managers in allocating limited resources.

Background Prairie–forest ecotones are ecologically important for biodiversity and ecological processes. While these ecotones cover small areas, their sharp gradients in land cover promote rich ecological interaction and high conservation value. Our objective was to understand how historical and current fire occurrences and human development influenced the Palouse Prairie–forest ecotone. We used General Land Office survey field notes about the occurrence of bearing trees to locate historical (1870s to 1880s) prairie, pine savanna, and forest at the eastern edge of the bioregion. We combined LANDFIRE Existing Vegetation classes to contrast historical land cover with current land cover. We reconstructed historical fire occurrence (1650 to 1900) from fire-scarred trees. We used fire and lightning records from 1992 to 2015 to interpret the role of people and lightning. Results Historically, the ecotone was a matrix of prairie with extensive savanna and some forest. More than half of the ecotone area was prairie, which is now dominated by agriculture, with some residential development. The 16% of the landscape that was pine savanna is now forest or shrubs, agriculture, perennial vegetation under the Conservation Reserve Program, or developed; no savanna now exists. Forests covered 12% of the ecotone and these are still mostly forest. Fires were historically frequent, occurring on average every 5 to 8 years at most sites. Lightning was not frequent but could likely have been sufficient to ignite fires that could spread readily given the rolling terrain and long fire season. Conclusions Fire was far more frequent historically than currently. Conservation, restoration, and other ongoing land-use changes will likely result in more continuous vegetation and hence fuel for fires. Lightning and people may ignite fires that therefore spread readily in the future. Understanding the past and potential future of fire in the Palouse Prairie bioregion may help us live with fire while conserving ecological values here and in similar prairie–forest ecotones.

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