Although several invasive species have induced changes to the fire regime of invaded communities, potential intraspecific shifts in fire-related traits that might enhance the invasion success of these species have never been addressed. We assumed that traits conferring persistence and competitiveness in postfire conditions to downy brome, a quintessential invasive species of the Great Basin (North America), might be under selection in areas with recurrent fires. Therefore, we hypothesized that populations from frequently burned regions of the Great Basin would have (1) greater tolerance to fire at seed level, (2) higher relative seedling performance in postfire environments, and (3) greater flammability than unburned Central European populations that evolved without fire. Seeds were collected from three introduced populations from frequently burned regions in North America and three introduced populations of rarely or never burned sites from Central Europe. We performed (1) germination experiments with seeds subjected to the effect of different fire components (heat shocks, smoke, flame, ash), (2) pot experiments analyzing the effect of postfire conditions on the early growth of the seedlings, and (3) a series of flammability tests on dry biomass of plants reared in a common garden. All seeds tolerated the low-temperature treatments (40 to 100 C), but were destroyed at high heat shocks (140 and 160 C). Only the 100 C heat treatment caused a difference in reaction of seeds from different continents, as the European seeds were less tolerant to this heat shock. We found significantly increased seedling height and biomass after 4 wk of growth under postfire conditions in American populations, but not in European ones. American populations had enhanced flammability in three out of five measured parameters compared to European populations. In summary, these intraspecific differences in fire-related traits might contribute to the persistence and perhaps invasiveness of the frequently burned North American downy brome populations.
Nomenclature: Downy brome, Bromus tectorum L. BROTE.
Management Implications: We provide evidence that North American downy brome populations from frequently burned environments have enhanced postfire biomass accumulation and flammability compared to European populations that are rarely or never exposed to fire. Seeds of downy brome, however, do not tolerate severe fires with high heat shocks.
Our results suggest that management should focus on changing the fire regime toward less frequent, but more intense, fires to stop this highly invasive species. Therefore, lengthening the fire-return interval by creating firebreaks, which would amplify the intensity and size of subsequent fires, may be one option for controlling this invasive species. However, such bands of fuel breaks, where fuels are removed by cutting several swaths to reduce fire hazard, can serve as refuge and seed source of downy brome. The association of fuel breaks with roadways might minimize the total landscape impact.
To prevent the germination of downy brome, persistent cover of the ground by native vegetation must be achieved by reintroducing fire-tolerant perennial grasses and resprouting shrubs such as rabbitbrush, as closed vegetation makes for very limited alien seedling recruitment opportunities. The fire management practice should target the historical, natural fire regime that favors native species reestablishment. These native species would produce higher fuel loads and a continuous fuel bed that might also increase the intensity and decrease the frequency of fires.
Although our results may suggest that the introduction of European, non–fire-adapted populations of downy brome to the United States might be an option, they would perhaps be outcompeted in short tim