Disturbance, resource pulses and invasion: short-term shifts in competitive effects, not growth responses, favour exotic annuals.
Increased resource availability and resource pulses often promote invasion by exotic invasive plants, but the relative importance of increased resource supply for invaders with different life histories is likely to vary. It is also unclear whether increased resources allow invaders to outgrow their native neighbours or alter the outcome of competition. Understanding mechanisms by which different invasive life histories respond to resource supply will help to target the most important species to control. We found that aerially dropped wildfire retardant on intermountain grassland provided a very high pulse of N and P and caused dramatic increases in the abundance of annual exotic invaders already present in the system. Field experiments with retardant stimulated similar shifts to dominance by exotic annuals and decreases in native perennial grasses, but elicited no responses by native annual species. In a greenhouse experiment, the growth response of both exotic and native species to retardant was not related to the intensity of mean competitive effects or responses. Concomitantly, gram per gram competitive effects differed substantially among species and treatments. N-caused shifts in the strength of competitive interactions were far better predictors than growth responses of the effects of retardant in the field. For example, Centaurea stoebe, an invasive perennial forb, grew disproportionally more than any other species in response to retardant when grown alone, but retardant significantly shifted competitive outcomes with this invader in favour of exotic annuals and the native bunch grass Pseudoroegneria spicata. Synthesis and applications. Our results indicate that an extreme nutrient pulse did not create conditions whereby invaders outgrew native neighbours or other exotics, but instead altered the competitive playing field in ways that resulted in considerable changes in community composition. Given the minuscule scale of retardant drops on the landscape, our results do not suggest changing the way retardant is used to combat wildfire but do suggest that localized post-retardant weed control may be important so that retardant drops do not become point sources for exotic invasion. Furthermore, our results suggest that under high N conditions, invaders with annual life histories should be prioritized for control.