Does disturbance enhance the competitive effect of the invasive Solidago canadensis on the performance of two native grasses?

The impact of invasive species on native plant communities can strongly depend on habitat disturbances. Thus, the joint study of invasion and disturbances are necessary to distinguish whether invasive species (1) are just 'passengers' of major environmental changes, (2) are the real cause (drivers) of native species decline, or (3) do disturbances and invasive species additively suppress native species (back-seat drivers). Therefore, we experimentally explored both the single and additive effect of competition by an invasive species and fire as disturbance on the performance of native species. We examined the responses of two native rhizomatous perennial grass species (Elymus repens and Brachypodium pinnatum) to competition with European invasive and American native Solidago canadensis. This was done under burned and unburned conditions, a novel disturbance type in this system. We found that competition with S. canadensis had a very strong negative effect on the performance of B. pinnatum irrespective of disturbance. In contrast, disturbance and competition had a cumulative negative influence on the performance of E. repens, with competition having greater effect than burning. Fire reduced the number of shoots of European S. canadensis individuals, but did not affect the frequently burned American populations. However, these differences did not translate into increased competitive ability of European populations compared with American ones. Thus, the competitive superiority of S. canadensis irrespective of continent of origin explained the performance loss in B. pinnatum ('driver' model); whereas reduced performance after burning of grass species and competitive superiority of the invasive species jointly decreased the performance of E. repens ('back-seat driver' model).