The role of disturbance in invasive plant establishment in a changing climate: insights from a drought experiment.
Climate change and disturbance are two major factors affecting the establishment of invasive species, yet few studies to date have assessed the individual and interactive effects of these two factors in a common setting. Disturbance has often been found to facilitate the establishment of invading species, while climate change may affect them positively or negatively through altering abiotic conditions, or indirectly by modifying species interactions. In a full-factorial field experiment in a semiarid temperate grassland in Central Hungary, we studied the effects of drought (40% rain exclusion throughout the year) and soil disturbance on the emergence, survival and aboveground biomass of four invasive plant species that represent different life forms and that are of concern in the region and at a broader scale. We added seeds of Ambrosia artemisiifolia (annual forb), Cenchrus incertus (annual grass), Asclepias syriaca (perennial forb) and Ailanthus altissima (deciduous tree) in drought and non-drought plots with and without soil disturbance. Ailanthus germinated poorly irrespective of treatments. Disturbance facilitated while drought suppressed seedling emergence in the other three species. Ambrosia was more sensitive to disturbance, while Cenchrus was more responsive to drought. Asclepias achieved substantial emergence in disturbed non-drought plots only, as drought strongly suppressed its emergence even in the presence of disturbance. Seedling survival and late-season aboveground biomass of Ambrosia and Cenchrus were positively affected by disturbance but were unaffected by drought, while no Asclepias seedling survived until late summer. Our results highlight that both drought and disturbance may considerably impact the establishment of invasive plants, with potential interactive effects, but responses may greatly differ among species and life stages. Overall, our findings in this study suggest that although drought may negatively affect seedling establishment, a drier climate may not suppress or eliminate invasive species if soil disturbance is present. They also highlight the importance of including disturbance in studies assessing the potential effects of climate change on plant invasions.