Drought and Plant Invasions Along an Aridity Gradient

Abstract

Questions: Biological invasions significantly impact plant communities, affecting and threatening biodiversity at regional and local scales worldwide. The invasion process may be influenced by climate change, particularly by the increasing frequency of extreme droughts. Here we evaluate (1) the level of exotic plant invasion along an aridity gradient and its relationship with environmental variables, and (2) the cumulative effect of prolonged droughts on native plant communities and the invasion of exotic species. Location: Rangelands located along an aridity gradient from the arid Patagonian steppe to the humid Pampa region in Argentina, South America. The study sites (11) encompass a wide range of mean annual precipitation (170–950 mm year−1) and diverse vegetation physiognomic characteristics. Methods: We conducted floristic surveys to assess plant community composition, diversity, and the cover of native and invasive exotic species in (1) 11 sites located along an aridity gradient, and (2) a 4-year drought simulation experiment established in nine sites. At each experimental site, rain-out shelters intercepted 50%–60% of incoming rainfall to simulate drought conditions. We then used linear mixed models to analyze the effects of drought on the cover, richness, and dominance of the plant community. Results: Our findings reveal (1) a positive relationship between plant invasion and rangeland humidity at the regional scale. In the humid extreme, invasive exotic species dominated the plant community, whereas in the arid extreme native species were more abundant than invasive exotic species. (2) Experimental droughts reduced native plant cover in arid and semi-arid rangelands and promoted the cover, dominance, and richness of invasive exotic species in humid counterparts, reducing native species richness. Conclusions: This study suggests a potential intensification of biological invasions in response to increased occurrences of droughts predicted by climate change. Management strategies for invaded humid rangelands are imperative, and early warnings are crucial during dry years in arid and semi-arid rangelands, where droughts could enhance the abundance of invasive exotic species.