Is the differential response of riparian plant performance to extreme drought and inundation events related to differences in intraspecific trait variation?

Abstract

Previous research on the impacts of extreme events has focussed mainly on plant performance. Selective effects of extremes suggests that appropriate traits to withstand extremes, or the ability to modulate traits, may increase the competitive advantage and survival of a species. We tested how multiple extreme events affect riparian plant performance, and whether this relates to intraspecific trait variation. We hypothesised that species tolerant to extreme environmental conditions would show highest intraspecific trait variation. We conducted a greenhouse experiment where 25 wet to drought-tolerant species (grasses and herbs) were subjected to single and double 10 day drought and inundation events, and all combinations thereof. We measured plant performance and multiple traits. Performance was severely reduced, with clear synergistic, non-additive effects of extreme events. Adverse effects were stronger for wet-tolerant than drought-tolerant species and for grasses than herbs. Three traits were identified as predictors of species responses to extremes. Despite known trait variation for these species, no variation occurred under these conditions, indicating that responses are difficult to explain from impacts of mean environmental conditions. Our results indicate that multiple events may lead to quantitatively different impacts than single events, which hampers predicting plant response to a future climate.