Semi-arid regions with Mediterranean-type climates harbor exceptional biodiversity, but are increasingly threatened by invasion by exotic species. However, it’s unclear whether predicted climatic changes in these regions, including both drought and increasing precipitation variability, will promote or slow further invasion. Studies in semi-arid systems highlight the importance of antecedent conditions, or lag effects, for predicting plant growth, but the influence of lagged drought effects on the growth of native versus exotic species remains largely unexplored. We imposed experimental rainfall treatments (normal rainfall, moderate or severe drought) in adjacent areas dominated by native shrubs or herbaceous vegetation, and quantified peak growing growing season biomass of native shrubs or herbaceous vegetation, and quantified peak growing season biomass of native and exotic herbaceous species. The treatments were applied during an unprecedented drought in California (2012-2016), and we subsequently quantified recovery from the drought treatments in the following year of high rainfall (2017).
Results/Conclusions
With drought, exotic biomass declined more steeply than native biomass, but exotic biomass returned to high levels in the high-rainfall recovery year. During the drought, prior year’s rainfall did not influence plant growth, and litter cover had a positive effect on exotic, but not native, biomass. In the recovery year, native biomass was positively correlated with previous rainfall while exotic biomass was negatively correlated; these responses were not explained by other antecedent factors (litter cover or seedbank). These results show that invasion in this system was promoted by high rainfall following drought. More generally, this study demonstrates how the influence of antecedent conditions can vary between plant functional groups, and can also depend on environmental context.