Priority effects are affected by precipitation variability and are stronger in exotic than native grassland species

Abstract

Exotic perennial grassland species often green up earlier than their native counterparts, allowing them to gain an advantage by dominating resources early (priority effects). Precipitation variability is expected to increase with climate change, and may alter the strength of priority effects. We hypothesized that exotics will have stronger priority effects than natives, precipitation variability will impact the strength of priority effects, and precipitation variability will impact the priority effects of native species more than those of exotics. We seeded one of five native or five exotic grassland species from the Central U.S. spanning multiple functional groups 28 days prior to a native seed mix. Priority effect strength was determined by how much establishment and diversity was reduced in the mix compared to controls (no species seeded before mix). We crossed these priority effect treatments with three water variability treatments, one low variability, and two high variability with alternate timing. Exotic species had stronger priority effects than natives, and decreased diversity and establishment from the seed mix. High variability precipitation when the growing season began dry significantly increased priority effects compared to low variability and high variability beginning wet. We found no significant evidence for a more pronounced impact of precipitation on native species, but trends suggest future studies may reveal significant interactions. Although future research in the field over multiple growing seasons is needed, our results suggest priority effects of exotics in Central U.S. grasslands are independent of precipitation timing and therefore likely to persist under changing climates.