The high variability of California’s stable annual-dominated grasslands defies prediction of community trajectories by traditional frameworks of succession. This makes management goals such as maintaining rangeland forage, restoring natives, and controlling noxious weeds (medusahead – Elymus caput-medusae & goatgrass – Aegilops triuncialis) challenging. Priority effects (i.e. when the first species to arrive alters the trajectory of the community) are an assembly mechanism currently being explored to better understand community change. However, as grassland communities influenced by the yearly variation in drivers such as litter and precipitation, we need long-term, field based studies to determine the importance of priority. For each of the three main grass groups in California grasslands (native perennials, naturalized exotic annuals, noxious annual weeds), we investigated whether a) temporal priority leads to maintained cover long-term, b) priority leads to resistance to invasion by other plant groups, and c) how strong priority effects are against extreme drought. We measured composition annually for 12 years in experimental grassland plots that were seeded with various mixtures of native, naturalized, and noxious invasive plants that were allowed to be naturally invaded by species included in the study design. Plots experienced the extreme drought of 2011-2014 and the historic wet year of 2017.
Results/Conclusions
In native-only-planted plots, natives survived severe climatic swings – drought decreased their cover, but they recovered to high abundance afterward. This dip in cover decreased their resistance to naturalized grasses but the natives were able to keep the noxious weeds out, likely due to their functional similarity in phenology of soil moisture use. Naturalized exotic-only-planted plots had similar trends in cover in relation to the drought but were not able to resist invasion by the noxious weeds. These results suggest that planting native grasses and giving them priority will lead to long-term native persistence and continuous noxious weed suppression, even under the extreme conditions of climate change. Thus native grassland restoration should be a priority on the invasion front of these damaging noxious weeds.