Extreme climate events, such as severe drought, could alter the competitive balance between native and invasive species. Droughts act on multiple scales, both temporal and spatial, making understanding the effects of these events complicated. Within the salt marshes of San Francisco Bay, Lepidium latifolium (white top) is an aggressive invasive plant of particular management concern. During California’s historic drought (2012-2015), we observed a die-back of Lepidium populations across the Bay. To understand the connection between invasive plant dieback and drought, we took two approaches. First, we established a precipitation manipulation in winter 2016 within a salt marsh in the south San Francisco Bay, California, USA. We applied four precipitation treatments (N=6/treatment): rain exclusion, rain exclusion control (rain excluded, plots hand irrigated after rain events), rain addition (2” of water added), and unmanipulated control. Second, we examined connections between salinity changes at the Bay scale and Lepidium die-back by pairing data from three Lepidium invaded sites (monitored 2014-2017) spanning a salinity gradient within the Bay with Bay water salinity data (USGS data set). Both experimental and long term plots were assessed for stem count and percent cover of all native plant species
Results/Conclusions
Between 2014 and 2015, near the peak of California’s drought, we observed a significant decrease in Lepidium stem count, stem height, and percent cover (p<0.05 for all). By 2017, Lepidium populations had not recovered to 2014 levels. We found our rain exclusions significantly influenced salinity following rain events, with exclusion plots having significantly higher soil salinities than control plots following a rain event. However, after excluding 1,786 gallons of rain from plots, we found rain exclusions did not significantly decrease Lepidium stem density. Utilizing generalized additive mixed models (GAMMs), we found Bay water salinity three years prior to the growing season of interest significantly altered Lepidium stem density. This lag effect was express through increased stem densities three years after a water year with less saline Bay water and decreased stem densities three year after water years with highly saline Bay water. Changes in native plant cover were primarily explained by changes in Lepidium stem density, though Bay water salinity three years prior was also significant in our GAMM models. Legacies of extreme climate events may be interpreted incorrectly or overlooked if temporal or spatial scale are not appropriate to detect lags in ecosystem responses.