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 ecosystem engineer. During California’s historic drought (2012-2016), 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 manipultion in winter 2016 within a salt marsh in 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) with Bay water salinity data spanning a salinity gradient within the Bay. Both experimental and long term plots were assessed for stem count and height of Lepidium and percent cover of all 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 numbers. This pattern persisted in 2017, indicating it is unlikely there is a lag in Lepidium response to decreased local precipitation. Our analyses of Bay water salinity changes and patterns in Lepidium stem die-back reveal sites experiencing the highest Bay water salinity conditions over multiple years had the most pronounced Lepidium stem reductions. This suggests, in tidal marshes, longer temporal scales (multi-year) and larger geographic scales (Bay salinity as opposed to plot-level salinity) could be key to understanding the impact of extreme drought on invasive plants. As climate continues to shift and become more variable, understanding these dynamics will be critical to effectively managing invasions and preserving important tidal habitats.