Tidal salt marshes are valuable coastal ecosystems that are increasingly threatened by anthropogenic stressors such as invasion by non-native species, climate change, and nutrient pollution. Lepidium latifolium is an aggressive non-native plant invading salt marshes on the west coast, displacing native plants and altering ecosystem processes. From 2012-2015, California experienced extremely dry conditions that stressed tidal salt marsh ecosystems. In a 3-year field experiment from 2014-2016, we tracked the effects of this extreme drought on L. latifolium invasion and used field experiments to test how nutrient addition alters these dynamics at four salt marsh sites across a salinity gradient in San Francisco Bay, CA, USA. We conducted our experiment in paired native and invaded salt marsh plots (n=12 pairs per site), half of which were treated monthly with 12.5 g of N and 1.25 g of P for 1.5 years during the most intense period of the drought and one subsequent wet winter. For three years, we measured L. latifolium stem density and height, percent cover of native plants, and percent cover of bare ground.
Results/Conclusions
We documented a dramatic die-back of invasive L. latifolium during extreme drought, including large reductions in stem density (47%-100%) and height (19%-47%). We found nutrient application lessened the effect of drought on L. latifolium stem density, but not height. In native salt marsh plots, extreme drought reduced native plant percent cover by 8%-23%, but nutrient addition strongly mitigated this impact. Interestingly, we found native plants in areas invaded by L. latifolium did not suffer reductions in percent cover due to drought, perhaps because they were simultaneously benefitting from the die-back of the invader. Further, native plant percent cover actually trended upwards during the drought in plots that were both invaded by L. latifolium and receiving nutrients. Drought significantly increased percent cover of bare ground by 9%-23%, which may provide future opportunities for invasion, but nutrient addition mitigated this effect. These results indicate extreme drought is an important driver of change in salt marsh plant communities and may impede spread of L. latifolium. Further, nutrient loading appears to help both native and invasive plants cope with extreme drought stress. Additional work is needed to determine whether nutrient addition overall promotes or impedes invasion in salt marshes during drought.