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  1. Abstract

    Extreme climate events, such as drought, are becoming increasingly important drivers of plant community change, yet little is known about their impacts on invasive plants. Further, drought impacts may be altered by other anthropogenic stressors, such as eutrophication. We found drought dramatically reduced density of invasiveLepidium latifoliumin salt marshes, and this die‐back was mitigated by nutrient addition. In a 3‐yr field experiment (2014–2016) conducted during an unprecedented drought (2012–2015), we tracked the effects of drought and nutrient addition on the plant community. We conducted this research at four salt marshes across a salinity gradient in the San Francisco Bay, California, USA. We manipulated paired native and invaded plots, one‐half of which were treated monthly with N and P for 1.5 yr during the most intense period of the drought and one subsequent wet winter. In addition, we monitored unmanipulatedL. latifolium‐invaded transects within our freshest and most saline sites throughout the three years of our manipulative experiment and one additional wet winter. We documented a dramatic die‐back of invasiveL. latifoliumduring extreme drought, with reductions in stem density (52–100%) and height (17–47%) that were more severe at low salinity sites than high salinity sites. We found nutrient application lessened the effect of drought onL. latifoliumstem density, but not height. In native plots, extreme drought reduced native plant cover (4–24%), but nutrient addition mitigated this impact. Interestingly, native plants in invaded plots did not suffer reductions in cover due to drought, perhaps because they were simultaneously benefiting from the die‐back of the invader. Our results show drought negatively impacted both native and invasive plants and this impact was stronger on the invader, which experienced persistent declines two years after the end of the drought. However, by mitigating the effect of drought on invasive plants, nutrient addition potentially erased the advantage drought provided native plants over invasive plants under ambient nutrient conditions.

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  2. Abstract

    Climate change is projected to increase the frequency of extreme drought events, which can have dramatic consequences for ecosystems. Extreme drought may interact with other stressors such as invasion by non‐native species, yet little research has explored these dynamics. Here, we examine the physical mechanisms and temporal scale underlying a dieback of an invasive non‐native plant,Lepidium latifolium,in tidal salt marshes of the San Francisco Bay, California, USA, during an extreme, multi‐year drought occurring from 2012 to 2015. Using generalized additive mixed models (GAMMs), we explored the relationship between eight years of estuarine salinity data and five years ofL. latifoliumdensity data from three marshes spanning a gradient of salinity across the San Francisco Bay. We found a significant time‐lagged (3 yr) effect of estuarine salinity onL. latifoliumdensity, with high salinities preceding reductions inL. latifoliumdensities and low salinities preceding increases. The most dramatic change in stem density, a 54% reduction in 2015, was preceded by a salinity increase of 43% from 2011 to 2012. We found theL. latifoliumdecline was driven by impacts on mature, rather than young, plants. Additionally, we tested the importance of local precipitation in drivingL. latifoliumdensities in a one‐season rain exclusion experiment. We found 100% exclusion of precipitation during one rainy season (January–mid‐May) did not have a significant impact on densities of mature stands ofL. latifolium. Our finding that estuarine salinity was a key driver ofL. latifoliuminvasion dynamics suggests sea level rise, like extreme drought, may hinderL. latifoliuminvasion, as it will also raise estuarine salinities. Further, our study highlights the importance of temporal lags in understanding climate change impacts on biological invasions, which has received very little study to date.

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