Biological Invasions In California Wetlands

Author : Rachel Diane Wigginton
Publisher :
ISBN 13 : 9781658413510
Total Pages : pages
Book Rating : 4.4/5 (135 download)

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Download or read book Tidal Marsh Plant Invasion written by Rachel Diane Wigginton and published by . This book was released on 2019 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Invasive species pose a significant threat to ecosystems and native biota, and wetlands habitats are particularly prone to plant invasion. Restoration of wetland systems after removal of invasive plants is complicated by the fact that many wetland invaders act as ecosystem engineers. Climate change is projected to increase the frequency of extreme drought events, which can have dramatic consequences for ecosystems, yet little is known about their impacts on invasive plants or marine systems more generally. Further, drought impacts may be altered by other anthropogenic stressors, such as eutrophication. We explored the interactions among plant invasion and restoration, eutrophication, and extreme drought through three field studies in the wetlands of the San Francisco Bay, CA, USA. First, we studied the impacts of invasion on wetland restoration. Though wetland restoration is common, revegetation after removal of an invasive plant has not often been studied. We examined the recovery of the plant and epifaunal communities within wetlands, which were formerly invaded by the invasive plant ecosystem engineer, hybrid Spartina. We used a landscape scale revegetation program to compare recovery at marshes where invader eradication was paired with revegetation, marshes where only eradication occurred, and native marshes that had never been invaded. We found that after only 1.5 years, revegetated areas had aboveground plant cover comparable to native Spartina marshes. Alternatively, belowground plant biomass at revegetation and eradication only sites remained significantly lower than native sites throughout the course of the study. We found no evidence of epifaunal community recovery in either revegetated or eradication only marshes, apart from a single site where gastropod abundances in revegetated areas were significantly elevated over eradication only areas. Our findings that short-term plant recovery occurred over a different timeline than epifaunal community recovery could have significant implications for revegetation programs, which are often performed to restore physical habitat for vertebrate species of concern that rely on trophic support from invertebrate food sources. Additionally, as wetland restoration is an important component of climate adaptation for sea level rise and carbon sequestration, understanding the dynamics of invasive plant control in these restored systems is of primary importance. Next we examined the compounding stressors or invasion, drought, and eutrophication on wetland plant communities. We found drought dramatically reduced density of Lepidium latifolium, an aggressive invasive plant, and nutrient addition mitigated this effect. In a 3-year 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. We manipulated paired native and invaded plots, half of which were treated monthly with N and P for 1.5 years during the most intense period of the drought and one subsequent wet winter. In addition, we monitored unmanipulated L. 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 invasive L. latifolium during 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 on L. latifolium stem 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. Finally, we examined the physical mechanisms and temporal scale underlying a die-back of invasive L. latifolium during the extreme drought. Using generalized additive mixed models (GAMMs), we explored the relationship between eight years of estuarine salinity data and five years of L. latifolium density data from three marshes spanning a gradient of salinity across the San Francisco Bay. We found a significant time-lagged (3 years) effect of estuarine salinity on L. latifolium density, with high salinities preceding reductions in L. latifolium densities 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. Additionally, we tested the importance of local precipitation in driving L. latifolium densities in a one-season rain exclusion experiment. We found 100% exclusion of precipitation during one rainy season (January to mid-May) did not have a significant impact on densities of mature stands of L. latifolium. Our finding that estuarine salinity was a key driver of L. latifolium invasion dynamics suggests sea level rise, like extreme drought, may hinder L. latifolium invasion, 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.