Abiotic Limitation Of Invasive Plants In The High Salt Marsh Transition Zone

Author : Megan Amanda Kelso
Publisher :
ISBN 13 : 9780438930025
Total Pages : pages
Book Rating : 4.9/5 (3 download)

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Download or read book Impacts of Extreme Drought, Eutrophication, and Plant Invasion on Salt Marshes and Blue Carbon written by Megan Amanda Kelso and published by . This book was released on 2018 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Coastal salt marshes are ecologically rich, productive systems that provide many benefits including flood protection, water filtration, and habitat for coastal fish, invertebrates, and shorebirds. In addition, they are one of the most effective natural systems at sequestering carbon, storing it three times as densely as most forest systems per unit area. These valuable ecosystems are increasingly threatened by anthropogenic stressors such as invasive species, eutrophication, and climate change. Each of these stressors can have large independent impacts on ecosystems and may also have important interactive effects. In this dissertation, I explore the interacting effects of extreme drought associated with climate change, eutrophication, and invasion by an aggressive non-native plant called Lepidium latifolium on tidal salt marshes and their ability to sequester carbon. In chapter 1, my collaborators and I explored how the combined stress of extreme drought and elevated nutrients altered invasion dynamics of L. latifolium in tidal salt marshes of San Francisco Bay, CA, USA. From 2012-2015, California experienced an unprecedented drought that stressed tidal salt marsh ecosystems. In a three-year field experiment from 2014 to 2016, we tracked the effects of this extreme drought on L. latifolium invasion using field experiments to test how nutrient addition altered invasion dynamics at four salt marsh sites along a salinity gradient in San Francisco Bay. We documented a dramatic die-back of invasive L. latifolium during the extreme drought, including large 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 addition 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 (4%-24%), 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 increased during the drought in plots that were both invaded by L. latifolium and received nutrients. These results indicate extreme drought is an important driver of change in salt marsh plant communities and may impede the invasion and spread of L. latifolium. Further, nutrient loading appears to help both native and invasive plants cope with extreme drought stress. In chapter 2, we examined the physical mechanisms and temporal scale underlying the dramatic die-back of L. latifolium documented in chapter 1. Extreme drought affects estuarine tidal salt marshes both by reducing local rainfall and by raising estuarine salinity through reduced fresh-water inflows. We tested the impact of local rainfall on L. latifolium by experimentally manipulating rainfall for one year in plots at a salt marsh in south San Francisco Bay. Surprisingly, we did not detect a significant impact of local rainfall on density of the invader, despite diverting 100% of the rainfall that would have fallen into experimental plots during the rainy season (January to mid-May). Next, we explored the relationship between bay salinity and invasion using eight years of monthly water column salinity data and five years of L. latifolium density field data at three salt marsh sites. We found a significant time-lagged (three years) effect of bay salinity on L. latifolium density across our three sites, 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. Our finding that water column salinity was a major driver of L. latifolium invasion dynamics suggests sea level rise, like extreme drought, may hinder L. latifolium invasion, since rising sea levels will also raise estuarine salinities. Further, our study highlights the importance of temporal lags in climate change impacts on biological invasions, which has received very little study to date. In chapter 3, we examine the effects of L. latifolium invasion and eutrophication on the valuable ecosystem service of carbon storage provided by coastal salt marshes, known as “blue carbon” storage. Specifically, we measured standing stock of carbon and rates of decomposition, an important underlying process driving future rates of carbon storage. We found invasion by L. latifolium overall decreased blue carbon, a surprising result that contradicts the general assumption that invasive plants store more carbon than native communities. We identified a synergistic interaction between invasion and nutrient addition on rates of below-ground decomposition, where nutrient addition amplified the accelerating effect of invasion on below-ground decomposition. We found nutrient addition increased carbon in above-ground and below-ground biomass. Our results demonstrate that anthropogenic stressors can alter blue carbon in independent and interactive ways. Taking a multi-stressor approach to studying blue carbon ecosystems will improve predictions of the permanence of blue carbon storage and might explain some of the observed variability in carbon storage among sites, two of the largest technical hurdles impeding the development of comprehensive blue carbon policy. Coastal wetland ecosystems are under threat from multiple concurrent stressors and understanding their combined impacts on these ecosystems and the valuable carbon storage service they provide is critical for effective management and policy development.