Summary 1.A fundamental challenge to invasion ecology is to determine what factors cause an exotic species to spread rapidly long after the initial introduction. The increase of a resource (e.g. nitrogen) could trigger an expansion, but in other instances, species have overcome biological limitations (e.g. an Allee effect) like accumulating sufficient genetic diversity for successful reproduction. Understanding the ecological mechanisms governing plant invasions, such as nutrient enrichment or Allee effects, can be used to improve invasive plant management. 2. We used the invasive, introduced grass Phragmites australis as a model to evaluate the role of nutrient enrichment and Allee effects in invasion. Based on recent studies that demonstrated the importance of sexual reproduction in this plant's spread, we chose to focus our efforts on reproductive output. We examined the effects of patch-level genetic diversity on viable seed production across watersheds of the Chesapeake Bay, USA, with differing levels of anthropogenic development (a proxy for nutrient enrichment). In an outdoor mesocosm experiment, we treated Phragmites plants originating from forested and developed watersheds with elevated vs. ambient nutrients and cross vs. self-pollination and determined the effects on viable seed, floret and inflorescence production. 3. The proportion of viable seeds produced at field sites varied widely and was not directly related to watershed development. Instead, seed viability was positively related to patch-level genetic diversity, and patches in more developed watersheds had higher genetic diversity. Also, plants in larger patches produced a higher proportion of viable seeds. In the mesocosm experiment, seed viability was substantially higher for out-crossed plants. Elevated nutrients resulted in greater floret and inflorescence production, particularly for plants originating from developed vs. forested watersheds. 4. These findings have important management implications: small populations should be controlled before they accumulate sufficient genetic variation for prolific viable seed production, and landscape-scale nutrient management could further limit reproductive output. 5. Synthesis and applications. Our research shows how nutrient enrichment and a weak Allee effect can interact across multiple scales to impact invasion success and how understanding the ecological mechanisms governing plant invasions can be used to better inform invasive plant management.

The number of patches of non-native Phragmites australis in brackish tidal wetlands in the Rhode River subestuary increased from 5 in 1971-72 to 212 in 2007, and the area covered by the patches increased more than 25 times during the same time interval. Genetic analysis of the patches showed that the expansion has primarily been from seed, and genetic similarities between patches indicate that most crosspollination occurs within a distance of 50 m. Comparison of patches in different parts of the subestuary indicate that the expansion of Phragmites australis has occurred at the scale of the entire subestuary and not the scale of subsections of the subestuary dominated by differing upland land-uses.

Summary1. The distribution of genetic variation can be interpreted to understand the timing and mechanisms of invasive species spread. Allee effects, positive relationships between fitness and density or number of conspecific individuals, can play a substantial role in determining the time lag between initial introduction and invasive spread and can produce genetic patterns in invading populations that can be interpreted to learn about factors affecting invasion mechanisms. 2. We examined the distribution of genetic variation in the invasive wetland grass Phragmites australis in the Chesapeake Bay, USA. We used microsatellite analysis to examine the reproductive mode (clonal vs. seed) by which the invasive haplotype of P. australis has spread and the distribution of genetic variation within and among brackish wetlands in nine subestuaries of the Chesapeake Bay. Watersheds associated with the subestuaries were dominated by forests, anthropogenic development or mixed forests and development. 3. Our results suggest that the invasive haplotype of P. australis has spread primarily sexually by seed, rather than clonally, and genetic diversity of patches within subestuaries increased while genetic similarity decreased with increasing development in the surrounding watershed. 4. This suggests a pattern whereby greater genetic diversity of patches may promote more rapid spread due to recruitment of multiple seedlings into a disturbed patch. 5. Synthesis. Evaluation of patterns of genetic distribution can help to identify factors affecting invasion in different environments and so inform management.

The invasion and expansion of the introduced haplotype of Phragmites australis across North America is of growing concern. Previous studies in the Chesapeake Bay region found that Phragmites was more abundant, had higher foliar nitrogen, and produced more viable seeds in brackish wetland subestuaries with more anthropogenic development of the watershed. Here, we focus on a different scale and address issues related to the invasion of Phragmites within a single subestuary, the Rhode River. We evaluated patterns in seed viability, foliar nutrient concentrations, patch size, and genetic variation in ten Phragmites patches in wetlands that occur in the side of the subestuary that is surrounded by forest and 10 patches in wetlands that are in the side of the subestuary that has extensive anthropogenic development. Seed viability varied from 0-60% among the 20 patches but did not differ significantly between patches on the developed vs. forested sides of the Rhode River. Foliar nutrients also did not differ between patches on the two sides of the Rhode River. Seed viability, however, was negatively related to foliar nutrients. Most Phragmites patches consisted of >1 genotype. Larger patches had multiple genotypes, and patches with more genotypes produced more viable seeds. Our study indicates that the Rhode River subestuary behaves as one system with no differences in the measured Phragmites variables between the forested vs. developed sides of the watershed. Our findings also suggest a cyclical process by which Phragmites can spread: larger patches contain more genetic diversity, which increases the chances for cross-fertilization. The subsequent increased production of viable seeds can increase local levels of genetic diversity, which can further facilitate the spread of Phragmites by seed.

Documented and forecasted trends in rising sea levels and changes in storminess patterns have the potential to increase the frequency, magnitude, and spatial extent of coastal change hazards. To develop realistic adaptation strategies, coastal planners need information about coastal change hazards that recognizes the dynamic temporal and spatial scales of beach morphology, the climate controls on coastal change hazards, and the uncertainties surrounding the drivers and impacts of climate change. We present a probabilistic approach for quantifying and mapping coastal change hazards that incorporates the uncertainty associated with both climate change and morphological variability. To demonstrate the approach, coastal change hazard zones of arbitrary confidence levels are developed for the Tillamook County (State of Oregon, USA) coastline using a suite of simple models and a range of possible climate futures related to wave climate, sea-level rise projections, and the frequency of major El Niño events. Extreme total water levels are 123Nat Hazards (2015) 75:2081-2102 DOI 10.1007/s11069-014-1417 more influenced by wave height variability, whereas the magnitude of erosion is more influenced by sea-level rise scenarios. Morphological variability has a stronger influence on the width of coastal hazard zones than the uncertainty associated with the range of climate change scenarios.

No abstract