Northeastern Naturalist Vol. 24, No. 3 R.M. Martin 2017 235 2017 NORTHEASTERN NATURALIST 24(3):235–238 Effects of Warming on Invasive Phragmites australis and Native Spartina patens Seed Germination Rates and Implications for Response to Climate Change Rose M. Martin1,* Abstract - The introduced Eurasian subspecies of Phragmites australis (Common Reed) is a common invader of North American coastal wetlands where it outcompetes native high-marsh species such as Spartina patens (Saltmeadow Cordgrass). Although Common Reed is known to colonize coastal marshes via clonal replication, recent research has indicated that germination from seed may also be an important mechanism by which this species spreads in saltmarsh systems. Sexual reproduction via outcrossing introduces genetic diversity into populations; thus, an increase in seed germination may have implications for the plant’s invasiveness. I tested the effect of temperatures projected to occur by the year 2100 on germination rates of 2 species: invasive Common Reed, and Saltmeadow Cordgrass. Projected end-of-century temperatures doubled Common Reed germination, but inhibited Saltmarsh Cordgrass germination. The potential for variability in responses to warming among Common Reed and Saltmeadow Cordgrass populations at larger geographic scales precludes generalization of results of this study without further investigation. However, my results suggest that warming may differentially affect germination of Common Reed and a native species it commonly displaces. This finding may have ecological implications depending on how these and other invasive and native species respond to continued climate change. Introduction Sexual reproduction by outcrossing increases genetic diversity (Barrett et al. 2008), which determines the ability of a species to adapt to environmental stressors. Increased reproduction by seed rather than clonal spread may be one response to global temperature changes. Phragmites australis (Cav.) Trin. ex Steud. (Common Reed), in particular the non-native Eurasian subspecies, frequently colonizes and dominates wetland ecosystems (Saltonstall 2002). Common Reed’s success may be partly determined by reliance on sexual as well as asexual (clonal) reproduction (Kettenring and Mock 2012). Environmental conditions, including temperature and salinity, may affect seed germination rates for Common Reed (Greenwood and MacFarlane 2006); a warming climate may alter germination patterns and, therefore, genetic diversity in Common Reed populations. The aim of this experiment was to test germination rates of Common Reed and a native coastal marsh species it often displaces, Spartina patens (Ait.) Muhl. 1Department of Biological Sciences, University of Rhode Island, 120 Flagg Road, Kingston, RI 02881. *Current address - ORISE Participant at EPA Atlantic Ecology Division, 27 Tarzwell Drive, Narragansett, RI 02882; rose.m.martin.31@gmail.com. Manuscript Editor: Thomas Philbrick Northeastern Naturalist 236 R.M. Martin 2017 Vol. 24, No. 3 (Saltmarsh Cordgrass), under conditions predicted to occur by the end of this century (IPCC 2013). Methods I conducted the experiment in vented growth chambers (Conviron® Model PGR15, Conviron, North Branch, MN). I set the daytime temperature to 32.8 °C and the nighttime temperature at 22.8 °C based on current regional summer averages. To simulate end-of-century temperatures predicted for the northeastern US, I increased the temperatures in the climate-change treatment chamber 4.5 °C based on the median of the upper bounds of model-predicted temperature increases for the year 2100 (IPCC 2013). I set the chambers to hold the humidity at ambient levels in the treatment and control chambers, and set light-period durations (averaging 393.7 μmol irradiation) at 15 h of light and 9 h of darkness. I sowed seeds of each species on the surface of clean, sieved Metro Mix commercial potting mix (Sungro, Agawam, MA) in 20-cm diameter nursery pots. I evenly distributed 300 seeds of each species among 10 pots for each of the 2 temperature treatments (present-day temperatures and year-2100 predicted temperatures). Common Reed seeds were harvested from a New York population, and I purchased Saltmeadow Cordgrass seeds of northeastern US origin from Environmental Concern, Inc. (St. Michaels, MD). Water was provided by regular misting. I counted seedlings for 2 weeks after first germination. I employed 2-factor ANOVA (species x temperature simulation) and post-hoc Tukey HSD tests to test treatment effects on germination rate. Statistical analyses were performed in JMP 10.0 and interpreted at a significance level of α = 0.05. Results and Discussion There was significant interaction between species and temperature simulation (F1,39 = 26.54, P < 0.01; Fig. 1); Common Reed and Saltmeadow Cordgrass germination rates responded differently to increased temperature. The Common Reed germination rate more than doubled under increased temperature conditions, while the Saltmeadow Cordgrass germination rate was significantly reduced. Under present-day temperature conditions, the germination rate of Saltmeadow Cordgrass was 6 times higher than that of Common Reed. In contrast, under the year-2100 temperature simulation, the germination rate did not differ significantly between the species. These results indicate distinct germination responses of invasive Common Reed and native Saltmeadow Cordgrass to the warming conditions predicted to occur by the end of the century. Changes in germination rate may be linked to degree of ecological success. Although Common Reed requires low-salinity soil conditions in order for successful germination to occur (Wijte and Gallagher 1996), establishment of populations by seed along the upland edge of saline coastal marshes could allow for encroachment of a genetically diverse population into coastal wetlands. Already a successful colonizer of disturbed coastal wetland systems at the expense Northeastern Naturalist Vol. 24, No. 3 R.M. Martin 2017 237 of native vegetation communities (Silliman and Bertness 2004), Common Reed may benefit from the genetic diversity that would arise from increased sexual reproduction, particularly since the species has limited self-compatibility (Lambert and Casagrande 2007). In contrast, Saltmeadow Cordgrass currently faces a confluence of interacting threats from sea-level rise (Donnelly and Bertness 2001), shoreline development (Bertness et al. 2002), and Common Reed invasion into the high marsh (Silliman and Bertness 2004). A decrease in germination rate in response to warming temperatures could potentially affect the response of Saltmeadow Cordgrass to changes affecting its habitat. The germination responses of the seeds tested provide insight into possible future effects of climate change on Common Reed and Saltmeadow Cordgrass. However, germination rates and responses to warming may vary among populations and across latitudinal gradients. The responses of the populations I tested may not have been representative of others throughout the range. Therefore, there is a need for future research into the impacts of climate change on germination rates of Common Reed and native marsh grasses to incorporate populations spanning a variety of latitudinal and environmental gradients. Figure 1. Saltmeadow Cordgrass and Common Reed mean germination rate ± standard error under present day and predicted year-2100 temperatures. Letters represent results of Tukey HSD tests. Bars not sharing the same letter are significantly di fferent. Northeastern Naturalist 238 R.M. Martin 2017 Vol. 24, No. 3 Acknowledgments I thank Dr. Serena Moseman-Valtierra for experimental-design advice and review of an earlier version of this manuscript, and 2 anonymous reviewers for their helpful comments. Dr. Laura Meyerson provided Common Reed seeds, and Ivy Burns, Tori Moebus, and Sean Kelly helped with experimental setup. Dr. Lisa Tewksbury and the University of Rhode Island Plant Sciences Department provided access to greenhouse facilities. 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