Positive interactions occur between Phragmites australis lineages across short term experimental nutrient regimes
Introduction
The introduction of a non-native lineage of the wetland grass Phragmites australis to North America has led to a dramatic increase in its relative abundance at the expense of native plants (Saltonstall, 2002; Silliman and Bertness, 2004; Escutia-Lara et al., 2012; Yuckin and Rooney, 2019). Once established, invasive Phragmites forms large mono-specific stands that can reduce animal and plant diversity (Able et al., 2003; Minchinton and Bertness, 2003), and alter biogeochemical cycles (Bernal et al., 2017; Mozdzer and Megonigal, 2013).
The aggressive expansion of this introduced lineage was likely facilitated by changes in land use (Lelong et al., 2007; Meadows and Saltonstall, 2007; Silliman and Bertness, 2004), increased nutrient pollution (Packett and Chambers, 2006; Saltonstall and Stevenson, 2007), and the plant’s own ecophysiological and genetic traits that contribute to its invasiveness (Mozdzer and Zieman, 2010; Mozdzer et al., 2013; Pyšek et al., 2018). Compared to a native American lineage Phragmites australis subsp. americanus, the invasive European lineage has greater phenotypic plasticity (Mozdzer et al., 2013), nitrogen uptake capacity (Packett and Chambers, 2006; Mozdzer and Zieman, 2010), ventilation efficiency (Tulbure et al., 2012) and salt tolerance (Vasquez et al., 2005). These traits have allowed invasive Phragmites to become dominant in many wetland habitats, particularly in areas susceptible to disturbance and nutrient enrichment (Kettenring et al., 2015; Sciance et al., 2016) where it can gain a competitive advantage over native vegetation (Bertness et al., 2002; Holdredge et al., 2010).
Management of invasive Phragmites is costly and not always effective. Current eradication strategies primarily rely on herbicide application, mowing and burning which are used at local scales, and have uncertain results in the long-term (Hazelton et al., 2014). In order to achieve an effective control it is necessary to implement a watershed-scale approach and it needs to specifically target nutrient management (Rickey and Anderson, 2004; Packett and Chambers, 2006; Hazelton et al., 2014; Kettenring, 2011).
Carbon rich soil amendments are widely used in wetland restoration projects in an attempt to increase organic matter (Scott et al., 2020), and have further been evaluated as a tool to control invasive species across different landscapes (Blumenthal et al., 2003; Eschen et al., 2006; Rashid and Reshi, 2010). Carbon additions can stimulate microbial uptake of the newly added carbon together with soil available nitrogen. As nitrogen gets immobilized into the microbial biomass, it becomes temporarily unavailable to plants and other organisms resulting in a reduction in net nitrogen mineralization (Blumenthal et al., 2003; Perry et al., 2010; Rashid and Reshi, 2010). Many plant invasions, including that of Phragmites, are facilitated by excess nitrogen (Perry et al., 2010), therefore limiting available nitrogen through carbon amendments could serve as an additional management strategy (Hazelton et al., 2014). On the other hand, nitrogen additions would be expected to promote growth of invasive Phragmites likely in detriment of native plants (Holdredge et al., 2010).
The goal of our study was to assess the competitive interactions between native and invasive Phragmites under carbon (sawdust) or nitrogen (urea) amendments, and evaluate the potential of carbon additions as a tool for control of the invasive lineage and for restoration of native Phragmites. We established a greenhouse competition experiment with native and invasive Phragmites and added either sawdust or urea to assess plant competition outcomes. We predicted that sawdust addition would promote nitrogen immobilization, and favor native Phragmites, which is considered to be a low-nutrient specialist (Holdredge et al., 2010). Urea addition would instead promote nitrogen mineralization, and have a greater effect on aboveground biomass production of the invasive lineage than the native, making it a more effective competitor.
Section snippets
Greenhouse set-up
Phragmites rhizomes were collected at random into two 5-gallon buckets in a freshwater tidal wetland in the Patuxent River in Maryland, USA on March 2015 from an invasive (38°40'1.16"N, 76°41'57.48"W) and a native (38°42'17.32"N, 76°42'9.11"W) stand. Sampled rhizomes likely represented multiple genotypes, as suggested in a study of subestuaries of the Chesapeake Bay that found 92 % of invasive Phragmites patches have at least 2 genotypes, while over half of them have at least 4 distinct
Results
Unexpectedly we observed predominantly positive interactions (i.e., facilitation) when native and invasive Phragmites were grown together in the same pots. The calculated Relative Interaction Index values across urea and sawdust treatments were significantly greater than 0 for aboveground (t30 = 15.4, p < 0.001) and belowground biomass (t27 = 32.47 p < 0.001) (Fig. 1). This was also observed for both components of aboveground biomass (leaves and stems) and belowground biomass (rhizomes and
Discussion
To our considerable surprise, we found facilitation to be the predominant interaction between native and invasive Phragmites across soil amendments, as reflected by greater above and belowground biomass of both lineages in mixture planting treatments (Fig. 1). Interactions between native and invasive Phragmites have been assumed to be strongly in favor of the invasive at the expense of the native, and several studies have attributed the displacement of native plants to competitive exclusion by
Author statement
Martina Gonzalez Mateu: Data curation, Formal analysis, Project administration, Investigation, Methodology, Visualization, Writing-original draft.
Stephanie Yarwood: Conceptualization, Funding acquisition, Methodology, Resources, Supervision, Validation, Writing - review & editing.
Andrew Baldwin: Conceptualization, Funding acquisition, Methodology, Resources, Supervision, Validation, Writing - review & editing.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
We would like to thank Zach Berry, Diane Leason, Amy Kuritzky, Jessica King, Lindsay Wood, Josh Gaimaro, Christine Maietta, Gianna Robey, Amr Keshta and Patricia Delgado for their help to carry out this research. This project was supported by a grant from the Maryland Water Resources Research Center (2014MD314B), Maryland, USA.
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