Negative Impacts of an Invasive Pasture Grass on a Native Wetland Herb Increase with Hydroperiod

Many wetland ecosystems face the compounding challenges of invasive species and changing precipitation regimes. However, these effects and their interactions are not experienced uniformly by native plants across a given wetland due to temporal and spatial heterogeneity in both environmental and plant demographic factors. While hydroperiod is known as a key factor influencing plant community composition in wetlands, less is known about how native species’ demographic traits may vary across a hydroperiod gradient and how this variation could impact interactions with non-native species. To fill these gaps, we conducted a plant demographic study in a wet meadow in south-central Idaho, United States, to understand: (1) how the demographic traits (survival, growth, flowering) of a native perennial geophyte Camassia quamash vary across a hydroperiod gradient, and (2) how the impacts of an invasive, rhizomatous pasture grass, Alopecurus arundinaceus , on this native species may vary across this same gradient. We found that C. quamash exhibits higher survival, but reduced flowering in drier upslope areas of the wet meadow, while A. arundinaceus had a greater per-unit negative impact on C. quamash in longer hydroperiod areas. This study is one of very few to assess how plant demographic traits vary with hydroperiod in wetlands and we suggest that variation in native plant demographic traits across environmental gradients should be considered in the study of species interactions in wetlands and beyond.