Could nitrogen deposition and warmer temperatures make the Rocky Mountain alpine vulnerable to non-native plant species invasions?

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While long-term nitrogen deposition and variation in summer temperature directly impact alpine plant community ecology, non-native plant invasions are a possible indirect effect of these factors on native plant communities.  We explored whether nitrogen deposition and increased temperature could promote upward expansion of invasive species into subalpine and alpine ecosystems in Colorado’s Front Range.  First, we conducted a survey along roads from upper montane-alpine to determine the abundance and upper elevation limit of non-native species.  These data were used to assess whether certain traits are associated with high elevation establishment along roads.  We then used Bromus tectorum (cheat grass) and Taraxacum officionale (common dandelion) to test whether nitrogen deposition and warmer growing season temperatures promote germination and growth in a growth chamber experiment.  While there were no treatment effects on germination, some Bromus traits responded positively to the warm temperature only in the nitrogen treatment.  Taraxacum growth did not respond to either treatment, and yet this species is a successful alpine invader in the Rocky Mountain alpine.  These results indicate multiple factors could promote upward expansion of invasive species’ ranges and the relative importance of these factors will likely be differ among species based on plant traits.