Climbing high: Probability of occurrence and patterns of invasion of Linaria dalmatica along an elevation gradient

Background/Question/Methods

Movement of non-native plant species into mountainous areas is an increasingly discussed topic.  Although these areas have traditionally been viewed as resistant to plant invasions, nearly 1,000 non-native plant species have been identified in mountainous areas world-wide.  General theories of plant invasion in mountain systems have been developed, but information at the species level is limited.  This information is critical to advancing our knowledge of plant invasions and for providing information to land managers.  This study was designed to investigate the patterns and consequences of Linaria dalmatica invasion in mountainous areas of the Greater Yellowstone Ecosystem (GYE) in the context of previously noted trends in: 1) probability of occurrence (PO) for the species and 2) non-native species richness in mountain systems.  Eighteen study sites were established along three mountain roads in the Absaroka-Beartooth Range (Montana and Wyoming, USA) in 2008.  Climate/environmental data were collected at each site in addition to demographic data for L. dalmatica.  Our objectives were to: 1) determine if L. dalmatica is currently limited by climate along an elevation or environmental gradient, and 2) determine how this species influences or is influenced by the vegetative communities and environmental conditions along an elevation gradient.

Results/Conclusions

L. dalmatica was found within the study area from 1,750 m to 2,300 m elevation.  Elevation and climate/environmental variables were used as primary predictors in analyses.  Predicted PO of this species had a hump-shaped relationship with elevation (p < 0.001), but no relationship with stem density.  Stem density of L. dalmatica showed a hump-shaped pattern with elevation in 2008 and 2009 (p < 0.05).   By 2011, stem density increased linearly with elevation (p < 0.05).  Cover of L. dalmatica was negatively associated with native species richness (p = 0.001) and perennial species cover (p = 0.01), but had no significant association with elevation.  PO data for this species suggest that there may be a climatic limit preventing this species from spreading to higher elevations, but we were not able to confirm this using demographic and climate data.  However, differences in the influence of elevation on stem density, PO, and cover for this species suggest that climate may primarily limit the establishment of the species in new areas, but once the species is established, the vegetative community is more influential on its expansion.  Thus, the relationship between PO and establishment success for this species needs to be investigated further.