Non-native plant species in mountainous areas: A case study of Linaria dalmatica in the Greater Yellowstone Ecosystem

Non-native plant species are increasingly being viewed as a threat to sub-alpine and alpine ecosystems, and the need to determine the causes and mechanisms of non-native plant invasions in mountain systems has been recognized. This study was initiated to provide: (1) useful information for managers in the local area and, (2) information that can be used in concert with data from similar studies to help elucidate the causes and mechanisms of non-native plant species invasions in mountain systems at the global scale. Eighteen populations a non-native species (Linaria dalmatica) and 14 populations of a closely related native species (Castilleja miniata) were surveyed along an elevation gradient in the Greater Yellowstone Ecosystem. Climate, environmental, and species demographic data were collected from each site from 2008--2011. Climate and environmental predictors varied along the elevation gradient, with stem density of the two test species being explained by different sets of predictors. Vital rates of L. dalmatica were found to be more variable and more highly associated with climate and environmental predictor variables than those of C. miniata. The population growth rate of L. dalmatica did not show any consistent trend with elevation, but did appear to be influenced by extremely cold temperatures and predictors related to the vegetative community. Probability of occurrence of L. dalmatica was related to elevation. Percent cover of L. dalmatica was not associated with elevation, instead being strongly associated with vegetative community characteristics such as percent perennial cover. The current range of L. dalmatica was broader than the majority of native species in the study area, and the non-native plant community in the study area was significantly more nested with increased elevation than the native plant community. These results suggest that L. dalmatica is a broadly adapted species. Overall, our data fail to provide conclusive evidence of climatic limits for this species, but suggest that it may be limited from upward expansion by extremely cold winter temperatures. Until conclusive evidence of climatic limitation is provided, the upper populations of this species should be monitored and managed to prevent further spread into sub-alpine/alpine environments.