Range limits and population dynamics of non-native plants spreading along elevation gradients

• Six-year regional monitoring of distribution of non-native flora across the Swiss Alps.
• Multi-year, fine-scale spatiotemporal population dynamics of Erigeron annuus and Solidago canadensis along elevation gradient.
• High elevation range limits of spreading species are characterised by very high population turnover and source-sink dynamics.
• Monitoring changing species distributions with global change should include information on colonization and extinction events.

Monitoring the elevation limits of non-native species is a potentially sensitive means of detecting effects of environmental change on invasion dynamics and species ranges. The aim of this study was to investigate temporal changes in the distribution of non-native plant species along elevation gradients in the Swiss Alps by repeating, in 2009, a regional survey from 2003 of 230 sites ranging in elevation from 200 to 2400 m a.s.l. We also studied the fine-scale spatiotemporal population structure of two of the non-native species – Erigeron annuus and Solidago canadensis – along an elevation gradient in a heterogeneous landscape.Most non-native species in the Swiss Alps rapidly decline in probability of occurrence as elevation increases. We found little change in the elevation ranges limits of species in time, suggesting that most species are not rapidly expanding at their high elevation range limits. For most species, populations were more dynamic (colonizations and extinctions) at the upper range limit where occurrence rapidly declined. Population turnover was negatively correlated with probability of occurrence at the regional and local scale. At low elevations, where probability of occurrence was higher, the number of individuals in a population was also greater. At the local and regional scales, E. annuus and S. canadensis had similar range limits. At the local scale, propagule production of both E. annuus and S. canadensis was greater in the core of their distributions at lower elevations, and distance to nearest neighbor increased as occurrence decreased.Our data demonstrate that range limits of non-native species at high elevation are associated with high population turnover, which results in a transition zone characterized by source-sink dynamics. Populations within this zone exhibit reduced probability of occurrence, and smaller patches. This result has important implications for the monitoring of spreading species along environmental gradients. To understand these limits and predict range expansion, multi-year monitoring and demography data that includes information on colonization and extinction events will be needed.