Microclimate variability in alpine ecosystems as stepping stones for non-native plant establishment above their current elevational limit

Alpine environments are currently relatively free from non-native plant species, although their presence and abundance have recently been on the rise. It is however still unclear whether the observed low invasion levels in these areas are due to an inherent resistance of the alpine zone to invasions or whether an exponential increase in invasion is just a matter of time. Using a seed-addition experiment on north- and south-facing slopes (cf. microclimatic gradient) on two mountains in subarctic Sweden, we tested the establishment of six non-native species at an elevation above their current distribution limits and under experimentally enhanced anthropogenic pressures (disturbance, added nutrients and increased propagule pressure). We found a large microclimatic variability in cumulative growing degree days (GDD) (range = 500.77 °C, SD = 120.70 °C) due to both physiographic (e.g. aspect) and biophysical (e.g. vegetation cover) features, the latter being altered by the experimental disturbance. Non-native species establishment and biomass production were positively correlated with GDD along the studied microclimatic gradient. However, even though establishment on the north-facing slopes caught up with that on the south-facing slopes throughout the growing season, biomass production was limited on the north-facing slopes due to a shorter growing season. On top of this microclimatic effect, all experimentally imposed anthropogenic factors enhanced non-native species success. The observed microclimatic effect indicates a potential for non-native species to use warm microsites as stepping stones for their establishment towards the cold end of the gradient. Combined with anthropogenic pressures this result suggests an increasing risk for plant invasion in cold ecosystems, as such stepping stones in alpine ecosystems are likely to be more common in a future that will combine a warming climate with persistent anthropogenic pressures.