Frost as a potential barrier to the invasion of evergreen trees in Southern Switzerland

Climate change causes plant species to migrate towards the poles and consequently modify the biodiversity, ecosystems and ecosystem services of their new habitats. In European forests, a large amount of non-native plant species introduced through globalization find suitable conditions to survive and spread. In Southern Switzerland, the invasion success of several non-native evergreen species in natural forests questions the capacity of these species to continue to spread northward, making it necessary to identify and understand the responsible mechanisms to accurately predict future forest composition. Whereas invasion success of a species is generally explained by an enhanced photosynthetic performance compared to local species, extreme cold temperatures have been identified as an important limiting environmental parameter to the northward spread of tree species. In this study, we measured the frost resistance (T50) and photosynthetic performance of three native (Ilex aquifolium, Hedera helix and Taxus baccata) and four non-native (Trachycarpus fortunei, Prunus laurocerasus, Laurus nobilis and Cinnamomum glanduliferum) evergreen species of southern Alps forests during their dehardening and hardening periods. We further used the measured frost resistance of non-native T. fortunei to compute its future potential distribution in Switzerland with a climate envelope model. Native evergreen species showed a highly seasonal frost resistance which reached more negative values than non-native species in winter. On the other hand, non-native species’ frost resistance was constant throughout the year. The results of our climate envelope model showed that T. fortunei was limited by the minimum daily temperature, but that most of the territory in Switzerland already has a climate warm enough to accommodate this species. Our study reveals that minimum temperature is unlikely to significantly limit the spread of T. fortunei in the future decades in Switzerland, raising additional concerns about the capacity of invasive evergreen species to colonize forests in the region.