Drivers of broadleaved evergreen species spread into deciduous forests in the southern Swiss Alps

The spread of non-native species into natural communities is a noticeable phenomenon linked to global change. Drivers of such invasions, however, may differ according to specific regional environments. Here, we aim at disentangling the role of selected climate and non-climate drivers on the spread of native (Hedera helix and Ilex aquifolium) and non-native (Prunus laurocerasus and Trachycarpus fortunei) evergreen species in mature deciduous forests in southern Switzerland. Covers of target evergreens were determined using 200 quadratic plots of 100 m(2), distributed on a regular grid of 100 x 100 m and spanning a steep climatic gradient ranging from -0.4 to 3.0 A degrees C in the average temperature of the coldest month. Species covers were used as response variables to be related to climate, stand structure, disturbances, propagule pressure and geomorphology proxies by performing tobit regressions. Propagule pressure resulted to be the main driver of occurrence for three considered species, while meso-climate and stand structure played only a secondary role. In contrast, the presence of native I. aquifolium was mostly correlated with the temperature of the coldest month, though with an overall low impact on cover. Our study demonstrates different drivers of invasion for native and non-native species with similar life history traits. In particular, differences emerge from ecological requirements (niche) and propagule pressure. The ongoing spread of evergreen broadleaved species at the regional scale demonstrates how evergreens invade a largely empty ecological niche, mainly issued by land-use change rather than climate warming.