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

Abstract

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 nonnative (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 × 100 m and spanning a steep climatic gradient ranging from −0.4 to 3.0 °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 mesoclimate 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.

Low litter cover, high light availability and rock cover favour the establishment of Ailanthus altissima in forests in southern Switzerland

“…However, the distribution of tree species is determined not only by the availability of suitable microsites for their establishment, but also by seed production and dispersal (Nathan and Muller-Landau 2000;Turnbull et al 2000). Limitations in propagule pressure are often particularly important for expanding non-native tree species (Pyšek et al 2009;Terwei et al 2013;Conedera et al 2017;Sullivan and Franco 2017;Dyderski and Jagodziński 2018). Accordingly, the distance to the next seed source played a role for the occurrence of A. altissima (Table 3), which is in line with previous studies (Radtke et al 2013;Höfle et al 2014).…”

Non‐native palms (Arecaceae) as generators of novel ecosystems: A global assessment

“…Ongoing global warming will promote the spread of non‐native palm species in the subtropics, warm temperate regions of the Northern Hemisphere (>30°N) and higher elevations (Reichgelt et al., 2018). The expansion of the Chinese windmill palm ( T. fortunei ) in Southern Switzerland (Figure 1d) is an example where global warming expanded palm niche space, increased urbanization increased propagule pressure and less intensive forest management allowed establishment (Conedera, Wohlgemuth, Tanadini, & Pezzatti, 2018; Fehr & Burga, 2016; Walther et al., 2007).…”

Ancient refugia and present‐day habitat suitability of native laurophylls in Italy

“…Human impact could be another factor that might have contributed to limit the area occupied by the native laurophylls at present. Native laurophylls are late‐successional species (Barbero & Quézel, ) requiring mature forest stands with relatively closed canopies for a successful colonization (Conedera, Wohlgemuth, Tanadini, & Pezzatti, ; Grund, Conedera, Schröder, & Walther, ). They might therefore have faced a drastic reduction of suitable niches for their spreading due to the long history of anthropogenic forest clearance in Mediterranean countries (Grove & Rackham, ).…”

“…We suggest that the abandonment of agricultural open areas and the reduction of some traditional forest management (e.g., wood pastures), inducing the increase and ageing of forests (Bodin et al., ; Falcucci, Maiorano, & Boitani, ), could have led to an increased availability of suitable sites for the expansion of all late‐successional species, also including the native laurophyllous ones. Moreover, the availability of free niches, combined with the increasing propagule pressure of native laurophylls, is likely to facilitate their spreading (Conedera et al., ). Apart from a land‐use related facilitation of the spread of laurophylls, this process of expansion is likely to be enhanced in future by expected milder winter temperatures too (Walther, ).…”