The influence of site factors and proximity of adjacent vegetation on tree regeneration into roadslopes

Many studies have shown evidence of very rapid natural colonization of trees on roadslopes. Tree colonization on roadslopes can be a useful tool in the ecological restoration of these degraded areas. However, little is known about the characteristics of the tree species capable of colonizing these novel habitats. Moreover, it is necessary to know how tree species respond to the two main limitations in these areas: microsite availability (i.e., roadslope characteristics) and seed arrival, which is related to propagule source, dispersion vector and landscape characteristics. The present study aims to investigate the natural colonization of tree species on roadslopes, as well as the factors determining their occurrence. We identified all tree individuals on 150 roadslopes, along 51km of a motorway. A total of 1143 individuals belonging to 18 species was recorded. Most individuals found resulted from a natural colonization process, although we found various fruit trees (present in 7 roadslopes) and Robinia pseudoacacia (2 roadslopes), which are probably associated with unintentional human-mediated seed dispersal. Only the six most abundant species were analysed in detail: Populus nigra, Quercus ilex, Quercus pyrenaica, Fraxinus angustifolia, Salix spp. and Ulmus pumila. Each roadslope was characterized both by site variables and by surrounding variables. We analysed the effect of the descriptive variables on the occurrence and density of each species on the roadslopes, and we subsequently used decision trees (classification and regression trees) to analyse the combined effect of the different predictors considered. Our results show successful tree colonization on the roadslopes, although they appear to indicate limitations to colonization associated with the availability of propagules. Furthermore, tree species dispersed by animals required a continuous flow of seeds favoured by an appropriate community of seed dispersers and a suitable landscape structure. By contrast, wind-dispersed tree species basically need favourable site characteristics on the roadslopes.