Analysis of the riparian habitat invasion by three tree exotic species in Spain

Abstract

This study provides a diagnosis and assessment on the invasion of Spanish riparian habitats by Ailanthus altissima (Mill.) Swingle, Robinia pseudoacacia L. and Ulmus pumila L. The riparian corridors of the Henares and Oria rivers and a stretch of the middle Ebro River were surveyed to locate all the stands of these three species. Origin (planted/spontaneous), size, density and habitat characteristics (biotic, abiotic and human influence) of every stand were considered to determine the degree of invasion of each zone and species. Data on stand and habitat characteristics were also collected. The degree of invasion was greater in the Henares River riparian corridors, where human influence and propagule density were higher. Each species showed a greater degree of invasion in a different study area. Spontaneous stands of the Henares and Ebro riparian vegetation were smaller than those of the Oria River. All three species coexist in habitats with high light availability, low competition and strong human influence; however, they may also occur in areas with a very dense canopy. Differences in habitat characteristics between planted and spontaneous stands could largely be explained by specific dispersal mechanisms operating at each species.