Distribution of evergreen broad-leaved woody species in Insubria in relation to bedrock and precipitation

An increasing number of evergreen broad-leaved species has naturalised in deciduous lowland forests near the lakes of southern Switzerland and northern Italy (Insubria), favoured by rising winter temperatures of the last few decades. We investigated how the abundance of these species varies across the Insubrian region, and whether distribution patterns can be explained by the responses of particular species to local environmental constraints. Species composition, cover values and maximal size of exotic and indigenous evergreen broad-leaved species were recorded at 22 study sites throughout the Insubrian region and analysed in relation to the regional precipitation gradient and geological bedrock. The survey revealed distinct differences in the distribution and composition of evergreen broad-leaved species: some species were restricted to the moister western part of Insubria, probably due to low drought tolerance, whereas other species were mainly found in the drier eastern part of Insubria or on calcareous rock, probably due to competitive exclusion from more fertile sites. These ecological preferences generally correspond to those reported from the core range of the species, but some local limitations suggest a restricted ecological breadth of species occurring at their northern range margin.