Plant distribution patterns and ecological gradients in the Southwest Finnish archipelago

This paper reports on a multivariate analysis of a large data set on the occurrence of 900 vascular plant species in over 1500 localities in the Southwest Finnish archipelago, which was collected by the Finnish botanist Ole Eklund. Four major types of islands are distinguished as to island size and degree of human impact. Subdivisions are based mainly on contrasts between the presence or absence of limestone in the bedrock, and of wetlands. The major floristic gradients could be related to the following factors: (1) decreasing island size and degree of human impact and increasing maritime influence; (2) increasing occurrence of limestone. (3) a trend from open habitats with high insolation to forested habitats; and (4) increasing amount of wetlands and rock pools. For the floristic characterization of island types a system of socio-ecological species groups was developed based on Ellenberg's indicator figures for the flora of Central Europe. The old controversy, as to the explanation of the distribution of plans species in the archipelago, between Palmgren ('dispersal is the limiting factor') and Eklund ('environmental differentiation is decisive') is discussed and related to the scale of observation. Our results suggest that environmental differentiation is of prime importance, but on the local scale the species composition of islands may be partly governed by dispersal processes.