North Atlantic Islands with native and alien trees: are there differences in diversity and species-area relationships?

Abstract

Questions Are there differences in species composition and richness between islands that were reforested more than 70 yr ago with the introduced Pinus mugo compared with islands supporting the native Pinus sylvestris? Do the results depend on autocorrelation in geographical space and species ordination space? Species richness is expected to increase as a function of the size of an island; are the responses to island size different between P. mugo and P. sylvestris islands. Does the land-use history have an impact on the current species composition and richness pattern? Location The archipelago is in the oceanic section of the Atlantic bioclimatic zone, west Norway. This archipelago was part of the ancient and widespread treeless heathland found along the European west coast. Methods Data on vascular plants were compiled from the forested islands, and their differences in species composition were analysed by ordination. The hypotheses were tested by means of t-tests and generalized linear models, the spatial component was accounted for by means of Moran's I and spatial autoregression with the moving average approach. This was done both in geographical space and species ordination space. Results There are more vascular plants on the islands with introduced P. mugo than on the islands with native P. sylvestris. The latter have rather homogenous undergrowth dominated by bryophytes. This may explain lower richness on islands with native forest and why island size is not correlated with species richness on these islands. In contrast, P. mugo is easily wind-felled in autumn storms, which keeps rocky microhabitats exposed to air and new forest habitats are created. Species that are associated with the previous land-use system (grazing) prevail on islands with introduced pine, and thus contribute to higher plant richness. Conclusions The difference in species richness and island species–area relationship (ISAR) between P. mugo and P. sylvestris islands may relate to the same underpinning causes. Species from the old land-use system have survived on P. mugo islands, but not in the late-successional forest with a more closed canopy that has developed on P. sylvestris islands. Thus habitat and species richness is higher and increases with area on P. mugo islands but not on P. sylvestris islands.