Patterns of variation in vascular plant species richness and composition in SE Norwegian agricultural landscapes

Abstract

Plant species richness and composition were studied in 16 1 km 2 agricultural landscape plots in SE Norway, in which a total of 2201 patches of 18 types (built-up areas omitted) were identified and mapped. Plots differed with respect to area covered by ploughed land (6–93, mean 47%) and woodland (1–90, mean 41%), while the area of boundary transitions, pastures and semi-natural land was low to moderate in all plots (2–22, mean 12%). Landscape complexity varied considerably among plots (from 31 to 285 discrete patches). A total of 738 species were recorded in the total studied area of 14.02 km 2 , more than twice as many as in comparable studies with plot-based sampling. The frequency distribution of species in patches were strongly right-skewed with a median frequency of 1%, showing that in the modern agricultural landscape species are patchily distributed and that it is very common to be rare. Species frequencies in plots were bimodally distributed, demonstrating existence of a large group of regionally widespread, landscape-scale core species. Two major floristic gradients were identified by ordination of patch species composition: (1) a gradient from species with preference for open sites and fertile soils to species with tolerance for shaded sites and infertile soils, reflecting variation from intensively used areas (ploughed land, with prominence of weeds) via boundary transitions, extensively used (pastures) and abandoned (semi-natural) land, to woodland. Within ploughed land this gradient reflected variation in intensity of use, within abandoned land it reflected time since abandonment and the re-growth succession under closing canopies. (2) A gradient related to preference for dry versus moist sites. Strong between patch variation in species composition within patch types suggested that factors other than those used to delimitate land type were also important determinants of the species composition. Patterns of variation in species composition and species richness were found to differ among scales and the implications of this result are discussed.