Plant species richness in grasslands: the relative importance of contemporary environment and land-use history since the Iron Age

The richness of vascular plants in all patches of dry semi-natural grassland within a landscape was investigated. The patch level richness was scaled up to the level of parish to match data on land-use intensity from Iron Age through historical time. Three measures of diversity were obtained: the local species pool, an area-independent diversity measure (the slope of the species number vs log area regression line), and the β-diversity, A gradient through the study area in the density of grassland and a corresponding clinal variation in grassland plant species diversity were found. Explanatory models were built by partial least squares regression and conventional stepwise multiple regression. Data on contemporary environmental conditions in the grassland patches and in the parish as a whole were added first, and then data on continuously older times in sequence. In the stepwise regression analyses the variables were also added in the opposite sequence. The results show that contemporary conditions are able to explain the major part of the variation in all diversity measures. Variables concerning former land-use do, however, add significantly to the explanation of variation in local species pool and in the area-independent diversity, but not in β-diversity. It is concluded that patterns of land-use intensity from the Iron Age and onwards have contributed significantly to the shaping of local species pools, and thereby the richness of grassland communities.