Long-term changes in calcareous grassland vegetation in North-western Germany – No decline in species richness, but a shift in species composition
Résumé
We aimed to answer the question of whether the species richness and composition of calcareous grasslands in North-western Germany had changed over the last 70 years as a result of atmospheric nitrogen (N) deposition. In total, 1186 plots of Festuco-Brometea (alliance Bromion erecti) grasslands from the sub-oceanic regions of the country were compiled (1061 plots from literature sources spanning a time period from 1936 to 1996, 125 new plots from 2008). Environmental descriptors recorded for each plot included geographic coordinates, altitude, heat index (combining slope and aspect), mean Ellenberg indicator values for light, soil moisture, soil pH and soil N, and cumulative N deposition (the latter being highly positively correlated with the year of sampling). In a Detrended Correspondence Analysis, the sample plot scores along axis one were highly correlated with the mean Ellenberg N-values, those along axis two were significantly affected by the year of sampling. In a general linear model, species richness of vascular plants showed a markedly hump-shaped relationship with mean Ellenberg N-value, whereas it was weakly affected by year (cumulative N load). Species with a significant negative trend over time were more often (than expected by chance) habitat specialists of dry grasslands, small, light-demanding and winter-green or evergreen with smaller seeds and scleromorphic leaves. In contrast to what has been found for acidic grasslands, N deposition in calcareous grasslands did not result in a decline in species richness, most likely because calcareous grasslands are water- and phosphorus-limited, and are well-buffered in terms of soil pH. To prevent a further change in species composition towards more mesophytic communities, grassland management by the site managers needs to be intensified.