| Summary: | Chronic nitrogen (N) deposition is a threat to biodiversity that results from eutrophication of sensitive ecosystems. We studied long-term monitoring data from 28 forest sites with a total of 1335 permanent vegetation sampling units from northern Fennoscandia to southern Italy to analyse temporal trends in species cover and diversity. We found that the cover of oligotrophic species decreased the more the measured N deposition exceeded the empirical critical load (CL) for eutrophication effects (p = 0.002). Although species, which prefer nutrient rich sites, did not experience a signifi cantly increasing cover (p = 0.440), they had a higher proportion among colonizers than oligotrophic species (p = 0.030). The observed response is, to our knowledge, the fi rst detection of a N deposition effect on temporal trends of vascular plants of forest fl oor vegetation in a European-wide long-term monitoring data set. Contrary to species cover changes, neither the decrease of species richness (alpha and gamma diversity) nor of homogeneity (beta diversity) correlated with nitrogen CL exceedance in our dataset. We conclude that oligotrophic plant species are on the downgrade in European forest ecosystems that are sensitive to eutrophication effects.
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