Currently legislated decreases in nitrogen deposition will yield only limited plant species recovery in European forests

Atmospheric nitrogen (N) pollution is considered responsible for a substantial decline in plant species richness and for altered community structures in terrestrial habitats worldwide. Nitrogen affects habitats through direct toxicity, soil acidification, and in particular by favoring fast-growing species. Pressure from N pollution is decreasing in some areas. In Europe (EU28), overall emissions of NO x declined by more than 50% while NH3 declined by less than 30% between the years 1990 and 2015, and further decreases may be achieved. The timescale over which these improvements will affect ecosystems is uncertain. Here we use 23 European forest research sites with high quality long-term data on deposition, climate, soil recovery, and understory vegetation to assess benefits of currently legislated N deposition reductions in forest understory vegetation. A dynamic soil model coupled to a statistical plant species niche model was applied with site-based climate and deposition. We use indicators of N deposition and climate warming effects such as the change in the occurrence of oligophilic, acidophilic, and cold-tolerant plant species to compare the present with projections for 2030 and 2050. The decrease in N deposition under current legislation emission (CLE) reduction targets until 2030 is not expected to result in a release from eutrophication. Albeit the model predictions show considerable uncertainty when compared with observations, they indicate that oligophilic forest understory plant species will further decrease. This result is partially due to confounding processes related to climate effects and to major decreases in sulphur deposition and consequent recovery from soil acidification, but shows that decreases in N deposition under CLE will most likely be insufficient to allow recovery from eutrophication

Nitrogen deposition affects forest floor vegetation across Europe - evidence from long-term monitoring. Oral presentation at International Symposium Ecohydrology, Biotechnology & Engineering, Book of Abstracts (17-19th September 2013 – Łódź)

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