Vegetation responses to long-term alkaline cement dust pollution in Pinus sylvestris-dominated boreal forests - niche breadth along the soil pH gradient

Questions How resilient is boreal forest vegetation growing on acidic podzols to the environmental change caused by alkaline kiln dust pollution? How adaptive is the vegetation, when species pool and time for dispersal do not limit? How constant is the species niche breadth along the soil pH gradient? Location Region of 30km around the Kunda cement plant, northern Estonia (59 degrees 30N, 26 degrees 32E), which has been continuously polluted last 140 years. Methods We sampled 20 forest stands on sandy gleyic podzols at various distances from the pollution source, leeward and windward of prevailing winds. Data processing: principal components analysis for gradient generalization, indicator species analysis for grouping species along the pollution gradient and generalized linear model analyses to estimate species reaction to pollution, considering covariates. Results The impact of alkaline dust accumulated over a century persisted despite resolute reductions of pollution. Forest soil conditions changed 10km leeward and 5km windward from the source: the litter pH level changed from 3.6 to 4.5 in unpolluted forests to 7.17.4 in the heavily polluted forests, and soil Ca content increased ten-fold. Soil alkalization has induced a remarkable succession from typical boreal vegetation toward vegetation of boreo-nemoral or calcareous habitats. We detected a linear increase of species richness from habitats on acidic soils (unpolluted) to newly formed alkaline soils (polluted habitats), reflecting the species pool composition and size, which is known to be large for calcicolous species. The establishment of several endangered calcicolous species shows the lack of dispersal limitation and points on their narrow soil pH niche. The niche breadth of species along the pH gradient varied among species, but only a few species appeared to be pH generalists. Conclusions Species ranking along the pH and/or Ca gradients indicates (1) the existence of different ecological guilds in terms of ecological niche use, and (2) the formation of a new community is dictated by the tolerance level of each species. This supports the individualistic community assembly rule suggested by Gleason. We find that such polluted regions with disturbed communities have high scientific value as long-term ecological experiments, but this might lead to conflict between the peculiar biodiversity complex and human well-being.