Vascular plants as indicators of nitrogen enrichment in soils.

The quantification of species along environmental gradients has long been the subject of study regarding natural plant communities and, more recently, ecosystems disturbed by anthropogenic emissions of eutrophying and acidifying substances. We studied 527 sites in southern Swedish deciduous forests divided into two regions, with mean nitrogen depositions of 17 and 9 kg ha^-1 y^-1. The higher deposition in the more exposed region has resulted in higher nitrogen mineralisation and nitrification rates. The objective was to identify nitrogen indicators, which we defined as species with a skewed distribution in relation to the net potential nitrogen mineralisation in the soil. Soil pH was introduced as a co-variable as it may relate to other factors limiting plant growth and survival in these ecosystems. We tested the distribution of the net potential mineralised ammonium (minNH₄⁺), nitrate (minNO₃^-), total inorganic nitrogen (minNH₄⁺+NO₃^-) and the ratio of nitrification (percentage of minNH₄⁺+NO₃^- as nitrate; %minNO₃^-) in 77 herbs. The number of species that showed a skewed distribution differed for the nitrogen variables studied in the following order: %minNO₃^- > minNO₃^- > minNH₄⁺≫ minNH₄⁺+NO₃^-. As many as 21 species in region 1 and 17 species in region 2 were selected as indicators for one or several of the nitrogen variables. A fern, five graminoids and seven forbs were found to be indicators of low %minNO₃^- and two ferns, two grasses and 13 forbs were found to be indicators of high %minNO₃^-. We correlated the Ellenberg N indicator values with species with and without skewed distributions, and found that the former group was best related to the N values. As with most indicators, the species identified here are probably best used in combination with other measures to demonstrate the effects of nitrogen deposition on vegetation. [ABSTRACT FROM AUTHOR]