Species Composition of Subalpine Grassland is Sensitive to Nitrogen Deposition, but Not to Ozone, After Seven Years of Treatment

Abstract

Reactive nitrogen (N) and ozone (O3) are the most widespread atmospheric pollutants with significant implications for conservation of semi-natural vegetation; their combined effects have, however, not been tested in long-term field experiments. To investigate these effects on the species composition of a subalpine Geo-Montani-Nardetum pasture, 180 turf monoliths were exposed for seven years to five N loads (0, +5, +10, +25, +50 kg N ha−1 y−1) in combination with three O3 levels (ambient, 1.2 or 1.6× ambient concentration) in a free-air fumigation experiment at 2000 m a.s.l. in the Central Alps. Aboveground biomass of grasses, forbs, sedges, and legumes, as well as individual species abundance was recorded annually. N addition caused strong changes in community composition and slightly reduced Shannon diversity: Sedges (Carex sempervirens and Carex ornithopoda) tripled their fractional biomass at the expense of legumes (Trifolium alpinum), grasses (Agrostis capillaris, Briza media, Festuca spp.), and forbs, the latter of which responded inconsistently. Compositional changes were significant with +5 kg ha−1 y−1; at all levels of N, however, changes ceased after 5 years. Elevated O3 and the combined O3 × N exposure had no effect on functional group productivity. Overall the results reveal high N sensitivity of the subalpine grassland, but low sensitivity to O3, singly or in combination with N. Thus, in the longer term any input of N above the current ambient deposition may cause a shift in the plant community composition of these ecosystems which are considered hotspots for biodiversity.