Influence of NH3 and SO2 on the growth and competitive ability of Arnica montana L. and Viola canina L

Abstract

The effects of atmospheric NH₃ and SO₂ separately and in combination on the growth and competitive ability of three species of the Violion caninae alliance were investigated. Growth and nutrient concentrations of Viola canina and Arnica montana in mixed culture with Agrostis capillaris were examined in relation to that in monoculture. Seedlings were transplanted into heathland topsoil in pots and placed in open-top chambers for 9 months from autumn to summer, where they were exposed to ambient air, 90 μ m^-3 SO₂ , 50 μg m^-3 NH₃ and to the combination of NH₂ and SO₂ . In the NH₃ + SO₂ treatment, a more-than-additive increase in nitrogen and sulphur concentrations was observed indicating co-deposition. NH₃ influenced the nutritional status of V. canina the most, increasing the nitrogen, phosphorus, and magnesium concentrations and reducing those of potassium and calcium. NH₃ fumigation significantly stimulated shoot growth of all three species and root growth of A. capillaris, while SO₂ reduced only the root growth of A. capillaris. The relative yield of V. canina was reduced by 20-30% in the presence of the air pollutants. The relative yield of A. montana was stimulated by 30-40% in treatments including SO₂ compared with that in ambient air or NH₃ alone. The competitive ability of both dicotyledons in mixed culture with A. capillaris was strongly reduced by NH₃ and was unaffected by SO₂ . The consequences of exposure to NH₃ and SO₂ for the survival and maintenance of threatened species in heathland vegetation are discussed.