INFLUENCE OF NH3 AND (NH4)2SO4 ON HEATHLAND VEGETATION

Heathland is currently being threatened by nitrogenous atmospheric deposition, mainly NHy (NH3 and NH4+). Deschampsia flexuosa and Calluna vulgaris in monocultures were well able to use NHy for biomass production, although in the case of C. vulgaris only shoot growth was stimulated and C. vulgaris increased its sensitivity to drought and the heather beetle (Lochmaea suturalis). In a mixed culture of C. vulgaris and D. flexuosa, the latter species was the better competitor for NHy. The relation between stimulated biomass production and increased stress sensitivity also applied to several other heathland species. A general effect threshold for adverse effects of NH3 on plants has been established with a toxicological model: 270 and 8-mu-g m-3 for a daily and annual mean, respectively. Heathland species are relatively sensitive to NHy.