for herbaceous species. There is insufficient evidence to provide a separate CLE NH3 for forest trees, but the value of 3 ± 1 µg NH 3 m -3 is likely to exceed the empirical Critical Load for N deposition for most forest ecosystems.

Heathland is currently being threatened by nitrogenous atmospheric deposition, mainly NH y (NH 3 and NH 4 "). Deschampsiaflexuosa and Calluna vulgaris in monocultures were well able to use NH y 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 NH y • 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 NH 3 on plants has been established with a toxicological model: 270 and 8 J.1g m -3 for a daily and annual mean, respectively. Heathland species are relatively sensitive to NHy"

No abstract

No abstract

No abstract