Impact Type: Direct exposure to pollutant
Available evidence suggests that ferns are sensitive to elevated ammonia concentrations (Schumann and Mills 1996, Pitcairn et al. 1998). Measurements of Pitcairn et al. (1998) showed changes in Dryopteris dilatata abundance near to intenstive livestock farms.
Burkhardt et al. (1998) have shown that the critical level for N deposition is normally exceeded before the critical level for NH3 exposure is reached. While relatively shade tolerant, carbon fixation is fundamental to N assimilation, thus overgowth by trees and more nitrophilous vegetation may weaken the capacity of ferns to tolerate NH3. Some ferns e.g. Dryopteris dilata (Broad bucker fern) are nitrophites (nitrogen lovers) - these will be more tolerant of NH3 and may respond positively.
|Habitat/ Ecosystem Type||Critical Load/ Level||Status||Indication of exceedance||Reference|
3 µg NH3 m-3 annual mean (uncertainty of 2-4 µg NH3 m-3)
Direct visible injury; species composition changes. Ecosystems where sensitive lichens and bryophytes are an important part of the ecosystem integrity, the critical level is set at 1 µg NH3 m-3.