Impact Type: Direct exposure to pollutant
Twinflower (Linnaea borealis), which is adapted to very nutrient poor sites (Hill et al. 1999), is listed in the UK BAP. It is restricted to pine woodland or shaded rocky places in eastern and northern Scotland, where it has under gone substantial decline in the 20th century. The reason for this is unclear.
Westlund and Nohrstedt (2000) have shown that in Sweden the use of urea in the treatment of cut tree stumps substantially increases local ammonium concentrations and causes severe damage to understory vegetation. L. borealis was shown to suffer 100% damage in all cases. A critical level of 8 µg m-3 has been given, but L. borealis is expected to be more sensitive to N deposition than NH3 concentrations.
|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.