Impact Type: Direct exposure to pollutant
Usnea spp. are highly sensitive to SO2, and are also believed to be sensitive to acidic deposition (Seaward & Hitch 1982). Hawksworth & Rose (1970a) suggest that some species can tolerate up to 40 µg m-3 SO2 (U. subfloridana), U. ceratina and rubiginea can tolerate up to 35 µg m-3 SO2, and that U. florida under 30 µg m-3 SO2, with U. artculata and filpendula only found in area with pure air.
There is very little literature publi shed on the effect of air pollution on this genus.
|Habitat/ Ecosystem Type||Critical Load/ Level||Status||Reliability||Indication of exceedance||Reference|
|Forests and semi-natural vegetation||
20 µg SO2 m-3 annual mean and half-year(Oct-March) mean
|UNECE, 2004||quite reliable i.e. the results of some studies are comparable||
Low temperature appears to enhance the negative effects of SO2, and the lower critical level of 15 µg SO2 m-3 is used where the effective temperature sum (ETS) (i.e. the sum of temperatures) above 5oC is below 1000 degree days (d.d) (Ashmore et al., 1994).