Impact Type: Direct exposure to pollutant
Polytrichum commune may be susceptible to damage by O3 in winter (Potter et al 1996b), if the exposure continues for more than four days (Lee et al. 1998). Potter et al. (1996b) observed no tissue death or visible damage, but there was significant reduction in growth and photosynthesis during winter O3 exposure (150 ppb).
Petersen et al. (1999) have shown that under laboratory conditions O3 at 50 ppb can stimulate the early growth and developmental phases of the gametophyte of P. commune. If this is occurring in nature, then P. commune, a common and abundant bryophyte, may be out-competing other species associates at current ambient oxidant levels. However, if O3 levels were to increase as currently predicted, the growth and competitiveness of P. commune could be impaired (Petersen et al. 1999).
The magnitude of O3 impacts on bryophytes in the UK is currently very uncertain. The largest O3 concentrations occur at high altitude sites, and the most severe bryophyte impacts are therefore expected for species at Scottish mountain sites. However, with the exception of the experimental studies by Potter et al. (1996a,b), there has been little demonstration of actual species responses in the field.
|Habitat/ Ecosystem Type||Critical Load/ Level||Status||Reliability||Indication of exceedance||Reference|
AOT40 3000ppb hours over 3 months or AOT40 5000ppb over 6 months
|UNECE, 2010||expert judgement i.e. only limited or no data are avaliable for this type of receptor||
AOT40 is the Accumulated concentration Over a Threshold of 40 ppb. If an hourly average ozone concentration exceeds 40 ppb the difference between the concentration and 40 ppb is added to a running total. The units are therefore ppb multiplied by hours. For natural vegetation, the AOT40 is summed for the daylight hours for a period of three months. Daylight hours are defined as when solar radiation exceeds 50 W m-2. The daylight hours are when plant stomata are normally open.
Flux-based critical levels, based on biomass reduction, are also available for local and regional assessment but are not yet incorporated into APIS. See critical levels chapter of the UNECE Mapping Manual.