Impact Type: Direct exposure to pollutant
By nature of their morphology and physiology, bryophytes are more likely to be susceptible to damage from ozone than higher plants (Potter et al. 1996b), and in particular to short-term acute ozone episodes (Potter et al. 1996a). However, the effect of O3 on bryophytes in the UK is currently very uncertain. The highest O3 exposures (concentration x time) occur at high elevations because concentrations remain high throughout the day and night (UKPORG 1997, Lee et al. 1998). Therefore if there is an effect on bryophytes it will be in the montane areas. However, with the exception of the experimental studies of Potter et al. (1996a,b) and others (Foot et al. 1996, Gagnon & Karnosky 1992, Lee et al. 1998), there has been little demonstration of actual species responses in the field to realistic combinations of concentration and time.
So far only four species of bryophytes have been shown to respond to O3 (reduction in photosynthesis, and/or increase membrane leakage). These are Pleurozium schreberi, Polytrichum commune, Racomitrium lanuginosum and Sphagnum recurvum (Potter et al. 1996b, Lee et al. 1998). S. capillifolium, S. cuspidatum and S. papillosum did not respond to increased O3 doses. None of the fumigation studies have reported death of bryophytes due to O3 exposure, and recovery of physiological activity was often observed when fumigation stopped (Potter et al. 1996a). Experiments so far published have only been carried out on common and abundant species. There is a need for more research, especially on rare Atlantic species, especially in a community contect where there is competition from other species (Potter et al 1996b).
|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.