Lichen growth and vitality may be influenced by environmental stress within a short time frame (e.g. within one growing season). Permanent quadrats are established using stainless steel screws to locate a quadrat frame with scale and colour reference bar. Ladders may be used to locate samples which are difficult to reach and where lichens are rare owing to stemflow / nutrient effects. Digital photographs are taken of the quadrat and species recorded in quadrat and on the whole tree. Rates of lichen growth, health and changes in assemblage composition are assessed at typically yearly intervals. Using image analysis software changes in area of thallus can be calculated and expressed as a percentage growth or loss of thallus area, which may be correlated with pollutants and other environmental parameters (Purvis et al., 2002). Images can be stored for subsequent investigation.
Between 1986-1990 monitoring of species of Lobaria across conservation sites in the vicinity of atmospheric recording stations across Britain showed a characteristic pattern of changes associated with pollution in those areas with high levels of acid deposition (Looney et al. 1990 and Wolseley et al. 1990) However, where acid deposition was not high there was considerable variation in response of specimens to local environmental conditions (Wolseley et al. 2001). Growth rate responses are expected to be correlated to atmospheric N supply, and provide the basis for changes in species composition, such as measured with the Dutch and Twig lichen diversity methods for nitrogen biomonitoring. The photographic method and measurement of growth rate changes is therefore expected to be a more sensitive and faster responding parameter than the methods based on presence of characteristic species. By examining changes in time, the method provides a technique to look at changes in air pollution with time.
The application of this method in Burnham Beeches led to analysis of changes in area cover of Parmelia spp. in relation to various physicochemical data in order to interpret which factors are responsible for the changing lichen floras at the site in the vicinity of London (Purvis et al. 2002). Subsequent analysis of specimens showed an accumulation of nitrogen over time in specimens of Parmelia sulcata which were colonised by alien algae and Physcia, a 'nitrophytic' species (Purvis et al. (in press).
Long term monitoring of (saxicolous) lichens growing on rocks on the Sarsen stones at Fyfield Down in Wiltshire was set up to evaluate changes in animal stocking rates at a conservation site where stock numbers were increased. No digital analysis was undertaken, but this is possible at a later date (O?Dare & Coppins, 1994).
Suitability to indicate atmospheric concentrations:
The approach is expected to be highly sensitive to NH3 concentrations, as well as possibly NOx, although the latter is less certain. At present, however, the quantitative relationships are not well established, due to the range of other factors affecting growth rates of individual lichens.
Suitability to indicate atmospheric depositions:
An association between growth of Parmelia sulcata, traffic levels, regional NO2 levels, black smoke emissions, sticky pad readings (but not SO2 levels) was identified. However, the method cannot be used on its own to indicate deposition without additional studies (e.g. chemical analysis).
Suitability to indicate environmental impacts:
By definition the method monitors an impact of N on ecosystem functioning. It provides detailed information on the dynamics of species/inidviduals within a permanent quadrat. Suitable for monitoring local conditions within an area of conservation interest and to compare growth and/or loss of rare species. However, the method does not monitor status of the local population unless these are quantified outside the quadrat. The method is also subject to local changes in environmental conditions independent of N deposition.
Sensitivity to other factors:
Permanent quadrats are subject to tree loss and to changes other than impacts of nitrogen. Large numbers of quadrats would circumvent the loss of samples.
The approach has the shortest time constant of the lichen diversity methods, since changes in growth rate precede changes in species composition. For fast growing species, change may in principle be detected within a year. In practice, there is no standard time interval between taking photographs, as this depending on the species? growth rate and habitat. Once a year is usually adequate in disturbed urban habitats and highly enriched habitats typified by a high turn over of species.
Applicable to monitoring of lichen communities on a variety of substrata including epiphytic (tree bark growing), saxicolous (bare rock growing) and terricolous (bare soil growing) lichens. At present the limitations are in assessing nitrogen deposition in local areas as a reference to further establish the method. Such quadrat monitoring is most more applicable to local scale temporal changes and is therefore not suitable for indicating N concentrations where recording of data is on a large scale.
Expertise in field:
Specialist staff are required to identify lichens and situations that should be monitored. Appropriate equipment for standardized digital photography is also required.
Expertise in laboratory:
Image analysis software required to calculate growth rates.
Cost (per unit sample): £10-100
Cost Comment: External specialist time needed for quadrat location and setting up: 6 man days for 21 quadrats e.g. Burnham), for quadrat recording and photography: 4 man days for 21 quadrats, and for analysis of digital images - (7 man days for 21 images). Time taken will vary with numbers of species present.