Epiphytic lichen diversity may be impaired by air pollution and environmental stress. A combination of lichen diversity and frequency on selected trees is therefore used as a measure of environmental quality. In order to avoid bias over quadrat location, a prescribed method of selecting trees and of sampling trunks is provided which takes into account directional effects on each tree. The resulting LDV value is used to define different zones of environmental quality. All species are recorded apart from some that are difficult to identify and/or are easily overlooked. Multivariate analyses of the sampled data together with physicochemical and other data can be used to interpret environmental gradients, to identify indicator species, and to detect changes in relation to aspect.
Lichen diversity and frequency is assessed on trees of a single species or bark type distributed at intersections of a geographical grid across the area being surveyed. Up to 12 trees at each grid intersection are sampled using 4 quadrat segments comprising 5 contiguous 0.1 m x 0.1 m quadrat squares placed at 1.50 m height on the tree trunk at 4 cardinal points (North, South, East, West). All species present within the 0.1 m x 0.1 m quadrats are recorded and frequency is assessed for each aspect to determine total diversity per trunk and diversity with respect to aspect.
This method was compiled by a panel of experts to allow comparison of results over a wide geographic region.
This method has now been tested in urban and rural sites in Britain and also in rural sites in Italy. Preliminary investigations show that it is difficult to fulfil the sampling criteria in urban and rural sites where tree species, structure and exposure are variable.
In a survey of London parks 5 Fraxinus trees in 6 sites showed that mean lichen diversity per site increased with distance from the centre of London. No correlations with N concentration or deposition or directional analysis were carried out (Davies et al., 2002). In one large woodland site (220 ha, Burnham beeches, UK) directional quadrats placed on 17 Quercus trees showed great dissimilarity of lichen communities on oak with greater similarity with location than with aspect. Areas of wood pasture were associated with acidophyte lichen species and low bark pH. More open areas were characterised by mixture of acidophytes and nitrophytes and higher bark pH. (Purvis et al. 2002).
Brunialti et al (2002) compared methods of sampling trees and quadrats and demonstrated that this method avoided clustering effects, and provided directional information.
In the present study (Pitcairn et al. 2003), 5 trunks of Pinus sylvatica and Picea spp were selected in 6 sites along a gradient from a point source of ammonia.Lichen decreased with moderate NH3 concentrations, but increased with higher levels, due to the replacement of acidophyte species with nitrophyte species.
Suitability to indicate atmospheric concentrations:
Not well suited because of the complex relationships with atmospheric N concentrations.
Suitability to indicate atmospheric depositions:
Not well suited because of the complex relationships with atmospheric N deposition. Recent work using directional quadrats in an area where ammonia concentration was monitored suggests that in close proximity to the source of NH3 there is directional influence due to higher NH3 deposition on the source side of the trees.
Suitability to indicate environmental impacts:
By definition changes in lichen biodiversity represents an impact of N. However, more extensive survey work and further analysis of lichen data are required in relation to atmospheric nitrogen to define appropriate nitrophyte and acidophyte indicator species within this system.
Sensitivity to other factors:
Variation in ecological conditions of light, bark structure and chemistry. Presence of other pollutants. Tree age/girth may also be a factor where trees carry a relict lichen flora from former conditions or else have undergone changes to bark chemistry.
This method can be used to assess distribution of zones around a point source or for long-term recording of changes over time on marked trees. Lichen biodiversity on tree trunks represents a consequence of the previous pollution climate over several decades. A 3-5 time year period between surveys is recommended.
The method is only applicable where sufficient number of trees of the same species and fulfilling stringent criteria are available for sampling.
Expertise in field:
Specialist/trained staff are required for sampling, and for data interpretation.
Expertise in laboratory:
Limited chemical analysis of specimens is required for the identification of species & in anaylsis / interpretation of data.
Cost (per unit sample): £unknown
Cost Comment: Setting up of a grid will depend on area to be sampled. Once the grid is established, it takes c. 2 days of external specialist time to locate suitable trees and record releves on c. 5 trees. A total cost needs to account for travel and data analysis time of external specialists.