Twigs provide a new substrate every year and colonising lichen communities are strongly affected by existing climatic and atmospheric conditions as well as by availability of propagules. Data on lichen diversity from a standard sampling procedure along woodland and or hedgerow margins allows a comparison of lichen communities in the vicinity of a range of environmental conditions, particularly those associated with agricultural conditions. The procedure is repeatable allowing an assessment of changes over time.
Twigs (5-10) of exposed trees are randomly selected along a boundary (woodland, hedgerow or parkland) where twigs are accessible. Lichens recorded on annual increments, or on 1-5 years and 5+ years, or presence on whole twig up to c. 10 yrs. Mean diversity for each site calculated and PCO used to identify shifts in lichen communities and their association with environmental conditions (Wolseley et al, 1999, Wolseley, 2002a, 2000b). This method simplified to produce an easy to use key to lichens on twigs (Wolseley et al. 2002) combined with survey methodology and data collection on a website www.nhm.ac.uk/botany/lichen/twig.
The twig method may be used with the Nitrophyte ? Acidophyte species lists van Herk (1999) (see Dutch Method) to provide a robust indication of atmospheric ammonia and nitrogen deposition.
Preliminary investigation of annual increments of Quercus petraea twigs along boundaries of an SSSI exposed to different land management practices showed that colonising lichen species and communities varied with surrounding land management. The characteristic acidophyte communities of Usneion and Pseudevernion of woodland glades and moorland were replaced by species of the Xanthorion. (Wolseley and Pryor, 1999).
In a further project, Fraxinus twigs were sampled along a transect across Pembrokeshire from a point source from oil refineries. The mean frequency of lichen species on twigs at each site showed a positive correlation with distance from source and prevailing wind direction (Purvis et al. 1998).
In the vicinity of point sources of ammonia in Norfolk and Devon twigs of Quercus were sampled at sites at known distances from the point source. In both sites twig flora showed a better correlation with distance form source than the flora of the trunks, which showed little overall change with distance from source (Wolseley and James , 2002). These results were improved by weighting nitrophyte (NIW) and acidophyte (AIW) species.
Lichen communities on twigs of Betula species at Earlston and Gordon moss were sampled to assess changes with distance from source and NH3 deposition near a poultry farm in Scotland, scoring for Nitrophyte and Acidophyte species (Pitcairn et al. 2003, this report). The complete disappearence of acidophyte species occurred with lower levels of NH3 for twigs than for trunks, suggesting that the twig method may be more sensitive.
Suitability to indicate atmospheric concentrations:
There is currently a shortage of data, however the preliminary results suggest that a good relationship with NH3 concentrations exists between the NIW and AIW species. With further data, this method may prove to be the most robust lichen diversity method to indicate NH3 concentrations. There is a lack of data on the relationship to oxidized nitrogen (NOx and HNO3) concentrations.
Suitability to indicate atmospheric depositions:
The close correlation with NH3 concentration suggests a parallel response to N deposition. However, it is expected that oxidized nitrogen and wet deposited ammonium may have different effects to NH3, since these do not increase bark pH.
Suitability to indicate environmental impacts:
By definition the method reflects an ecological impact of atmospheric nitrogen. It is also highly sensitive to very levels of NH3 concentration (e.g. 1 ug m-3, Pitcairn et al. 2003, this report).
Sensitivity to other factors:
Variation in available tree species may affect results through variation in the natural pH of bark and bark surface structure. Environmental conditions especially shading may affect the results obtained, although no shading effect on NIW-AIW was detected in the present study by a poultry farm in Scotland.
Lichen biodiversity on twigs represents a much shorter timescale of pollution climate than trunks. More data are necessary, but twig diversity may represent pollution climate over periods as short as 3-5 years.
Currently the method is only applicable in areas where native trees are a component of the landscape. Initial testing has focused on Oak, but the recent work has shown that the method can also be applied to birch and spruce. The method is less applicable for trees species with higher bark pH.
Expertise in field:
Requires a specialist to collect data.
Expertise in laboratory:
Some laboratory work is required in order to identify sterile or small samples.
Cost (per unit sample): £unknown
Cost Comment: Selection and laying out of random samples for each site and sampling of 10 twigs requires 1-2 days of specialist time depending on species diversity on twigs. The total cost also needs to account for travel and data analysis time of specialists.