Lichen diversity and cover is assessed on trunks of specified trees and weighted according to selected species that are classified as "nitrophytes" (species preferring nitrogen enriched tree bark) or "acidophytes" (species preferring naturally acidic clean tree bark). The method was developed in the Netherlands based on large-scale monitoring in conjunction with physicochemical measurements (van Herk 1999, 2002).
10 acid -barked trees between 1-2.5 m girth are selected in each site and all lichen species on the trunk are recorded up to 2 m. The abundance of lichens at each site is recorded using a 6 point scale from "only one thallus present" to "present on 6-10 trees with more than 10 cm2 per tree". Species are scored as nitrophytes or acidophytes according to van Herk, and the mean number of nitrophyte and acidophyte species found per tree is expressed as NIW and AIW values.
The method has been widely used in the Netherlands to map and monitor spatial patterns of ammonia (NH3) pollution. AIW and NIW values have been calculated in c. 50% of sites adjacent to sampling points of ambient NH3 air concentration. In these sites, a strong correlation of high NH3 with NIW values was found and a negative correlation with AIW values. (van Herk 1999, 2002).
This method was tested in the UK in a) Norfolk and b) Devon on Quercus species at sites in the vicinity of NH3 recording stations using species defined as nitrophytes and acidophytes by van Herk. In the vicinity of chicken sheds at a) NIW values showed a strong correlation with NH3 deposition except at high concentrations of ammonia suggesting critical level exceedances. In the vicinity of stock rearing sheds at b) there was little correlation of NIW values with distance from the sheds. However there was a strong negative correlation with AIW values. (Wolsey, and James, 2002).
For the present report (Pitcairn et al. 2003), lichens on available trees of Pinus sylvatica, Picea and Betula were sampled with distance from a poultry farm in Scotland. A close response was found with the NH3 concentration, with the method proving the most robust and sensitive of several methods tested. A variation of the approach applied to twigs (Wolsely and James 2000, see additional record), was shown to be similarly sensitive, but may indicate present N levels more robustly than the tree trunk lichen flora.
Suitability to indicate atmospheric concentrations:
A close correlation between NIW and AIW scores and atmospheric NH3 concentrations has been demonstrated in the Netherlands and preliminary testing in the UK, with the method able to detect around 1 ug m-3 NH3.
Suitability to indicate atmospheric depositions:
The close correlation with NH3 concentrations also implies a response to total nitrogen deposition. However, it is unclear whether the same response occurs for oxidized nitrogen and wet deposited ammonium as for gaseous NH3. It is expected that lichen response is most sensitive to NH3 due to the parallel effect on bark eutrophication and pH. (With some exceptions, NH3 inputs tend to raise bark pH, while NH4+ inputs tend to reduce bark pH).
Suitability to indicate environmental impacts:
By definition a change in lichen species composition represents an impact of N deposition. The method is very well suited and a robust response to ammonia levels has been shown.
Sensitivity to other factors:
This method appears most suitable for temperate climates. In warm and dusty climates (e.g. Mediterranean) nitrophytes are a natural constituent of the flora. Species lists of nitrophytes and acidophytes therefore require adjustment for different climatic regions. Certain species (e.g..Xanthoria) may develop on acid-barked trees as a result of the ?alkaline dust effect (Gilbert 1976) in the vicinity of quarries.
Further work is required regarding the development of the method in UK conditions.
Lichen biodiversity on tree trunks represents a consequence of the previous pollution climate over several decades. The twig-based modification is anticipated to reflect conditions over a much shorter period.
The method requires the occurrence of suitable tree species with naturally acidic bark (e.g. oak, birch, spruce), with tree trunks of appropriate diameter and age. While most work in the Netherlands has used hedgerow trees, with good light availability on trunks, the recent UK work has shown that the method can also work in transects through woodland, even where much lower light levels occur.
Expertise in field:
Specialist/trained personnel are needed for identification. While the method provides the most robust and sensitive lichen analysis response to nitrogen, it is also one of the most complex in terms of scoring and data analysis.
Expertise in laboratory:
S Basic chemical tests for species identification may be necessary, and there is specialist analysis required to calculate the NIW and AIW scores.
Cost (per unit sample): £unknown
Cost Comment: The work needs to be conducted by external specialists. Location of sites in the vicinity needs 1 day, and sampling of 10 trees per site needs 1.5 days. The total cost also needs to account for travel and data analysis time of external specialists.