Additional nitrogen supply both leads to a eutrophication of tree bark and changes in bark pH. In particular, enhanced levels of NH3 have been shown to increase bark pH. The environmental preferences of lichens in relation to these different conditions may be classified into two extremes: 'nitrophyte' lichen species prefer high supply of nitrogen and high bark pH, while 'acidophyte' lichen species prefer a low supply of nitrogen and the naturally low pH of clean bark.
In the Ellenberg approach (as applied by Wirth, 1991 for lichens), the relative preference of lichens as nitrophytes or acidophytes is incorporated in to an empirical scale of 1 to 7. The most nitrophytic species have the highest scores, while the most acidophytic species are given the lowest scores. Species with intermediate preference are scored in between. The lichen Ellenberg score for the site is calculated simply as the average of the Ellenberg values of the different species present. In more refined approaches, the result may be weighted by the relative frequency of the different species to estimate an ?abundance-weighted Ellenberg score.
The method may be applied to either tree trunks or twigs (see also Twig sampling method), with sampling on a range of tree species with naturally acidic bark (e.g. oak, birch, pine, spruce).
The method has been tested in an inter-comparison of nitrogen biomonitoring methods across a mixed woodland downwind of a poultry farm in Scotland. The advantage of the method is that it is relatively quick and simple to score and that, as far as possible, all lichen species are assigned an Ellenberg score. This means that even in situations with very low lichen diversity, the method can still be applied. The correlation with NH3 concentrations was better with sampling on twigs than on trunks (Pitcairn et al. 2003, this report).
The method has the advantage is that it is amenable to simplification to a restricted set of species that can be identified by non-specialists, which is of use to encourage wider public appreciation of lichen monitoring for nitrogen (Pitcairn et al. 2003, this report). Application of the simplified method at the showed that it is less sensitive to low NH3 levels, but provided a clear signal for NH3 concentrations >= 8 ug m-3, which is the currently established critical level for NH3.
Suitability to indicate atmospheric concentrations:
The method has shown a clear relationship to atmospheric NH3 concentrations above 1.7 ug m-3, but is less sensitive than the most detailed lichen approach (Dutch method/Twig method). The simplified Ellenberg approach is again less sensitive, but more suited to application by non-experts.
The extent of relationship to oxidized nitrogen concentrations has yet to be established.
Suitability to indicate atmospheric depositions:
The response of the lichens to NH3 concentrations implies a relationship also exists with nitrogen deposition. However, a differential sensitivity is expected between oxidized nitrogen deposition (HNO3, NO2, NO3-) and the different forms of reduced nitrogen deposition (NH3, NH4+).
Suitability to indicate environmental impacts:
By definition, the method represents an ecological impact of enhanced nitrogen. The simplified scheme (using only a limited set of easily identifiable species), is well suited to demonstrate effects to stakeholders, but has a lower sensitivity than a survey (by specialists) which scores all species present.
Sensitivity to other factors:
The varying bark characteristics of different tree species affects the occurrence of nitrophyte species, so caution is needed in applying the method between tree species. The scores obtained at a site may be expected to be modified under conditions with very high SO2 levels.
The lichen flora of trunks and twigs respond to atmospheric conditions on different time scales. The time constant of response for trunks may be a decade or longer (especially under conditions of improving air quality, with a slow recolonization of previously eutrophicated bark). The time constant of response for twigs is faster and may reflected atmospheric conditions over previous periods as short as 3-5 years.
The method is currently limited to the trunks and twigs of trees with naturally acidic bark.
Expertise in field:
Specialists are required to conduct a full survey identifying all lichen species. For the simplified approach, identification by agency staff is possible following training.
Expertise in laboratory:
The full identification of species requires some basic chemical tests by specialists. The numerical calculation of Ellenberg scores is very simple.
Cost (per unit sample): £unknown
Cost Comment: For the full species approach, two specialist days are required for identification of species and chemical tests, plus one person day for data analysis. Time for travelling to sites needs to be added. Costs may be reduced where several sites should be assessed in parallel.
The simple approach would require typically one day in the field by a non-specialist, but substantial further development of this approach is necessary before it can be considered ready for general use.