scope:

Introduction

Many natural and anthropogenic changes of nature progress continuously and over long periods of time. They can be recognized and interpreted only by the long-term comparison of consecutive data series with each other (Muhle and Poschlod, 1989). Methods which have the goal of continuously measuring certain environmental properties over a long period of time are summed up under the term "long-term observation" (monitoring).

One aim of such studies may be the observation of long-term changes of the levels of ambient air pollution on selected habitats by applying the standardized method of lichen mapping. Lichens are particularly well suited for this purpose because they react sensitively to a number of air pollutants on the one hand and on the other hand they are long-living organisms and as such are predestined to be used in long-term observations.

An eligible procedure for that is described in guideline VDI 3799 Part 1 "Measurement of the immission effects - Measurement and evaluation of phytotoxic effects of ambient air pollutants (immissions) with lichens; mapping of lichens for Assessment of the Air Quality (LGW)".

Lichen mapping is based on taking inventory of the condition of the lichen vegetation found within a largely standardized surveying grid on the bark of trees. The frequency of occurrence of lichens serves as measuring unit. The procedure is marked by a high level of practicability but is not suitable for pursuing the development of individual lichens over the course of time because it does not cover their size.

That gap is closed by the procedure which determines the size with the help of transparency films. It supplements or expands the above mentioned VDI guideline. A more differentiated documentation of the changes of the composition and the level of the ambient pollutants is made feasible by that procedure.

A method, which was applied by Hurka and Winkler (1973) as well as by Wirth and Brinckmann (1977) for the first time in efforts to study the effects of ambient pollutants on ecology, specifically, to determine the spatial variation of lichen distribution, was taken and modified for the procedure. The investigations which were carried out to get an inventory of ecological effects for Baden-Wuerttemberg included the use of transparency films in order to get additional records of the temporal changes in lichen growth on tree barks caused by ambient air pollution (Wirth, 1987; Wirth and Oberhollenzer, 1991; Wirth and Oberhollenzer, 1995). Comparable inventories were also presented by Wirth et al. (1999), Zimmer (1996 and 2000) as well as by Kirschbaum and Hanewald (2001).

Because the procedure for determining the size (surface area of lichens) is based on the guideline on lichen mapping the fundamentals of that method are first summarized.