This method enables integrated statements about general pollution stress over longer periods of time. Sulfur dioxide and dust were the primary urban pollutant components of stress factors on lichen in the past. Motor vehicle exhaust gases, ozone, and nutrient inputs are the relevant factors today. The dimensions of this influence can be measured by lichen.
A relationship has been detected between the specific community lichen existing at a site and degrees of air pollution. A rich variety of lichen species exists more frequently in areas with clean air. Both the number of species and the coverage are greatly reduced in areas with severe air pollution. There are even areas without any lichen cover at all. The absence of naturally appearing lichen in severely polluted areas limits the spacial differentiation of pollutant effects. This is why exposure monitoring methods are employed.
Lichen mapping for determining air quality is regulated in VDI Guideline 3799, Part 1. This ensures the general comparability of data. The evaluation of study findings do not allow any direct conclusions to be drawn about toxic influences on humans or plants. The evaluation can only serve as a point of orientation for general pollution load, especially if comparison with previous mapping shows an increase or decrease of lichen occurrence. An urgent need for improvement of air quality exists in areas determined to have "extremely high" pollution. These areas are characterized by occurrence of the toxic-tolerant crustose lichen Lecanora conizaeoides. In areas termed to be "severely loaded", as indicated by the presence of this lichen, residents are at increased risk of respiratory illnesses (Rabe and Beckelmann 1987).
The foliose lichen Hypogymnia physodes was employed to determine the total effect of pollutants in study areas which have no distinctive natural lichen vegetation. The basic measure of effect given in VDI Guideline 3799 Part 2 is the mortality rate of the thallus (lichen body) at the end of the exposure period. It is not usually possible to draw direct conclusions regarding the amount of pollutant inputs based on the study findings, because SO2 and other factors affect the lichen. A general derivation from the mortality rate is that increased damages to other plant species can be expected, as well as a loss of species in the ecosystem, with increased effects on lichen.
Analysis of Pine Needles
The Scots Pine (Pinus sylvestris) is a widely distributed, native tree species in Berlin and Brandenburg. It is very well suited for long-term passive monitoring studies because it is an evergreen. Pine needles were sampled for the Cadastre of Ecological Pollution Effects because pine needles have good accumulation and response characteristics. Pine needles are exposed to prevailing pollutions all year-round, in contrast to the 14-day growth cycle of rye grass. The analysed elements characterize a longer period of time, thus integrating seasonal fluctuations of pollutions to an mean pollutant level. High winter SO2 inputs can be especially well studied by the sulfur concentrations of pine needles. Winter inputs cannot be determined by exposing rye grass in summer.
No binding evaluation basis exists for pollutant levels in pine needles. A comprehensive review of existing comparative studies was made in order to derive classification limits for "low", "medium", and "high" concentrations in the Berlin area. These value categories characterize the prevailing ecosystem pollutant level from airborne inputs and soil factors. They provide indications for the pollution load of the natural environment, such as forests, but they are not to be held equivalent with threshold value for the protection of cultivated crops or human health.
Standardized Rye Grass Cultures
Rye grass (Lolium multiflorum) is often used in agriculture as fodder (feed crop). It is an accumulation indicator representative for other food and feed crops and is often used for an estimation of load on natural vegetation. The grass is exposed during the growing season to register accumulation of airborne pollutants. Rye grass contents indicate if there is a danger of contamination in the consumption of plants and crops in the study area.
No federal German evaluation procedures exist. Comparative studies were used as a basis to develop guidelines for tolerance values below which there are no expected toxic effects on natural vegetation (cf. Scholl 1974), or for grazing animals or humans from contaminated plants by way of the food chain (cf. BGA 1986, FMV 1990).
Green Kale Exposure
Green kale (Brassica oleracea acephala) is a recognized standard plant for determining effects of organic airborne pollutants and is used routinely by local agencies. High frost tolerance allows it to be used in active monitoring during autumn and winter, when other plants cannot be exposed and airborne pollution increases. Green kale is especially suited for detecting organic pollutants because these pollutants are usually lipophilic (fat-soluble) materials and accumulate greatly in the leaf wax layer.
Procedures for breeding plant material, exposure, harvesting, scheme of evaluation, and analysis of test plants have been extensively established and tested (cf. Arndt et al. 1987; Rademacher and Rudolph 1994; TÜV-Umwelt Berlin-Brandenburg 1995). No limit or guideline values valid for all Germany exist for organic compounds in vegetable foodstuffs or feedstocks. The pollution levels "low", "medium", and "high" were defined using values found in other green kale studies in Germany. The defined classifications are an empirical aid. They do not consider toxicological aspects predominately. They allow only general indications of pollutant levels in reference to human health.