Chlorophyll a fluorescence is often used to estimate various types of damage in lichens. In order to optimize the output and improve interpretations of such measurements the protocol for pretreatment and measuring is important. To study the effects of measurement conditions, the lichens Lobaria pulmonaria, L. scrobiculata, Xanthoria parietina and Parmelia sulcata were first stressed by high light intensities at 600 or 1000 µmol photons m−2 s−1 for 4 h. Then various conditions during recovery or pretreatment were used to optimize the detection of more lasting damage. Recovery from photoinhibition was incomplete in darkness, whereas light as low as 0·2 or 1·0 µmol m−2 s−1 resulted in complete recovery if the recovery period was long enough. Additionally, low intensity light given for1·5 h after one day in darkness caused rapid and complete recovery. In conclusion, before measuring maximal PSII efficiency (Fv/Fm) with chlorophyll fluorescence, it is important to let lichens recover in low intensity light and not in darkness, to optimize recovery from photoinhibition; dark adaptation can only be recommended if the photoinhibition status of the lichens is of interest.

Lichen element content is a reliable indicator for relative air pollution load in research and monitoring programmes requiring both efficiency and representation of many sites. We tested the value of costly rigorous field and handling protocols for sample element analysis using five lichen species. No relaxation of rigour was supported; four relaxed protocols generated data significantly different from rigorous protocols for many of the 20 validated elements. Minimally restrictive site selection criteria gave quality data from 86% of 81 permanent plots in northern Midwest USA; more restrictive criteria would likely reduce indicator reliability. Use of trained non-specialist field collectors was supported when target species choice considers the lichen community context. Evernia mesomorpha, Flavoparmelia caperata and Physcia aipolia/stellaris were successful target species. Non-specialists were less successful at distinguishing Parmelia sulcata and Punctelia rudecta from lookalikes, leading to few samples and some poor quality data.

Lichens have long been regarded as bioindicators of air pollution, and structural studies typically have indicated negative impacts in highly polluted areas. In this research, Parmelia sulcata and Platismatia glauca were collected from one clean and two polluted sites in the Pacific Northwest forests of the United States to investigate the anatomical and ultrastructural responses of relatively resistant lichens to moderate air pollution. Light microscopy of polluted materials revealed only slight increases in the algal cell proportions of the thallus, and a decrease in the fungal cells of the medulla. Using transmission electron microscopy, increased lipid droplets in the cytoplasm and an increase in the cell wall thickness of the photobionts were found in the polluted lichens. These results were compared with physiological data in which the net carbon uptake did not show any significant differences; however, the total chlorophyll content was heightened in the polluted samples. The increased total chlorophyll content and the absence of any changes in the algal cell proportions of the polluted samples suggest that the photobionts possessed a higher chlorophyll content per unit volume of the photobiont at polluted sites. The results also indicate that lichens have altered their storage allocation in different cellular compartments. This may be a result of symbiotic readjustment(s) between the photobiont and the mycobiont. In comparison with the physiological results from these two species, these changes do not represent damaging effects by low-level air pollution.