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Abstract:The annual course of irradiance was recorded at two vertical and even-aged neighbouring Quercusstems, one rich in L. pulmonaria, one without. Irradiance never exceeded 610μmol photons m−2s−1at the L. pulmonariasite, whereas the L. pulmonaria-deficient site could experience 2h daily 2000μmol photons m−2s−1, and 6h above 1000μmol photons m−2s−1during a clear day in early spring. Thalli of L. pulmonariawere transplanted to these two stems. During the first 40 days (April–May), transplants at the L. pulmonaria-deficient site developed severe chlorophyll degradation, and a substantial reduction in maximal PS II efficiency (FV/FM) even when measured after a 48-h recovery period at low light intensity. Extensive bleaching was formed along light-exposed sides of the tiny ridges on the upper side. Subsequent to this damage, FV/FMgradually rose to nearly normal levels during the following year. This apparent recovery was probably mainly due to irreversible loss of damaged chlorophyll, but also to some level of acclimation. No damage was observed in control transplants on the L. pulmonaria-rich tree, which were the only transplants gaining sufficient growth for new attachment to the new substratum during the 397-day transplantation period. Nevertheless, a fine-scale, but highly significant seasonal variation in FV/FMof control transplants reflected variations of even low irradiance levels. FV/FM, as measured after a 48-h recovery period at low light intensity, is an efficient method for recording permanent high light damages at and shortly after damage is formed. However, FV/FMis not a useful estimator of chronic long-term damage.

The F_V/F_Mratio ofLobaria pulmonariadecreased with decreasing pH in samples from a boreal forest influenced by acid rain in SE Norway, suggesting that low pH has an effect on the optimal quantum yield of PSII. The pH was measured of distilled deionized water in which lobes from different habitats were rinsed, probably indicating the ionic environment of the lichen. The best prediction of F_V/F_Mwas found in a multiple regression model including the explanatory variables: pH,a/b-ratio and chlorophyll degradation measured as the 666/666a ratio, all correlated positively with F_V/F_M, but not with each other. The commonly used chlorophyll degradation 435/415 ratio cannot be used forL. pulmonariabecause of an unidentified, habitat-influenced brownish pigment with high absorbance in the 415–435 nm wavelength range.