Lichenologist 32(3): 271–289 (2000) doi:10.1006/lich.1999.0265 Available online at http://www.idealibrary.com on HIGH-LIGHT-INTENSITY DAMAGE TO THE FOLIOSE LICHEN LOBARIA PULMONARIA WITHIN A NATURAL FOREST: THE APPLICABILITY OF CHLOROPHYLL FLUORESCENCE METHODS‡ Yngvar GAUSLAA* and Knut Asbjørn SOLHAUG* Abstract: The annual course of irradiance was recorded at two vertical and even-aged neighbouring Quercus stems, one rich in L. pulmonaria, one without. Irradiance never exceeded 610 mol photons m 2 s 1 at the L. pulmonaria site, whereas the L. pulmonaria-deficient site could experience 2 h daily 2000 mol photons m 2 s 1 , and 6 h above 1000 mol photons m 2 s 1 during a clear day in early spring. Thalli of L. pulmonaria were 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 (F V /F M ) 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, F V /F M gradually 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 F V /F M of control transplants reflected variations of even low irradiance levels. F V /F M , 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, F V /F M is not a useful estimator of chronic long-term damage.  2000 The British Lichen Society Introduction Due to its simple and non-destructive measurement, a depression in maximal PS II efficiency (F V /F M ) has become a sensitive and frequently used stress indicator of photosynthetic organisms subjected to various stresses, like excessive visible (Demmig-Adams et al. 1990a; Öquist et al. 1992) and UV-B irradiance (Ihle & Laasch 1996; DeLong & Steffen 1997), extreme tempera- tures (Robberecht & Junttila 1992; Jensen et al. 1997; Larcher et al. 1997), pollution components (Strand 1995; Gauslaa et al. 1996), or toxic chemicals *Department of Biology and Nature Conservation, The Agricultural University of Norway, P.O. Box 5014, N–1432 A r s, Norway. E-mail: yngvar.gauslaa@ibn.nlh.no ‡Abbreviations: F 0 =minimum fluorescence in the dark-adapted state; F M =maximum yield of fluorescence; F V =variable yield of fluorescence (i.e. F M –F 0 ); F V /F M =optimal quantum yield of PS II; PS II=photosystem II. 0024–2829/00/030271+19 r35.00/0  2000 The British Lichen Society