Differences in the susceptibility to light stress between epiphytic lichens of ancient and young boreal forest stands

1. Exposure to high light intensities (250 and 1000 mu mol photons m(-2)s(-1)) caused a rapid and strong decline in F-v/F-M in hydrated lichens. However, all species except the old-forest indicators Lobaria pulmonaria and Evernia divaricata recovered original values of F-v/F-M within 2-3 days at 18 degrees C. 2. Lichen indicators of long ecological continuity, especially within the threatened Lobarion community, were most susceptible to high light, also in evidence by increased chlorophyll degradation in hydrated thalli with increasing PFDs. Usneion species showed a moderate level of photoinhibition; those members of the family Parmeliaceae that are ubiquitous were the most resistant. 3. A reduction in temperature from 18 to 19 degrees C during high-light treatment caused a larger and more permanent reduction in F-v/F-M in hydrated thalli of Lobaria pulmonaria than of Parmelia sulcata. 4. F-v/F-M was permanently reduced in air-dry thalli subjected to high PFD; irreversible and very low levels of F-v/F-M developed within 2-3 days of high-light exposure for tested Lobarion species measured after 48-h recovery in moist conditions at low light. Ubiquitous Parmeliaceae species maintained fairly high F-v/F-M throughout the 7-day exposure; Usneion species were intermediate. The lichen indicators of long ecological continuity showed a large and highly significant chlorophyll degradation, whereas the remaining species had some kind of protecting mechanisms. 5. Clear-fellings of forests are likely to be detrimental for presently declining lichens of ancient forests by causing a sudden increase in PFDs on remaining lichen thalli.