Forest successional stage affects the cortical secondary chemistry of three old forest lichens.

Three epiphytic old forest lichens (Usnea longissima, Pseudocyphellaria crocata, and Lobaria pulmonaria) were transplanted along a natural shade-sun gradient comprising three successional stages in boreal spruce forests (dense young forest, open old forest, and clear-cut) for one summer. After harvest, extractable secondary compounds were analyzed by high-performance liquid chromatography, and the brown pigmentation in melanic species was quantified by reflectance measurements. Cortical compounds in all species increased from shady young forests to exposed clear-cuts. Usnic acid, the major cortical, secondary compound in U. longissima, showed consistently higher concentration in the clear-cut than in the two forested stands. Pseudocyphellaria crocata and L. pulmonaria, lacking extractable secondary compounds in the cortex, significantly increased their amounts of cortical melanins in well-lit stands. The medullary compounds showed more complex responses. Many were not influenced by environmental conditions during the transplantation, whereas the majority of those that responded showed the lowest concentration in clear-cut transplants. Only a few medullary compounds showed the highest concentration in the clear-cut, and at a low level of significance. The synthesis of UV-B-absorbing usnic acid and melanins seems to be part of an acclimation to increased light exposure. The medullary compounds in studied species barely function as solar screens despite their strong UV-B absorbance.