Seasonal and spatial variation in carbon based secondary compounds in green algal and cyanobacterial members of the epiphytic lichen genus Lobaria.

Acetone-extractable carbon based secondary compounds (CBSCs) were quantified in two epiphytic lichens to study possible effects of external factors (season and aspect) on secondary chemistry and to relate defense investments to biomass growth and changes in specific thallus mass (STM). At the end of four separate annual cycles starting in each of the four seasons, the cyanolichen Lobaria scrobiculata and the cephalolichen Lobaria pulmonaria (green algae as the primary photobiont and with localized Nostoc in internal cephalodia) were monitored in their natural forest habitats and after being transplanted at three contrasting aspects in open sites. Season strongly influenced most CBSCs. Medullary CBSCs in both species were twice as high in summer as in winter. Aspect hardly affected major CBSCs, whereas transplantation from forest to clear-cut slightly reduced these compounds. No major CBSCs in any species showed a trade-off with growth rate. Dry matter- as well as thallus area-based medullary CBSC contents increased with STM. The cortical usnic acid strongly increased with growth rate and followed spatial, but not seasonal variations in light exposure. Maximal CBSC levels during seasons with most herbivores is consistent with the hypothesis inferring that herbivory is a major selective force for CBSCs. Lack of trade-off between growth and defence investments suggests that these two processes do not compete for photosynthates.