Seasonal partitioning of growth into biomass and area expansion in a cephalolichen and a cyanolichen of the old forest genus Lobaria.

Growth in two old forest lichens was studied to evaluate how temporal (seasonal) and spatial (aspect-wise) partitioning of biomass and area growth respond to seasonal changes in light and climate. We monitored relative growth rates during annual courses in the cephalolichen Lobaria pulmonaria and the cyanolichen Lobaria scrobiculata transplanted in boreal clear-cut to five fixed aspects in winter, spring, summer, and autumn. For each annual set, growth was quantified in January-March, April-June, July-September and October-December. Mean biomass and area increased in all seasons, but growth was highest in July-September. Mass growth did not follow area increment during a year. As a result, mass per area (specific thallus mass (STM)) declined (L. scrobiculata) or stayed constant (L. pulmonaria) in the dark, humid October-December season, whereas it strongly increased in the dry, sunny April-June season. Aspect influenced growth in species-specific ways. Seasonality in biomass growth mainly followed light availability, whereas area growth was strongest during humid seasons. The substantial STM changes across seasons, species, and aspects can be explained as passive responses to seasonal climate. However, as STM, according to the literature, is a driver of water storage, recorded changes probably improve fitness by prolonging hydration in places or during times with high evaporative demands.