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Selective feeding by gastropods in Lobaria scrobiculata allows quantification of intrathalline anatomical layers

Total dry lichen mass per area (DM/A) and water holding capacity (WHC) are interesting anatomical and physiological parameters in ecological and taxonomic studies of lichens. These parameters are partly under genetic control (constitutive adaptation) as evidenced by considerable interspecific variation, and partly influenced by external factors (inducible acclimation; e.g. Gauslaa et al. 2006). Despite the simplicity in measuring such variables, their potential has not been fully exploited in lichenological science (see e.g. Gauslaa et al. 2009). Subdivision of the total biomass into biomass fractions of separate thallus layers can help scientists understand internal processes such as resource partitioning and biont interactions. However, this task is difficult, and has rarely been done. Some gastropods form species-specific grazing traces on lichen thalli (Baur et al. 2000), and some show strong grazing preferences for specific thallus structures (e.g. Gauslaa 2008; Asplund & Gauslaa 2010). The slug Arion fuscus feeds on the upper cortex and the underlying photobiont layer in some lichens such as the cyanobacterial Lobaria scrobiculata (Scop.) DC. (Asplund et al. 2010a). These two species are sympatric in Scandinavian forests. Following ecological studies (Asplund et al. 2010a, b), we investigate here the efficiency of natural treeclimbing gastropods to discriminate between different thallus layers of the forest lichen L. scrobiculata by consumption of the upper cortex and photobiont layer. Our main objective is to use these gastropod-grazed samples to quantify DM/A and WHC, and to estimate the dry density of 1) the upper photosynthetic part including the upper cortex and 2) the lower purely fungal parts of the medulla including the lower cortex. In this way, we can quantify thallus layers and some major internal water reservoirs.