Interspecific and intraspecific variations in water storage in epiphytic old forest foliose lichens

Cyanolichens (Lobaria hallii, Lobaria retigera, Lobaria scrobiculata, Pseudocyphellaria anomala) and cephalodial (Lobaria pulmonaria) and noncephalodial (Platismatia glauca) chlorolichens were sampled in dry open, mesic open, and mesic closed forests in central British Columbia. Specific thallus mass (STM), water-holding capacity (WHC), percent water content at saturation, and thickness of upper cortex, photobiont layer, and medulla plus lower cortex were measured. Whereas STM did not differ much between cyanolichens (9.8 +/- 0.1 mg dry mass.cm(-2)) and chlorolichens (8.0 +/- 0.1 mg dry mass.cm(-2)), cyanolichens had a WHC (20.3 +/- 0.2 mg H2O.cm(-2)) that was nearly twice that of the chlorolichens (10.8 +/- 0.2 mg H2O.cm(-2)). STM and WHC increased with light exposure at the studied sites, presumably as an acclimation to higher evaporation demands. Within species, WHC was strongly coupled to STM and increased substantially faster with STM in cyanolichens, compensating their inability to use humid air to restore photosynthesis. The photobiont layer was two times thicker in cyanolichens, whereas the thickest layer (the hydrophobic medulla) did not differ between cyanolichens and chlorolichens. Most interspecific variation (88.3%) in WHC could be accounted for in a multiple regression model where STM and photobiont to total thickness ratio were the most important independent variables. Photobionts thus play significant roles in the water economy of lichens.