Water storage in the lichen genus Usnea in Sweden and Norway : Can morphological and water storage traits explain the distribution and ecology of epiphytic species?

Lichens are poikilohydric and cannot control water uptake and loss, water relations could therefore impact their distribution. This study examines if morphological, anatomical, and water storage traits could explain distribution of epiphytic species in the lichen genus Usnea. Seven species from oceanic (Norway) and continental areas (Sweden) were studied. Total, internal, and external water holding capacity (WHC, mg H2O cm-2) along with relative water content (WC) were recorded by spraying the thalli with water and measuring mass after shaking and blotting. The specific thallus mass (STM, mg cm-2 - main driver of WHC) was calculated from images of wet thalli. Thickness of anatomical layers (cortex, medulla, and axis) was also measured. Pendent species had lower STM and water storage than shrubby species, most probably an adaptation to water uptake from humid air. Total, internal, and external WHC were higher in the shrubby species than in the pendent ones. The pendent species had the same internal WHC as earlier reports on Bryoria and Alectoria. External water storage decreased for all species as biomass increased. The ratio between total and internal water was twice as high as reported in foliose lichens. Variation in branch diameter was much higher in shrubby than in pendent species. The interspecific differences in water storage reflect regional differences in water sources – oceanic species had higher water storage than pendent continental species, but lower than the shrubby U. hirta. I conclude that both internal and external water storage help to explain distribution of Usnea in Norway and Sweden