The lichen cushion: A functional perspective of color and size of a dominant growth form on glacier forelands

Mat-forming lichens dominating high-latitudinal habitats vary in color and geometry. Widespread species are light greenish yellow (usnic acid) and reflect solar radiation, whereas melanic species absorbing most solar wavelengths are spatially more restricted. Color thereby influences lichens’ energy budget and thus their hydration and photosynthetically active periods. By using well-defined cushions from early successional stages on glacier forelands – three melanic^(m) and three usnic^(u) mat-forming lichens with hair-like branches (Alectoria ochroleuca^(u), Gowardia nigricans^(m)), hollow terete branches (Cladonia uncialis^(u), Cetraria muricata^(m)), and flat branches (Flavocetraria nivalis^(u), Cetraria islandica^(m)) – we quantified hydration traits and analyzed how color and cushion size affect water loss rate (WLR) and duration of active periods. Main findings: 1) WLR declined with cushion size and was highest in melanic lichens. 2) Active periods were longer for usnic than for melanic lichens and increased with size in all groups. 3) Size, color, and taxon nested in color significantly influenced WLR and duration of active periods in linear mixed models. 4) Hair lichen cushions had shorter active periods than growth forms with terete or flat branches due to their more open canopy architecture and lower water holding capacity (WHC). 5) WHC measured for isolated branches highly underestimated WHC for intact cushions.