Wind-acclimated thallus morphogenesis in a lichen (Evernia prunastri, Parmeliaceae) probably favored by grazing.

How does the morphogenesis of a widespread epiphytic lichen interact with the environment, leading to amazingly polymorphic and wind-tolerant fruticose thalli? A series of photographs showed that the normal branches were basically even and flat with isotomic-dichotomous branchings. Numerous deviations from this basic regular pattern were found and traced back to their origins in the asymmetry of the branches' cross sections. This was observed quantitatively (statistically testable) in the field, while the thalli were growing. This large variability in growth of single branches was a prerequisite for the polymorphism seen in complete thallus growth forms. The structure and distribution of the mentioned important asymmetries in branch cross sections can best be explained by the effect of grazing on cortex and phycobiont layer. Very small-scale growth observations and experimental simulation confirmed this. In thalli with many such small-scale variations in branch growth the overall thallus growth form became wind acclimated as a result of the selective effect of wind exposure on the thallus phenotype: branches growing windwardly grew more slowly. Such thalli were well protected from being dislodged by wind, and they desiccate slowly as demonstrated by a previous study.