Lichen-forming fungi and mosses form a major part of the terrestrial non-vascular vegetation and are thought to have a significant impact on global biogeochemical cycles, such as the nitrogen and carbon cycle. However, in order to draw quantitative conclusions about their ecosystem functions, it is essential to understand the metabolic processes underlying their growth. The dynamic water balance of lichens and mosses is a crucial factor in this regard since the metabolic processes of the organisms can only occur in phases with sufficient water saturation. Water can occur inside the cells (internal) in the symplast and in pores in the cell wall (apoplast water), and also externally in the capillaries of the intercellular space. It is poorly known, however, how atmospheric demand for water, related to water potential ( ψ ), affects the dynamic distribution of internal and external water in lichens and mosses and which consequences this may have for their water balance. Here, we examined water absorption of Pleurozium schreberi , Cladonia portentosa and Peltigera rufescens under a gradient of ψ in the laboratory. Results show that for all species, relative water content decreased with decreasing ψ. Both internal and external water contents thereby showed a consistent pattern across the range of ψ- values tested here. This indicates that, although a proportion of the internal water has already evaporated and therefore the turgor pressure is altered, in all three species a proportion of external water is retained by capillary forces even at low ψ .

Alien tree species have been introduced to Europe and often used as roadside trees. Currently, preference is given to species that are adapted to urban climate and drier conditions. Native epiphytic lichens are adapted to local tree species; however, little is known whether a shift toward more alien roadside trees would affect the diversity of epiphytic lichens within cities. We considered three genera of roadside trees that are common in Hamburg (Germany) Acer, Tilia, and Quercus, each with a native and an alien species per genus. Species numbers, frequency and diversity of epiphytic lichens were assessed and compared. Tree locations were grouped according to three classes of increasing traffic volume as a proxy for air pollution. Mean bark pH values have been recorded for each tree individual. The species pairs Quercus robur/rubra and Acer platanoides/saccharinum showed significant differences in the diversity and abundance of lichens, with the alien species showing lower values. The species pair Tilia cordata/tomentosa did not show any significant differences. The number of nitrogen-tolerant lichen species differed little among all tree species. Lichen species richness decreased with increasing traffic volume for all tree species pairs, thus demonstrating the inhibiting effect of traffic on the occurrence of lichens. Bark pH differed little between the species pairs yet high traffic volume resulted in an increased bark pH. In conclusion, two out of three alien species had negative effects on lichen diversity. Further tree species should be assessed to better estimate the effect of alien roadside trees on lichen diversity.