A revised Rydzak’s Drought Hypothesis for a changing environment

Air pollution is certainly a key limiting factor for the survival of lichens in urban environment, but some recent field studies underlined that water availability is also important, partially confirming what was a key point in the highly controversial Rydzak’s Drought Hypothesis. This study was aimed at understanding the interplay between air pollutants and air humidity (used as a first, rough proxy of water availability) in determining (i) the lichen diversity value (LDV) of urban communities, and (ii) the physiological responses of a macrolichen transplanted to urban environments. LDV was measured in highly standardized conditions in two cities of NE Italy with different levels of pollutants along transepts running parallel to or far from ditches. Healthy thalli of Flavoparmelia caperata were transplanted in three successive seasons to 10 sites located in two other cities of NE Italy, selected for their different climate and comparable levels of pollutants. The two data sets confirm the greater importance of meso- and microclimatic conditions with respect to the actual level of pollution, suggesting than when SO2 is negligible, one of the key factors for the survival of lichens in urban environment is certainly water availability, from which the activation of lichen metabolism strictly depends.