Is the length of the drying period critical for photosynthesis reactivation in lichen and moss components of biological soil crusts?

Lichens and mosses represent the macroscopic components of biological soil crusts (biocrusts). Their ability to exploit short periods of water availability and reversibly deactivate metabolism is crucial for their growth and survival. In this work we investigated photosynthesis reactivation, respectively after long (65–66 days) and short (15 days) dry periods, in lichen and moss species widespread in two Mediterranean environments (Portugal and Italy). Chlorophyll a fluorescence emission of the samples was investigated and the parameter Fv/Fm, an indicator of vitality of photosynthetic organisms, was used as a proxy for photosynthesis reactivation. The fruticose lichens Cladonia convoluta and C. rangiformis, and the moss Pleurochaete squarrosa, typical of Mediterranean environments, showed a significantly slower reactivation of photosynthetic activity when subjected to a longer period of drought. Conversely, the alien invasive moss Campylopus introflexus was not affected by prolonged dry conditions. The study showed that drought duration influences the reactivation of photosynthetic activity in terricolous lichens and mosses forming biocrusts in re-hydration cycles. These results indicate the likelihood of a reduction in biocrust productivity as a consequence of climate change in Mediterranean drylands.