High-light stress in wet and dry thalli of the endangered Mediterranean lichen Seirophora villosa (Ach.) Frödén: does size matter?

In the Mediterranean basin, coastal dune systems are a priority habitat for nature conservation. Seirophora villosa is strictly associated with undisturbed dune juniper formations and can be used as an indicator of the status of conservation. Light regime and water availability are generally the main ecological factors modified by habitat fragmentation that can be detrimental to the colonisation and survival of S. villosa populations in coastal dunes. This study aims to investigate how light regime and water availability affect individual specimens of S. villosa, by studying the relationship between photosynthetic activity and water content per thallus area in different sized S. villosa thalli and comparing susceptibility of hydrated and desiccated thalli to light stress. During dehydration, photosynthetic activity decreased, reaching low constant values in smaller thalli more quickly than in larger ones. During the exposure of dry thalli to high light, photoinhibition consistently occurred earlier in smaller specimens than in larger ones. Moreover, larger thalli that were kept dried recovered to pre-treatment values within 3 days, while smaller ones took 5 days to recover initial values. On the other hand, both large and small wet thalli were photoinhibited within 1 day and recovered to pre-treatment values within 4 and 6 days respectively. Our results showed that S. villosa thalli are susceptible to sudden increases in light exposure, especially in the case of small specimens, which after photoinhibition exhibited a reduced ability to recover. Habitat fragmentation therefore represents a significant threat to the species, particularly in dispersal and establishment phases.