Mediterranean sea cliff plants: morphological and physiological responses to environmental conditions

Aims The plants of Mediterranean sea cliff ecosystems are resistant to several environmental challenges. In the present study, six species typical of the coastal rocky cliffs have been analysed in order to evaluate their diverse morphological and physiological responses to their environment across the seasons, and to examine the strategy of the ecological group to which each species belongs. Since these species are widespread across the Mediterranean region, our aim was also to highlight their ecophysiological features in habitats where the direct influence of the sea is stronger. Methods The selected species are characteristic of the sea cliffs of Elba island (Tyrrhenian sea, Italy): the halophyte Crithmum maritimum, the semideciduous Helichrysum italicum and Lavandula stoechas, and the sclerophylls Myrtus communis, Quercus ilex and Rhamnus alaternus. Four morphological traits - canopy height, leaf area, specific leaf area and leaf dry matter content - and two physiological traits - leaf water potential (LWP) and photosynthetic efficiency (PE), measured before the dawn and at midday were analysed. Water potential was measured by a pressure chamber and photosynthetic efficiency was determined by the analysis of chlorophyll fluorescence. Plant performance was also evaluated by calculating chronic (PIchr) and dynamic photoinhibition (PIdyn). Important findings Crithmum maritimum showed high resistance to the recurrent dry periods, because of the high water storage capacity of its leaves and its PE declined markedly only in July, under the harshest climatic conditions. Semideciduous taxa utilise primarily an avoidance strategy, which aims at reducing the overall leaf surface, while sclerophylls mostly show a tolerance strategy towards the prevailing stressors, as demonstrated by LWP and PE, that are lower in the sclerophylls than in the semideciduous taxa during summer, due to osmoregulation and photoinhibition, respectively. Furthermore, variability of physiological parameters was higher in the sclerophylls than in the semideciduous taxa, because the former had to withstand wider oscillations of their LWP and PE. The sclerophyllous taxa underwent a slight loss of PE also in winter, likely owing to the combined action of low temperature and high irradiance. In Mediterranean sea cliff ecosystems, the stressful combination of high irradiance, high temperatures and low rainfall typical of the summer season may have been intensified by the shallow soil which displays a poor water storage capacity. On the other hand, winter stress, caused by high solar radiation and low temperatures, does not seem to seriously affect the performance of the studied species.