Abstract :
Porquerolles Island covers an area of 1 250 ha and appears from a bryological perspective as a vast complex of natural habitats that are more or less under human pressure. The present study is based upon 345 prospected sites and summarises some 2 397 floristic data. 117 taxa are known in Porquerolles. 21 taxa are liverworts, 2 are hornworts and the remaining taxa are mosses. Porquerolles appears richer in bryophytes than Port-Cros island, that exhibits a total of 101 taxa. 20 taxa mentioned in Port-Cros are nonetheless not known in Porquerolles. These 20 taxa predominantly inhabits calcicolous man-made habitats and 9 of them show a high patrimonial value. Conversely, 36 taxa known from Porquerolles have never been mentioned in Port-Cros. The great majority of these 36 taxa are from more or less man-made ground habitats but several are from moist rocky surfaces, relatively cool and shady banks, low-lying maquis and bark of broad-leaved trees. Only 6 out of these 36 taxa show a high patrimonial value. This general statement perfectly reflects the greater human pressure on Porquerolles island and the rarity of humid habitats of this island. Then the bryoflora of the archipela-go is credited with 135 taxa of bryophytes. The most frequent ones (Hypnum cupressiform evarcupressiforme, Trichostomum brachydontium, Weissia controversa, Scleropodium touretii, Rhynchostegiella litorea, Rhynchostegium confertum, Fissidens taxifolius, Tortella flavovirens, Bryum capillare and Bryum dichotomum + Frullania dilata-ta) are not numerous as they do not exceed the percentage of 8,5 % of the total. Rare taxa are, on the contrary very numerous and constitute the majority of records (57 %). Pottiaceae, Bryaceae, Brachytheciaceae and Orthotrichaceae are best represented fami-lies. Genera Bryum, Fissidens, Riccia, Cephaloziella, Didymodon, Grimmia, Orthotrichum, Tortula, Syntrichia and Entosthodon are species richest ones. 5 species are cited in the Red Data Book of European Bryophytes : Aschisma carniolicum, Cephaloziella calycula-ta, Ephemerum sessile, Fissidens ovatifolius and Tortula freibergii. 52 additional taxa are considered as patrimonial because of their regional or local rarity. A distribution map for each of the species seen in 2005 is provided.
The main bryophytic groupings of Porquerolles are: 2 corticolous groupings, 1 saprolignicolous grouping, 4 saxicolous groupings (1 heliophilous, 1 sciophilous, 1 chas-mophilous with Tortula freibergii and 1 grouping with Atrichum undulatum) and 5 ground groupings (1 halophilous, 1 grouping of more or less humid compacted substrates, 1 bank grouping and 1 grouping with Sematophyllum substrumulosum). Port-Cros seems richer with humid saxicolous or ground groupings.
From a bryogeographical point of view, Porquerolles Island is an excellent example of the diversity to be found in crystalline Provence. The Mediteranean taxa are largely dominant. Species widely distributed round the world form an important percentage but much less prominent than that of Mediterranean s.l. taxa. Atlantic taxa are poorly repre-sented.
The main factor influencing the evolution of bryoflora are forest fires, the increase in the number of urbanization works, agricultural use of island, shading off hydrographic net-work, polluted sea sprays, excessive tourist presence and proliferation of exotic bryophytes or vascular plants. These factors may show complex consequences that are briefly evaluated in our work.
The origin of the bryoflora of Porquerolles Island is probably the result of 1 – a very ancient stock surely altered during Pleistocene glaciation, of 2 – a more recent stock, pos-terior to the last glaciation, originating from the continent by means of atmospheric migra-tions, and of 3 – a still evolving very recent stock, undoubtedly linked to human activity which is dramatically increasing over all of the site.