The Alps - The Alpine chain hosts two peculiar biomes: 1) the Alpine biome (above treeline); 2) the subalpine-oroboreal biome (near treeline, dominated by Larix-Rhododendron, also including natural, closed Picea abies forests). In Italy, the limit of most arctic-alpine/oroboreal vascular vegetation lies somewhere in the northern Apennines, with the relevant exception of the Gran Sasso-Majella Massives in the central Apennines, which host a truly Alpine flora above treeline (but no well-developed oroboreal belt). Of particular interest are also some internal Alpine valleys with a continental climate, which host several "steppic" species. The terms "arctic-alpine" and "boreal-montane" are often used both for vascular plants and for lichens. In the former case its meaning is relatively clear, as it refers to plants occurring e.g. both in the Arctic zone and in the Alpine belt of mountains of the Temperate zone, and absent from mountain systems of more southern latitudes. In the case of lichens, the term "arctic-alpine" is much less clear: many "arctic-alpine" lichens do also occur in mountains of the subtropical or tropical zone, and several of them even show a bipolar distribution including parts of Antarctica, while e.g. the mountains of Calabria and even those of Sicily do still host several so-called "boreal-montane" lichens. Comparing the lichens of Greenland, the Alps and the Central Asiatic Mountains, Nimis (1997) suggested a relatively high affinity among the three biota, with a gradient of decreasing richness from south to north which might well correspond to a relatively recent colonization of formerly ice-covered areas by more "southern" lichens already adapted to the cold-dry conditions of mountain habitats. This suggests that the term "arctic-alpine", as applied to lichens, might prove to be deceiving. Pending further research, however, in this book this term will be adopted in a very broad sense, for all lichens occurring in and above the subalpine and alpine belts of the Italian mountains, and in the Arctic zone, irrespectively of their occurrence elsewhere. On the whole, the lichen biota of the Alps seems to hold an intermediate position between those of the Arctic and of the Himalayas: the Alps were less subject to glaciation than the Arctic zone at large, and this ensured the persistence there of several "Alpine" species which are widespread in widely distant mountain systems of the Northern, and sometimes even of the Southern Hemisphere. Any hypothesis on the ancient origin of this peculiar element is premature: we still need reliable data from several mountain systems worldwide. A lichen checklist of the Alps is presently in preparation (Bilovitz & al. 2013).
The high Mediterranean mountains host the Oromediterranean biome (above treeline outside the Alps and Abruzzi). The highest peaks of the Mediterranean mountains do neither have a truly Mediterranean climate, nor do they host a sclerophyllous vegetation. However, they are biogeographically so peculiar that the existence of an "Oromediterranean" vegetation belt is accepted by most authors, albeit under different denominations. In southern and insular Italy only a few mountains attain treeline. Some of them (e.g. the recent Etna Volcano in Sicilia, and the much older Gennargentu Massif in Sardegna) host a peculiar vegetation, dominated by thorny-shrubs of the genus Astragalus, Tragacantha-section. The thorny-shrubs formations of the Mediterranean mountains have an old history, perhaps dating back to the Messinian period, when the Mediterranean was a semi-desert, biogeographically connected with the Iranian-Turanian region (see Nimis 1981). The lichen biota of the few truly Oromediterranean peaks of Mediterranean Italy, while probably species-poor, could prove to be of high biogeographic interest, but still await exploration.
The montane beech forests. Beech (Fagus sylvatica) is often the dominant tree in the mountains of Italy. In the Alps it forms pure to mixed forests (with Abies alba in cool-humid situations), with a broad altitudinal range, from ca. 600 m, in contact with the submediterranean belt, to ca. 1800 m, in contact with the oroboreal belt. Along the Apennines, down to Sicily, albeit twarted and shrub-like, beech marks treeline, but it does not occur is Sardegna. During the glacial periods, beech forests and their flora were confined to refugial areas in southern Europe, especially in the Balkan and Italian peninsulas, and later expanded northwards, the present vascular flora of beech forests becoming progressively poorer from southern Europe to southern Scandinavia (Nimis & Bolognini 1993). What may now appear to be a typical example of central European vegetation, such as a German beech forest, is in reality a very much impoverished version of a biome that has its roots, and maintains its maximum diversity in the mountains of the Mediterranean Region. This holds true for vascular plants and for lichens alike. Many species of the deciduous forest belt, "central European" or "submediterranean" species, as they are often called, have colonised central and northern Europe from the south. The beech forests of Italy differ very much in their lichen component, depending on the degree of air humidity: some of them host an unusually rich, interesting, luxuriant lichen vegetation (e.g. with Mediterranean-montane species such as Melanelixia laciniatula, Physconia venusta and Ochrolechia balcanica), others - even those located in rainy areas (e.g. parts of Liguria and Friuli) - are almost a lichen desert. The balance between air humidity and precipitation in the liquid form can perhaps explain these dramatic differences.
The submediterranean deciduous forests (dominated by deciduous trees other than Fagus) occupy a wide area lying between the montane and the Mediterranean belts, covering most of the lowlands and hills of Italy. The potential vegetation is dominated by deciduous trees, especially Quercus and Carpinus, most forests having being substituted by coppices dominated by Ostrya and Fraxinus ornus, urban areas and cultivations. The glacial and post-glacial history of submediterranean forests is similar to that of beech forests, with a difference related to the thermic requirements of the dominant trees, which survived in warmer sites, mostly in lowland areas, especially in southern Italy. The vascular flora is richer in the South, poorer in the North (see Bolognini & Nimis 1993), while lichen richness, also in this case, mainly depends on air humidity and human disturbance. Before Roman colonisation, most of the Po-plain was covered by dense deciduous forests. Presently, it is an agricultural, industrial or densely urbanised area. The plains of the North, plus a narrow strip along the eastern side of peninsular Italy, are among the most densely populated parts of the country, where several lichen species do not occur because of pollution and/or almost total deforestation. In many textbooks, Italy is still being subdivided into three major parts; northern, central and southern. This subdivision, which is familiar to every Italian, is mostly based on historical premises. Biogeography, however, suggests that the Italian Peninsula, instead of being split from north to south, could be split from west to east. The term "Tyrrhenian Italy", used for defining a biogeographic region, first appeared in a paper by Nimis & Tretiach (1995): a multivariate classification of 20 regions of Italy and of the percent incidence of biogeographic elements in their lichen biota, separated the regions facing the Tyrrhenian sea (from Liguria to Sicilia, incl. Sardegna) into a distinct cluster, characterised by many suboceanic to oceanic, sometimes subtropical lichens. The western part of the Italian Peninsula has a mild-humid climate generated by Tyrrhenian maritime air masses, while the Adriatic coast, on the lee-side of the Apennines, is subject to cold-dry winds from Eurasia during winter. The inland extent of maritime influence differs according to the presence-absence of high mountains stretching parallel to the coasts, being more pronounced in Tuscany and Latium, where the humid Tyrrhenian air masses can reach the watershed of the Apennines.
Mediterranean Italy: this is the part of the country which would be potentially dominated by evergreen broad-leaved forests. This biome, apart from small enclaves in the Insubrian District and along the Trieste coast, is mainly present along the coasts of Liguria, the Peninsula and the islands, with a notable difference between the Adriatic and the Tyrrhenian sides of Peninsular Italy: along the Adriatic side, truly Mediterranean vegetation is almost absent or very localised north of Puglia, whereas along the Tyrrhenian side it reaches north to the coasts of Liguria, with wide inland penetrations e.g. in Tuscany. Due to the rugged morphology, truly Mediterranean vegetation is surprisingly reduced to a narrow coastal strip in some southern areas such as western Calabria. Contrary to the Iberian Peninsula - almost a small continent in itself - the narrow Italian Peninsula and its islands, bathed from all sides by the Mediterranean Sea, rarely experience extreme climates. There are, however, a few parts of Italy which have a really dry-Mediterranean climate. These are mostly located in southern Sicilia, southeastern Sardegna, and in parts of Puglia. In the lichenological literature the term "Mediterranean" has often been used exactly as far vascular plants. Many authors (e.g. Nimis & Poelt 1987) implicitly assumed the existence of a "Mediterranean element" in lichens, whose distribution patterns would be consistent with those of steno- or eurimediterranean vascular plants. Barreno (1991) was one of the first to question this assumption, suggesting that examples of truly "Mediterranean" distribution are far less frequent in lichens than in vascular plants. She pointed out that many terricolous "Mediterranean" lichens are distributed far beyond the Mediterranean Region, some of them extending throughout the Mediterranean, Irano-Turanian and Saharo-Arabian biogeographic provinces. The "Mediterranean" lichen element is difficult to define and quite heterogeneous, as it includes: (a) several, often not very well-known, coastal species restricted to the Mediterranean Region, (b) species with a Macaronesian-Mediterranean distribution which are not bound to a particularly humid climate, (c) a few species extending into other parts of the world with a Mediterranean climate, especially California, (d) some species restricted to the humid montane belt of the Mediterranean mountains. On the whole, there is a sharp contrast between the richness of the Mediterranean vascular flora, and the scarcity of truly Mediterranean lichens. The puzzling paucity, among lichens, of cases of truly "Mediterranean" distribution patterns was confirmed by the bioclimatic analysis of the ltalian lichen biota by Nimis (1993) and Nimis & Tretiach (1995). For example, steno- plus eurimediterranean species account for 28.5% of the vascular flora of Sicily (Nimis 1984), whereas the corresponding figure for lichens is only 8 % (Nimis & Tretiach 1995). Perhaps the richest habitat for truly "Mediterranean" lichens are humid rock outcrops, both siliceous and calcareous, along the coasts, which host peculiar and often geographically differentiated biota (see e.g. Roux 1991). The epiphytic vegetation, on the contrary, is much more homogeneous throughout the Mediterranean Region.
Summarising, the high diversity of lichens in Italy is mainly due to the presence of some biogeographic groups of species with different ecological requirements (Nimis & Tretiach 1995). The main ones are:
A) A mainly temperate group of lichens without particular suboceanic affinities which is well represented in all regions and is most frequent in the deciduous forest belts.
B) A suboceanic to oceanic group with subtropical affinities, bound to humid climates, which is most frequent along the western side of the Peninsula, in Liguria and in Sardegna, i.e. in Tyrrhenian Italy.
C) A "northern" group with arctic-boreal affinities, restricted to the highest mountains, most frequent on the Alps on acid substrata and becoming progressively rarer southwards.D) A rather poorly defined set of species whose hitherto known distribution is limited to the southern European mountains, which in Italy is mostly found on calcareous substrata in the alpine belt of the Alps.
E) Another poorly defined element restricted to the lowlands and lower mountains of the Mediterranean Region, sometimes extending to Macaronesia and to other Mediterranean areas of the world, which in ltaly has a mainly Tyrrhenian distribution and the highest diversity in the Mediterranean belt.
F) A small set of widespread xerophytic species, occurring in the most arid parts of Mediterranean Italy (Sardegna, Sicilia) and in the driest parts of the central and western Alps.
This picture reflects fairly well the climatic diversity of the country, from cold-alpine to warm suboceanic climates, with a prevalence of warm-temperate, moderately humid climates, and with an overall scarcity of truly arid climate-types, despite the summer drought period of some regions of the South.