Fitoclima della Sicilia sud-orientale

South-eastern Sicily, otherwise called Hyblean district, is geologically made of a calcareous plateau of Ceno-Mesozoic origin which is interrupted by volcanic formations in the mountains and in the northern part. The southern, eastern and northern sides of the plateau degrade towards the Sicilian Canal, the Ionic Sea and the Catania Plane, respectively. The north-western side is bordered by a Plio-Pleistocene transition area of sand and clay. The plateau is ploughed by several rivers in a sunburst pattern from Mt. Lauro (986 m), which is the highest part of the area. The Hyblean district is the most anciently part of Sicily colonized by man. This millenary presence has caused alterations and the disappearance of the original forest cover, which is still present, but with few fragmented strips. Consequently, the identification of climax vegetation results to be sometimes hard. In this paper we perform a bioclimatic study of the area according to Montero de Burgos & Gonzales Rebollar method (1983) based on a set of indices or Bioclimatic Intensities, among which there is the potential Bioclimatic Intensities (IBP), the actual one (IBR), the free one (IBL), the dry one (IBS), the conditioned one (IBC) and the cold one (IBF). The IBP estimates the maximum primary productivity of a climate without factors limiting plant growth; IBR estimates the actual productivity taking place when water availability is not sufficient to compensate for evapotranspiration requirements; IBL estimates forest productivity; IBS and IBF, finally, estimate the duration and intensity of vegetation inactivity periods due to summer draught or winter harshness. Based on the monthly values of the IBs, 33 stations in south-eastern Sicily were classified according to the hierarchical method using the dissimilarity coefficient of Bray & Curtis (1957) and the complete linkage technique. The results of the cluster analysis allowed us to gather the 33 stations of the Hyblean District into two groups, each divided, in turn, into three subgroups. In all the 6 subgroups correspond to the geographical division of the district into homogenous phytoclimatic areas, which shows clearly how the south-western area is well differentiated from the eastern one. Starting from the top of the Hyblean mountains, two different bioclimatic gradients develop, the one along the southern and south-western coastal line, and the other towards the north-eastern coast. Generally, stations in the south-western area result to be characterized by longer periods of draught, whereas those in the eastern area have lower primary productivity in winter months. The mountainous area is distinguished by the presence of IBF in winter or, anyway, by low IBL values. Distinctive phytoclimatic conditions characterize the Buccheri area which results thus to be different from all others. Next to Buccheri, on the top of Mt. Lauro, we find grasslands of Hypochoeridio-Lotetum conimbricensis Brullo, Grillo & Terrasi 1976, which are very close in ecology and floral composition to Cynosuro-Leontodontetum siculi Brullo & Grillo, 1978, which is largely represented on the Nebrodi range (northern Sicily). The potential vegetation of Buccheri area is deciduous oak forests of Mespilo-Quercetum virgilianae Brullo & Marcenò 1984; according to some authors, the uppermost area of Mt. Lauro could be interested by Turkey oak (Quercus cerris L.) woods of Arrhenathero nebrodensis-Quercetum cerridis Brullo, Minissale, Signorello & Spampinato 1996. The potential natural vegetation of the area immediately south-west of Buccheri, which is characterized by rather higher winter temperatures, is represented by Mespilo-Quercetum virgilianae although forest phytocoenoses belonging to Oleo-Quercetum virgilianae Brullo 1984, lacking the