Floristic and geobotanical interest of Sierra de Alcubierre (Monegros, Aragon, Spain)

Sierra de Alcubierre is the main orographic nucleus of the Monegros and the one which penetrates farthest into the most arid part of the Ebro valley. It constitutes a highly interesting biogeographical meeting place: the sharp contrast between the northern and southern slopes gives rise to widely differentiated microclimates offering a broad range of habitats for taxa and plant communities of very varied geographical origins and ecological conditions.
Provisional analysis has been carried out on the basis of the 578 taxa so far recognized in this territory in order to establish the percentage of the main chorologic Elements and Subelements. The floristic background is clearly Mediterranean (65%). Of particular significance, in the arid environments where they are located, are the eurosiberian species (8,2%): Atropa belladona, Filipendula vulgaris, f.ex.; also the submediterranean species (6%) are present: examples reported for the first time include Buxus sempervirens, Amelancher ovalis, Polygonatum odoratum, Lonicera xylosteum and Viola willkommii. Outstanding among endemics (8%) are certain territorially endemic taxa such as Allium pardoi, Nepeta beltranii, Vale- rianella multidentata and Reseda tutea subsp. vivantii. The discovery of noteworthy plants endemic to the Iberian peninsula, such as Arrhenaterum album, Bifora radians, Cirsiumferox, Senecio lagascanus, also deserves mention. A new taxon is described: Viola rupestris susbp. orioli-bolosii Molero, L. Sáez & Villar.
In terms of vegetation, the phytosociological method was used to identify the communities which provide the best definition of the Plant Landscape of the Range. Of special interest is the study of continental holm-oak forests, Quercetum rotun- difoliae, of which two new subassociations are described: senecietosum lagascani Molero, Sáez & Villar and viburnetosum tini Molero, Sáez & Villar. Impoverished gall-oak graves -Violo-Quercetumfagineae subass. viburnetosum tini Molero, Sáez & Villar-, altered by the presence of certain thermophilous coastal species, and finicolous and residual in character, are found in a few exceptional enclaves.
Finally, stress is laid on the need to introduce suitable measures for the protection of certain places in the Range, particularly the residual woods of the northern slopes, in view of the exceptional interest of the flora and landscape.