Acta Bot. Barc.,45 (Homenatge a Oriol de Bolos): 363-390 Barcelona 1998 INTERÉS FLORÍSTICO y GEOBOTÁNICO (MONEGROS, ARAGÓN)l Julia MOLER0 2 , Llorenc SÁEZ 3 & Luis VILLAR 4 ABSTRACT Floristic and geobotanical interest of Sierra de Alcubierre (Monegros, Aragon, Spain) Sierrade 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 northem and southem 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 choro- logic Elements and Subelements. The fioristic background is clearly Mediterranean (65%). Ofparticular 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 lutea subsp. vivantii. The discovery of notewor- thy plants endemic to the Iberian peninsula, such as Arrhenaterum album, Bifora radians, Cirsium ferox, 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 groves -Violo-Quercetumfagineae subass. viburnetosum tini Molero, Sáez Trabajo financiado con cargo al proyecto "Recursos fitogenéticos de Aragón", desarrollado por convenio entre la Diputación General de Aragón y el Instituto Pirenaico de Ecología (CSIC) durante los años 1995 y 1996. 2 Laboratori de Botánica. Facultat de Farmacia, Universitat de Barcelona. Av. loan XXIII. s/n. E-08028 Barcelona. 3 Departament de Biologia Vegetal, Botánica. Facultat de Biologia, Universitat de Barcelona. Av. Diagonal, 645. E-08028 Barcelona. 4 Instituto Pirenaico de Ecología, CSIC. Apartado 64. E-22700 laca (Huesca).