A COMPARATIVE-STUDY OF CISTUS LADANIFER SHRUBLANDS IN EXTREMADURA (CW SPAIN) ON THE BASIS OF WOODY SPECIES COMPOSITION AND COVER

The ecological structure and diversity of woody species in the Cistus ladanifer shrubland ('jaral'), one of the most widespread Mediterranean scrub systems in the western half of the Iberian Peninsula, was studied in Extremadura (CW Spain). 57 points were sampled for presence/absence and abundance (line interception) of woody species. Several diversity indices were calculated to elucidate the role of both C. ladanifer and bare soil in the jaral structure. The sites were classified by agglomerative hierarchical clustering and ordinated by Principal Component Analysis (PCA). Most of the examined scrubs have diversity within the range 1-3, evenness above 0.5 and species richness between 1 and 12. Clustering and PCA distinguished three groups of scrubs: A, those with a certain Atlantic influence and highest values of diversity and total cover, in which Lavandula stoechas subsp. luisieri, Erica umbellata and Halimium ocymoides are frequent; B, the most continental scrubs, growing on moderately acid soils with Cistus salvifolius and C. crispus, and, C, those as continental as group B but on more acid soils, characterized by Genista hirsuta. Differential species between groups basically coincide with those used in phytosociological classifications. Diversity data, together with affinity and PCA analyses, emphasize that the ecological structure of Mediterranean shrublands is strongly influenced by humidity and the annual temperature range.