Changes in mediterranean shrub communities with Cytisus purgans and Genista scorpius

Summary Shrub communities are important in the mediterranean landscape and linked very much with human activities. The permaneney and the spreading of these communities are related to biological characteristics of the dominant species which control the patterns of changes in structure, richness, types of life forms, and ability of colonization by trees. In this paper, changes in communities are analysed and compared for Cytisus purgans and Genista scorpius (both Papilionaceae). For both communities the schemes of origin and spreading are rather the same: overgrazing and fire play a prominent role. Changes in structure show different stages emphasized by the types of distribution of the chlorophyll containing stratum. There is a direct relation between changes in structure and changes in species richness and life form spectrum: richness decreases generally from the second or third year after fire. According to the dryness of the stand therophytes may or may not be important in the youngest stands. The colonization of the shrub communities by trees is dependent on several conditions, either on the shrub community itself, or on the plant-matrix surrounding the shrub community patch and also on animals; man, too, plays an important role. In conclusion we think that the above-mentioned changes in shrub communities fit the inhibition model proposed by Connell & Slatyer (1977); but, more generally it seems that the plant successions observed in our regions follow both, this model and the facilitation and tolerance models of the same authors. Linked with this fact, it seems that the successions show generally fluctuating values of richness as is suggested by Whittaker (1975). We think that plant succession must be well analyzed at different levels of organization and include the study of the influences man and animals may have.Plant nomenclature follows Fournier (1961); for birds see Peterson et al. (1967).We are very much indebted to Ch. Rimbault for her contribution to the translation of this paper.