Role of disturbance in maintaining a savanna-like pattern in Mediterranean Retama sphaerocarpa shrubland

Abstract. Can the interaction of episodic and chronic disturbances explain the maintenance of savanna-like patterns? We explored the morphological and spatial patterns of the leguminous shrub Retama sphaerocarpa in a Mediterranean environment in relation to disturbance. Various morphological variables of R. sphaerocarpa shrubland were found to be determined by a combination of two types of disturbance: (1) mechanical cutting: an episodic, heavy, short-term disturbance of anthropogenic origin for management purposes, and (2) herbivore activity primarily by rabbits: generally a chronic, more lenient, long-term disturbance. The intensities of these two types of disturbance were not correlated. Mechanical cutting effects on R. sphaerocarpa shrubland morphology predominated quantitatively over herbivore effects. Herbivores generally produced open shrubland with fewer, more scattered, thicker branched, larger R. sphaerocarpa shrubs. In contrast, intense sprouting after cutting produced a higher density of smaller R. sphaerocarpa shrubs with denser aerial biomass. However, heavy herbivory in abnormally dry periods produced some effects similar to those of mechanical cutting. The size of R. sphaerocarpa shrubs was positively related to seed production. Thus, the means of propagation depended upon the type of disturbance: episodic disturbances resulted in intense sprouting, whereas chronic herbivore activity resulted in the formation of thick branches that produced a large number of seeds. The combination of these two disturbances determine, in part, space occupancy patterns of dominant woody species in this Mediterranean landscape and similar savanna-type ecosystems. Investigations of environmental constraints on vegetation distribution and abundance should take into account the historical role of herbivores in shaping present systems.