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

Can the interaction of episodic and chronic disturbances explain the mainte nance of savanna-like patterns? We explored the morphological and spatial p atterns of the leguminous shrub Retama sphaerocarpa in a Mediterranean envi ronment in relation to disturbance. Various morphological variables of R. s phaerocarpa shrubland were found to be determined by a combination of two t ypes of disturbance: (1) mechanical cutting: an episodic, heavy, short-term disturbance of anthropogenic origin for management purposes, and (2) herbi vore activity primarily by rabbits: generally a chronic, more lenient, long -term disturbance. The intensities of these two types of disturbance were n ot correlated. Mechanical cutting effects on R. sphaerocarpa shrubland morp hology predominated quantitatively over herbivore effects. Herbivores gener ally 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 ae rial biomass. However, heavy herbivory in abnormally dry periods produced s ome effects similar to those of mechanical cutting. The size of R. sphaeroc arpa shrubs was positively related to seed production. Thus, the means of p ropagation depended upon the type of disturbance: episodic disturbances res ulted 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 p atterns of dominant woody species in this Mediterranean landscape and simil ar savanna-type ecosystems. Investigations of environmental constraints on vegetation distribution and abundance should take into account the historic al role of herbivores in shaping present systems.