Shrubland patchiness succession in mesic versus arid Mediterranean sites

There has been a growing interest in describing and defining the vegetation patch entity and its dynamics. Conceptual and computerized models suggest different frameworks for patch and patchiness evolvement. Our approach was to develop a conceptual model of patch and patchiness formation that is based on shrub development stages, the transitions between the stages, and the nature of the interactions between them. Within this framework, we have studied patch formation and dynamics in the dwarf clonal shrub Sarcopoterium spinosum at two sites, arid and mesic, along the Mediterranean-semiarid climatic gradient. We have combined shrub age and development analysis with genetic analysis (AFLP), in order to identify shrub developmental stages. With field and garden experiments we have examined the nature of the interactions between developmental stages. Finally, we have reconstructed the history of patch formation, by the analysis of the spatial-genetic and spatial-age structures of shrub patch population. Results of the genetic-demographic analysis reflected feedback mechanisms that were demonstrated in field and garden experiments and revealed the role of clonal vr. sexual recruitment in patch formation: Sarcopoterium spinosum patches are enhanced by positive feedback mechanisms that encourage within-patch sexual and clonal recruitment. Nevertheless, in mesic sites, there is greater contribution of clonal recruitment to patch expansion. However, in arid sites the relative importance of facilitation to patch dynamics increases, while in mesic sites the importance of interference increases. These processes sharpen the patchiness of the shrubland both spatially and temporally, consistent with Wilson and Agnew's suggested 'sharpening situation'.