Spatio-temporal scales of dispersal limitation in the recolonization of a semi-arid Mediterranean old-field

Dispersal and establishment limitations have been proposed as important mechanisms that control vegetation colonization processes. While many studies have emphasized the role of dispersal limitation in the dynamics of plant communities, little is known about the spatial and temporal scales at which dispersal limitation affects plant population dynamics. In this study we investigate spatial and temporal scales of dispersal limitation in a system of semi-arid Mediterranean old-fields recolonized by the dominating dwarf shrub Sarcopoterium spinosum (Rosaceae). Our analysis was based on a combination of two complementary approaches: 1) age analysis of S. spinosum shrubs growing at different distances from source populations, and 2) a corresponding analysis of changes in vegetation cover since recolonization onset (ca 55 yr ago), based on information obtained from historical aerial-photographs. Shrubs sampled in plots near the source populations were similar in their age distribution to the source populations, but significantly different from shrubs sampled far from the source populations. Increasing distance from source populations was associated with a decrease in median age of the shrubs, and in the density and relative frequency of adult shrubs (>15 yr). These patterns of changes in age structure occurred over distances of several tenths of meters. Young age groups (<5 yr) were scarcely represented in all plots, suggesting that establishment limitation was also important in determining recolonization rates. Analysis of the aerial photographs supported the results of the age analysis, showing evidence for both distance effect and establishment limitation. We conclude that dispersal limitation may have a long-term impact on vegetation patterns, even at small spatial scales, and that dispersal limitation interacts with establishment limitation in determining recolonization processes.