Spatio-temporal patterns of colonization and expansion of Retama monosperma on developing coastal dunes

Sand stabilization by human plantation is generally considered as one of the most influential factors in biodiversity loss of coastal dune. Frequently, the environmental impact occurs beyond the initial plantation areas due to some planted species are capable to expand to the surrounding dune areas due to sometimes these species exhibiting invasive behavior. Assessing how the spatial and temporal colonization and spread of a shrub species takes place is an arduous task as the initial establishment is influenced by several factors. An understanding of this could be of great importance to the accurate prediction of the course of invasions and establish management plans. In this study, we quantify and analyze the spatio-temporal expansion patterns of the shrub Retama monosperma in a coastal dune landscape. In conjunction with measurements of plant spatial distribution, maturity and dimensions, we use an age-analysis approach to analyze the age structure of the colonizing population and the characteristics of the spread of the R. monosperma population in the early end of a sand spit formed over the last 33 years. The age structure of the dune population suggests that recruitment has increased continuously over time as the population has increased. We found significant différences between age and diameter size structures of dune ridge populations due to their different colonization stages and, in particular, to the environmental conditions. The presence of long-distance dispersers is essential for the spread of R. monosperma in this fragmented landscape. Population expansion is achieved by means of a coupled pattern of radiating wave fronts and the continuous establishment of multiple "nascent foci" on ridges, which then expand and coalesce. R. monosperma displays lag-phase behavior in the early stages of colonization.