Variation of reproductive traits in Rhododendron ferrugineum L. (Ericaceae) populations along a successional gradient

Sexual and vegetative reproduction in the alpine species Rhododendron ferrugineum was studied along a successional sequence (meadow → open heathland → closed heathland) at two sites and in a wet heathland. This study aims to determine (1) the characteristics of sexual reproduction in R. ferrugineum populations (2) when and how these populations develop layering (adventitious rooting) and (3) whether reproductive traits and reproductive strategies develop relative to the degree of population closure and maturity. The variables used to describe sexual reproduction were inflorescence density (per m(2) of Rhododendron cover), number of flowers per inflorescence and per m(2) of Rhododendron cover, and seeds production (per fruit and m(2) of Rhododendron cover). Flowering and fruiting phenologies were also recorded. For describing clonal development, we investigated layering variables such as length and annual growth rate of prostrate stems, rooting occurence and ramet density. The results show that the direction toward which the clones extend is mainly determinated by the topography, and that layering steadily increases with increasing population closure and maturity. Reproductive potential of R. ferrugineum is enormous (0.4–2.4 million seeds m(-2)) but reproductive effort remains low with respect to total biomass of seeds (3–21 g m(-2)). Reproductive effort of R. ferrugineum populations could be reduced as a conterpart of layering development only when the shrub draws more matter and energy in layering stems than to aerial stems. The variations in reproductive traits observed on our sites could be due to primarily to phenotypic response to variable microhabitat features, rather than to genetically deterministic processes.