AGE STRUCTURE AND DYNAMICS OF RHODODENDRON-FERRUGINEUM L POPULATIONS IN THE NORTHWESTERN FRENCH ALPS

The ericaceous shrub Rhododendron ferrugineum occupies large areas at the subalpine level (1600-2200 m) of the northwestern French Alps. It frequently reaches 90-100% cover, accumulates 60-70t/ha (dry weight) a bove-ground biomass and is able to dominate many subalpine landscapes by outcompeting other species. We formulated the hypothesis that the s pecies shows a rapid and invasive seedling establishment and a fast po pulation development. We investigated the age structure and developmen t of two populations. Although classic colonization stages can be iden tified, and variation did occur depending on the site, R. ferrugineum populations showed a surprisingly slow development compared with other ericaceous species. Curves obtained from polynomial regression equati ons of cover on age allowed us to predict the future development of po pulations. Thus, depending on the site, it would take 150 to 250 yr to achieve total cover. We discuss the factors involved that could expla in this slow development and discuss which biotic and abiotic factors may be responsible for the large extension of this species in the suba lpine belt.