Secondary succession of Alnus viridis (Chaix) DC. in Vanoise National Park, France:: coexistence of sexual and vegetative strategies

In the western Alps, Alnus viridis expansion on subalpine grasslands brings major modifications in the functioning of ecological systems. The aim of this study was to assess which reproduction strategies were responsible for colonization and persistence of the shrub. Indices of vegetative and sexual reproduction were assessed in four 100 m(2) sites distinguished by the date of A. viridis settlement to determine the prevalent strategy as a function of age of the A. viridis stand. Results indicated that sexual allocation to reproduction was effective in all situations, whereas layering was absent in the site displaying the most recent A. viridis settlement. The number of cones per individual declined significantly from strictly sexual-related individuals to individuals displaying both reproduction strategies. (Mann-Whitney test: U = 132: P = 0.022). On these grounds we argue that A. viridis colonization process on secondary succession is supplied exclusively by sexual reproduction. In contrast, temporal persistence of dense stands is thought to require layering, which is also hypothesized to maintain a mechanism of inhibition towards arboreal coniferous species of late successional stages.