The replacement of Alnus glutinosa by Fraxinus excelsior during succession related to regenerative differences

Mechanisms of the replacement of one species by another during a primary succession were studied in the deciduous woodland of the outer archipelago of Stockholm, Sweden, where there are only two principal tree species. Alnus glutinosa (alder) was most abundant close to the sea shore, while the abundance of Fraxinus excelsior (ash) increased with increasing elevation above sea level, which due to the isostatic land upheaval corresponds to increasing age of the site. Regeneration of the two species, followed in permanent plots, was different. Alder did not regenerate from seeds in the woodland. No seedling survived 3 yr, even under conditions of thin canopy and removed field-layer, and most died within the first year. In contrast, survival of ash was 35% after 10 yr. Annual height increments of the ash juveniles were < 8 mm under a closed canopy , but growth of the juveniles increased rapidly when a gap was opened out by a tree-fall. Thus, ash, in contrast to alder, possesses characters advantageous for regeneration in gaps, which may explain the successive replacement of alder by ash in this woodland.