Age structure of woody species populations in relation to seed rain, germination and establishment along the river Dalälven, Sweden

Abstract

Three methodological approaches were combined to assess the population dynamics of woody species, particularly birch, growing along the shores of the Dalälven river in eastern central Sweden. 1) Seed rain, seed bank and seedling densities were estimated to document the current recruitment potential. 2) Seed burial experiments were performed and artificial gaps in the field layer created to obtain additional information on germination and establishment processes. 3) Age distributions were used for retrospective interpretation of woody species successions.

The zonation of the vegetation along transects from wet meadow into forest cannot be interpreted as a chronosequence. Storm gap formation, water level regulation and cessation of hay-making in the past were followed by episodic colonisation and regeneration of several woody species.

Although birch seed rain is most abundant close to parent trees and the half-life of the birch seed bank increases with decreasing submersion, seedlings occur abundantly everywhere along the transects. Seedling mortality, however, is 100% and the current environmental conditions in the wet meadows and in the forest do not allow any establishment. Long-lasting submersion, shading by trees and interference by species from the field layer hampered or prevented birch establishment.