Strategies in Growth of the Common Reed (Phragmites australis) as Related to Successional Stages in a Rapidly Varying Estuary
The common reed Phragmites australis (Cav.) Trin ex Steudel (= P. communis Trin.) is the dominant component in the Kokemäenjoki River delta, western Finland, an estuary where the changes in the vegetation are more rapid than in any other ecosystem in Northern Europe. Three phases in the successional development of Phragmites were characterized on the basis of decades-long follow-up. Of the biometric characteristics of the reed stands, the height, weight and density of individual shoots (ramets) were measured. In all the parameters studied, marked differences were seen between the successional stages. The growth and production of Phragmites australis is exceptionally high in the studied Kokemäenjoki River delta, due to the continuous supply of nutrients, and the suitable grounds in the estuary. The height and weight of the ramets were statistically significantly greater at the pioneer stage of succession than in the final, regressing phase. The average height of the shoot was 266.5 cm in the pioneer stage, 278.4 cm during the mature phase and 219.8 cm in the regressing phase. The height and weight values correlated significantly in all three successional phases. At the pioneer stage, however, the ramets were markedly more robust than in the other two phases. In the density of the monocultural stands, a clear self-thinning trend was seen in all the successional phases, i.e. the number of ramets per unit area is linearly reduced during the growing season. The average density of ramets was 151.9 individuals/m2 at the pioneer stage, 176.9 ind./m2 in the mature stage, and 147.1 ind./m2 in the regressing stage. As a conclusion, the variations between the successional phases in a rather uniform river delta were so notable that a detailed description and characterization of the sample sites is necessary in any analysis of wetland macrophytic vegetation.