Appropriate Nutrient Economy in Phragmites australis at Different Phases of Estuarine Succession

Abstract

The common reed Phragmites australis (Cav.) Trin ex Steudel – the dominant macrophytic plant species in the Kokemäenjoki River delta, western Finland – showed distinct and appropriate trends in the nutrient economy according to the previously determined successional phases of the vegetation development. The height and weight of individual aboveground shoots (ramets) decreased in the order: Pioneer stage > Mature stage > Regressing stage. The concentrations of the major nutrients nitrogen and phosphorus were determined only partly by amounts of these nutrients in the plants’ rhizospheres. The estuary is eutrophic, and the river water guarantees a continuous supply of nutrients, and thus the levels of nitrogen and phosphorus are high throughout the study area. The levels of nutrients correlated significantly with the contents of organic matter in the rhizospheres of the reed stands. On the basis of the relationships between the major nutrients’ concentrations in the leaf blades, nitrogen appeared to be the growth-limiting nutrients in the pioneer stage of succession in these estuarine habitats. The N/P-ratios in the three stages were as follows: Pioneer stage: 10.75, Mature stage: 13.59, Regressing stage: 14.67. In general, the values below 15 are considered to be nitrogen-limited. The actual levels of N and P in the leaf tissues were, however, high throughout the study. As evaluated by the concept of critical concentration, i.e. the level of a nutrient, which guarantees maximal growth potential, the habitats at all the three successional phases showed adequate level of nutrient availability for the maximal production of Phragmites. At the pioneer stage, where the rhizosphere resources of nutrients were poorest, the common reed showed an appropriate morphological adaptation. In the pioneering stands, the common reed produces considerable amounts of adventitious roots (water roots) on the underwater sections of the culms. In the pioneer stage, the average weight of the water roots was 675 mg/ramet in the fertile (flowering) shoots and 267 mg/ramet in the sterile (non-flowering) shoots, i.e. 2.6–5% of the total weight of the aboveground shoots. In the mature stage, the amount of adventitious roots was minor, but in the regressing stage – where growth enhancement and substantial flowering is essential for the future of the species – the reeds produced more adventitious roots again.