Secondary succession in a fallow central European wet grassland

摘要

This long-term study (23 yr) aims at specifying the characteristic features of secondary progressive successions in abandoned wet eutrophic grasslands as a precondition for better understanding causal relationships and improving the predictability of successions on such habitats The vegetation of inundated fen grasslands (Calthion) at the lower course of the river Wumme near Bremen (Northwest Germany) was studied annually on three permanent plots (each 10 x 10 m(2)) where hay making had ceased in 1983, 1985 and 1992, respectively In the initial succession phase of three to five years a fast increase of tall-growing, rhizomatous, deciduous reed species (e g Phalaris arundinacea) occurred This strongly changed vegetation structure and within-canopy light climate, which was the reason for the strong decrease in species diversity Rhizomes are hypothesized to be a key factor of high competitiveness in abandoned wet grasslands because this organ combines multiple advantageous functions low-risk vegetative propagation, nutrient storage and nutrient re-allocation between above- and below-ground plant organs, all these functions allowing for a gradual build-up of a high biomass Extinct grassland species (e g Senecio aquaticus) represent a contrasting set of plant traits such as small stature, short lifespan, prevailing generative reproduction and evergreen leaves. The subsequent succession phase commencing about five years after mowing was ceased was characterized by persistence of the established reed vegetation, which prevented tree colonisation until today. Secondary successions in abandoned wet eutrophic grasslands can be predicted on the level of plant communities and functional species groups considering changes in the habitat (esp. light climate), the presence of reed species with a specific set of plant traits in the "Initial Floristic Composition" and their potentially high competitive vigour.