Species Richness of Herbaceous Fen Vegetation in Broadland, Norfolk in Relation to the Quantity of Above-Ground Plant Material

Abstract

Relationships between species density and biomass of various components of aboveground vegetation in 34 stands of herbaceous fen vegetation in Broadland, Norfolk, are examined. Regression models relating species density to the other variables were developed. Species density was negatively correlated with the amount of aboveground plant material, particularly with total crop in Sept., but with standing crop in Sept. and with the amount of litter, especially that at the start of the season. Regressions indicated that species density halved as Sept. total crop increased by 1850 g m-2. Sept. standing crop by 1200 g m-2 and April litter by 1630 g m-2. High values of total crop biomass were always associated with species poverty. These high values were produced by the development of a large standing crop, or the accumulation of litter, or both. A large standing crop may be accumulated by a productive vegetation or, to some extent, by plants of lower productivity with long-lived leaves (e.g., Cladium mariscus, the litter of which is persistent). Overall, there was an increase in species density with decrease in mass of crop and of litter but an increase in variability so that species-poor and species-rich vegetation with small total crop occurred. Vegetation very rich in species occurred only when the aboveground Sept. total crop was less than .apprx. 1500 g m-2. Management of sedge beds (summer mowing) is necessary for high species density. Managed reedbeds may have a slightly lower species density than unmanaged ones. Species density in reedbeds was always lower than that in managed sedge beds. Possible factors controlling crop and litter amounts and possible mechanisms regulating species density are discussed.