Species richness of herbaceous fen vegetation in Broadland, Norfolk in relation to the quantity of above-ground plant material

(1) Relationships between species density and biomass of various components of above-ground vegetation in thirty-four stands of herbaceous fen vegetation in Broadland, Norfolk, are examined. Regression models relating species density to the other variables are developed. (2) Species density was negatively correlated with the amount of above-ground plant material, particularly with total crop in September, but also with standing crop in September and with the amount of litter, especially that at the start of the season. Regressions indicated that species density halved as September total crop increased by 1850 g m-2, September standing crop by 1200 gm-2and April litter by 1630 g m-2. (3) 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 also persistent). (4) Overall, there was an increase in species density with decrease in mass of crop and of litter but also an increase in variability so that both species-poor and species-rich vegetation with small total crop occurred. Vegetation very rich in species was found only when the above-ground September total crop was less than about 1500 g m-2. (5) 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. (6) Possible factors controlling crop and litter amounts and possible mechanisms regulating species density are discussed.