The Distribution and Status of Glyceria Maxima (Hartm.) Holmb. In the Region of Surlingham and Rockland Broads, Norfolk

Abstract

Glyceria forms an integral part of the primary succession from all sheltered open water areas under the influence of moving tidal waters and is the main peat-forming plant in early stages of succession. The primary community consists of a floating reed swamp which merges gradually into a semi-floating fen with increased peat deposition beneath the mat. Where tidal scour is appreciable, the outer edge of the floating mat is replaced by a narrow fringe of anchored Phragmites reedswamp. Glyceria is followed in the primary succession by Phragmites encroaching from behind by vegetative means. This replacement is due to physiol. rather than mechanical factors. Glyceria is absent from landlocked pools within the peat basin, being replaced entirely by Phragmites. The Glyceria community of the primary tidal regions is an interpolated phase, occupying part of a zone which in areas with no tidal movement would be dominated by Phragmites. Glyceria also is a prominent constituent of secondary anthropogenic fen areas, where fen has been maintained in parts which would normally be occupied by carr. Its dominance is favored by regular summer cutting and by a system of drainage which permits free circulation of tidal water between the peat strips. Glyceria is the main agent in the overgrowth of drainage dykes consequent upon neglect. Its relation to Phragmites in successive stages is the same as in primary succession. The limitation of Glyceria to areas under the influence of moving tidal water is more probably related to mineral salt supply than to possible aeration of the substrate. Both primary and secondary Glyceria communities contain relatively few subsidiary spp. Early mechanical prostration of long Glyceria shoots produces a mattress which seems to be effective in preventing invasion. The same factor seems to be responsible for the fact that an invasion by Phragmites is apparently always a prelude to the establishment of carr.