Compared with the Sedge Fen, the vegetation of the Charles Raven Reserve has been poorly documented. The following are the main sources of information:
Vegetation of the mere - Dr.S.M.Walters (late 1950s)
Small areas of grassland - Dr H.J.Harvey (early 1970s)
Characeae in new boundary ditches - Mrs J.Moore
(B.M.N.H., late 1970s)
Vegetation of boundary ditches - Dr D.E.Coombe (1979-80)
'A Survey of the Flowering Plants on the National Trust
Property to the South of Wicken Fen (i.e. Adventurers' Fen)'
- A.D. Mills (Report in Fen Archives, 1981)
General notes on flora and vegetation - Dr.J.M.Lock (1984-85)
The summary that follows is taken from Dr Harvey's paper to the Committee (1981), with modifications, and on more recent observations by Dr.J.M. Lock. Dr Harvey's account was based on his casual observations over the years. The vegetation is mapped in Fig.. A new survey is in progress in 1992.
Grassland
Ai - (Brett's Piece). Agrostis stolonifera (Creeping Bent) and Festuca rubra (Red Fescue), with much rush (Juncus spp.) The latter has been controlled by cutting and heavy autumn grazing. Some parts liable to flooding.
Aii - Similar to the above but with more Juncus. Once again, some control has been achieved by cutting and late grazing. Often very wet in places.
Aiii - (Trevelyan's Piece). Again rather similar, with much Agrostis and Festuca. Formerly badly infested with ragwort and thistles but these have been well controlled by hand pulling. The south-eastern corner is a shallow scrape, usually bare of vegetation but with a fringe of rushes.
Aiv - (Rothschild's Lapwing). Lolium perenne (Ryegrass) and Agrostis stolonifera and Festuca rubra. Very few rushes, and not much ragwort or thistle. Probably the best grazing area.
Av - (Evans' Fen). Earlier infestation by thistle and ragwort has now largely been overcome but constant control needed.
Avi - The lode banks have a diverse flora, including a number of species characteristic of better-drained sites.
Swamp
Bi - This area has now been divided into two. The western two- thirds has been fenced off and reed allowed to take over. The eastern third is wet grassland with a lot of rushes. At the time of the 1981 account the whole area was wet grassland with much rush and some reed.
Bii - This area contains the four old brickpits. The vegetation is wet cow-trampled grass with much rush. The cow trampling is probably an important component of the environment as it maintains open habitats for the Lesser Water-plantain (Baldellia ranunculoides).
Reed
Ci - (Commercial reed-bed). Virtually pure reed (Phragmites communis), with Calamagrostis (Small-reed) and Phalaris (Reed- grass) in slightly better-drained sites. Some invasion by willow bushes, mainly on the eastern edge.
Cii - (Conservation reed-bed). Reed is abundant at the north- east end, but thins out towards Commissioners' Drain and is then largely replaced by Juncus and Phalaris.
Ciii - (Outer Mere fringe). Reed, with patches of grassy vegetation on higher patches. This has largely developed since the 1980 report as a result of the exclusion of grazing from these compartments.
Civ - (Mere fringe). Reed, with occasional willow bushes. Some grassy vegetation on high spots. Reed is most vigorous close to the open water.
Scrub
This occurs mainly at the foot of the bank of Wicken Lode, and consists of Crataegus (Hawthorn), Rosa (Wild Rose) and Salix (Willow). Overgrown hedges along Harrison's Drove and elsewhere contain the same species, together with some Frangula (Alder Buckthorn), Rhamnus (Purging Buckthorn) and Viburnum (Guelder Rose).
Woodland
The only compact areas of trees (and these are small) are the Alder plantation on Evans' Fen and the Willow and Hawthorn thicket on the Mere Island.The hedges of the Conservation Reed Bed contain some large willows. Some dense thickets of Elm that survived the epidemic of Dutch Elm Disease are now over 6m high.
Ditches
Most of the ditches are choked with vegetation, and the ditch round the Mere is now heavily shaded by reed. In some places there are large patches of Rumex hydrolapathum (Great Water Dock), and there are a few clumps of sedge (Cladium mariscus). No records are yet available for the grid of new ditches created in the Commercial reed field in 1989.
Open water
The Mere is very poor in aquatic macrophytes, probably because of grazing and defaecation by waterfowl. The old brickpits contain a few species, particularly those ponds which are not excessively shaded. The ponds dug in 1969 and 1971 seem poor in aquatics. No records are yet available for the Borrow Pit (1989).
General
Dr. Harvey commented on the richness of the aquatic vegetation in his paper. Since then many of the ditches have become overgrown and there may be a case for a roding and slubbing programme to be started, along the same lines as the programme implemented on the Sedge Fen (see Sections A2.7, A4.2.5, A6.7). The new ditch close to the junction of Monk's Lode and Wicken Lode is rich is species, as is Commissioners' Drain, which used regularly to be cleaned by Swaffham Internal Drainage Board (until 1989).