TRANSACTIONS of the

NORFOLK & NORWICH NATURALISTS' SOCIETY

Volume 47 Part 1 2014 &

Volume 48 Part 1 2015 combined

Natural History Museum Library

TRANSACTIONS OF

ooo2266oa_ . . _ { NATURALISTS' SOCIETY

Volume47 Parti 2014 & Volume 48 Parti 2015 (published April 2016)

Editor: AR Leech Assistant Editors: S Harrap & NW Owens

Published by the Norfolk & Norwich Naturalists' Society www.nnns.org.uk

Contributions for Volume 49 (2016) should be sent to the Editor, Nick Owens, 22 Springfield Close, Weybourne, Holt, Norfolk NR25 7TB owensnw7@gmail.com, from whom notes for contributors can be obtained.

The Norfolk & Norwich Naturalists'Society has as a principal aim the investigation and recording of Norfolk's wildlife and to this end it publishes:

• An annual volume of Transactions, consisting of papers and notes on wildlife in the county.

• The Norfolk Bird and Mammal Report which contains systematic lists of observations on the county's birds and mammals, as well as relevant articles.

• The Norfolk Natterjack, a quarterly illustrated newsletter.

All of these publications are free to members, as are Occasional Publications on specific topics.

The Society also arranges lectures and field meetings which are planned to appeal to anyone interested in natural history. More specialist groups cover many aspects of the county's flora and fauna.

The subscription rate is £20 per year, which includes all members of a family living at the same address.

Membership enquiries should be made to: David Richmond, 42 Richmond Rise, Reepham, Norfolk, NR10 4LS, All other enquiries should be directed to the Secretary, Nick Owens, 22 Springfield Close, Weybourne, Holt, Norfolk NR25 7TB. Tel: 01 263 58841 0.

The Society gratefully acknowledges the support of the Sarnia Trust in the

production of this publication.

ISSN 0375 7226

© Norfolk & Norwich Naturalists' Society 20 1 4 Charity No. 29 1 604 No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electrical, mechanical, photocopying, recording or otherwise, without permission, except in accordance with the provisions of the Copyright Design and Patents Act 1988. Applications for permission should be addressed to the Secretary (see address see website: www.nnns.org.uk).

Printed by Barnwell Print Ltd, Aylsham, Norfolk NR1 1 6SU.Tel: 01263 732767

Vegetation development in the Broadland fens

Jo Parmenter

Introduction

The Broadland fens form part of a complex and extensive mosaic of waterways, shallow lakes, grazing marsh, woodland and arable land occupying approximately 300 km2 (George 1992). This area of land lies in the valleys of three major rivers; the Bure, Yare and Waveney, and their main tributaries, the Ant, Thurne, and Muckfleet. See Fig. 1 for map of Broadland fens.

Over two-thirds of the Broadland landscape is managed as agricultural land; either intensively managed pasture (grazing marsh) or' high quality arable land. Much of the remainder is seminatural vegetation or open water. The Broads National Character Area Profile (http://publications.naturalengland. org.uk/publication/1 1549064) records that over 7,000 ha of land is nationally designated as Sites of Special Scientific Interest (SSSI), with international designations including three Special Protection Areas (SPA), two Special Areas of Conservation (SAC) and two Ramsar sites. Key habitats include grazing marsh (c. 12,300 ha) and fen and reedbed (c. 10,400 ha), as well as the most extensive tract of wet woodland within eastern England. A number of iconic species are present in the Broads, including 95% of the UK population of Fen Orchid Liparis loeselii var. loeselii1 (Mason 2014), and the entire UK population of the Swallowtail butterfly Papilio machaon Both of these species are restricted to fen habitat.

Principal environmental and anthropogenic influences upon fen vegetation

Five principal uses have historically been made of the Broadland fens. These were peat cutting, reed cutting, sedge cutting,

1 See Appendix for English names of plants

referred to in the text.

>

rr

L» j

X 1

< - LU

P LD

csj

CO

<

litter/f odder mowing, and grazing: “On the

basis of the information available, turbary2 would seem to be the most economically important and widespread historic land use, followed by the cutting of various types of vegetation (litter, reed, sedge etc.), and lastly, grazing. Turbary was particularly prominent in the medieval period, producing the deep, water-filled basins which dominate the landscape to the present day, but peat cutting continued until the early 20th century. During the medieval and post-medieval periods the demand for litter and fodder is likely to have risen with the increasing population. By the late 18th century, it was possibly as important as peat cutting to the local economy and probably greatly exceeded the importance of reed and sedge cutting.

Peat has been cut for fuel in Broadland for over 1000 years. The increased use of peat as a fuel source from the late Saxon period onwards led to the creation of extensive turbaries in Broadland. A large proportion of the floodplain, and possibly over 75% of the land area in the middle and upper floodplains, has been cut for peat. Many of the fen communities which are found growing over the former turbaries are relatively recent in origin and the great majority of the colonisation of open water by fen vegetation has taken place over the last 175 years. The apparently sudden proliferation of fen communities over the medieval turbaries was in most cases in response to the accumulation of lake mud and sediment to the point when the basin became sufficiently shallow to enable colonisation by swamp species (Lambert 1965).

The shallowest of the former medieval broad basins, at Dilham and Sutton, have already succeeded to fen. Substantial areas of open

Trans. Norfolk Norwich Nat. Soc. 2014 47(1)

1

kilometres

Figure 1 . Location of fen sites within The Broads area. Key opposite.

2

Trans. Norfolk Norwich Nat. Soc. 2014 47(1)

The River Ant Fens

1 East Ruston Common, Honing Common & Dilham Broad

2 South Fen & The Holmes

3 Broad Fen & Smallburgh Fen

4 Barton Fen, Common Fen & Stalham Fens

5 Sutton Fen, Sutton Broad & Hand Marsh

6 Barton Broad Fens & The Heater

7 Catfield Fen

8 Reedham Marshes, Snipe Marsh, Hall Fen & Alderfen

The River Thurne Fens

9 Horsey Mere, Brayden Marshes & Long Gore Marsh

10 Skoyles Marsh, 100 Acre Marsh, Whiteslea & Meadow Dyke (North)

11 Hickling Broad Marshes & Mrs Myhills Marsh

12 Heigham Sound, Meadow Dyke (South)

& Sound Marshes

13 Mere Farm Marshes, Starch Grass & Martham Broad Marshes

14 Potter Heigham Fen & Martham Marshes

15 Womack Water Marshes & Shallam Dyke The Muckfleet Fens

16 Ormesby Common, Ormesby Broad Marshes & Hemsby Common

17 Rollesby Broad Marshes

18 Burgh Common & Filby Broad Marshes

The River Bure Fens

19 Coltishall Marsh & Blackmans Fen

20 Crostwick Marsh & Dobbs Beck Marshes

21 Belaugh, Wroxham & Jubys Farm Marshes

22 Snapes Water, Hoveton Marsh, Sedge Fen & Salhouse Marshes

23 Woodbastwick Marshes & Hoveton Little Broad & The Lows

24 Ranworth Broad Marshes & Horning Church Marshes

25 Horning Hall Marshes & Hulver Ground

26 Ranworth Marshes & Ward Marsh

27 Ranworth Flood, Leists Marsh & South Walsham Fen

28 Upton Fen & The Doles

29 AcleCarrs & FishleyCarrs

30 Decoy CarrAcle

31 Halvergate Marshes

32 Wickhampton Marshes & The Fleet Marshes

The River Yare Fens & Ronds

33 Whitlingham Marshes & Thorpe Marshes

34 Surlingham Church Marshes & Kirby Marshes

35 Surlingham Broad Marshes, Bargate & The Outmeadows

36 Bradeston & Strumpshaw Marshes & Strumpshaw Common

37 Surlingham Marsh, Wheatfen & Rockland Marshes

38 Ducans Marsh & Carleton Broad Marshes

39 Buckenham & Hassingham Carrs & Buckenham Ronds

40 Poplar Farm Marshes, Limpenhoe Marshes & Cantley Ronds

41 Hardley Flood, Chedgrave Common & Loddon Common

42 Thurlton Beck Marshes

43 Reedham Ronds

The River Waveney Fens & Ronds

44 Glebe Marshes & Burgh Castle Ronds

45 Belton Ronds

46 Belton Bog & Bradwell Doles

47 Fritton Ronds

48 Fritton Decoy & Ashby Warren

49 Herringfleet Ronds

50 Wheatacre Marshes

51 Flixton Decoy, Wicker Well & Summerhouse Water

52 Somerleyton & Blundeston Ronds

53 Burgh St Peter & Barnby Ronds

54 Barnby Broad & Marshes, North Cove & Wadehall Marshes

55 Oulton Marshes, White Cast Marshes & Sprats Water

56 Beccles Marshes, Wild Carr & Stanley & Alder Carrs

57 Roos Hall Marshes, Barsham Marshes & Geldeston Marshes

Trans. Norfolk Norwich Nat. Soc. 2014 47(1)

3

water at Hickling Broad, Ranworth Broad and Heigham Sound have also become grown over. The deeper basins, such as those at Trinity Broads and Fritton Decoy, are still relatively open. The colonisation of the Yare valley broads by swamp and subsequently fen vegetation has progressed much faster than elsewhere in Broadland, apparently because of the ability of Reed Sweet-grass Glyceria maxima to colonise quite deep water without the necessity of being rooted in the underlying substrate (Lambert 1965).

Before enclosure, much of the upper valley floodplain was held in common. Common fenland would have been used to supply a variety of commodities, including reed, sedge, litter and peat. The most essential types of fen produce were peat for fuel and fodder for feeding domestic animals, however the management regime had to take into account the differing requirements of a large number of people and common land thus often had to supply a wide range of commodities, giving rise to a mosaic of differing management regimes. Doling2 enabled individuals to exploit common fenland as they chose. In this respect, the effect of doling on fen management was not dissimilar to the effect of Parliamentary Enclosure.

Parliamentary Enclosure led to a complete restructuring of the landscape of the Broadland fens. Vast tracts of common wetland and heathland were enclosed and drained or otherwise improved. Much of the Broadland floodplain fen was enclosed into private ownership, with small areas designated as poor allotment. The common fenland was divided up into rectilinear parcels using dykes to denote ownership boundaries and to improve drainage. The resulting landscape was more ordered in appearance than that resulting from the earlier enclosures.

Private ownership gave increased control by individuals and enabled those

2 Doling was the allocation of strips of common land to a specific person.

individuals to make choices about the way in which they would manage their land. Enclosure increased exploitation of fenland for mutually exclusive purposes, such as turbary or litter cutting. It also encouraged a much more organised pattern of management and specialisation of land use towards particular marketable fen products such as reed and sedge, rather than continuing to manage one area of land for a number of different purposes. The decision whether to cut peat or whether to harvest fen crops was probably largely an economic one, although the existing vegetation would have to some extent determined the way in which the fen was managed.

Some areas of fen, particularly those which had undergone substantial terrestrialisation or those on solid peat, have been grazed in the past, usually in the drier summer months. It is also likely that animals were encouraged to graze in the areas which were drained to facilitate exploitation of the peat resource, while these areas were not actually being cut (Parmenter 2000). Grazing of fen vegetation results in the suppression of the taller or most palatable species, whilst low growing species and coarse rushes and sedges are favoured. These areas today are sometimes indicated by an increased frequency of rushes and often the presence of the sedges Carex disticha and C. panicea. Filipendula ulmaria and Calamagrostis canescens also tend to occur at a higher frequency in these areas. Other species, such as Phragmites australis , Carex paniculata , Cladium mariscus and Schoenus nigricans tend to be suppressed or eradicated by long term grazing management and may be reduced in frequency or absent from formerly grazed fen areas.

Superimposed upon the mosaic of concentric bands of successional communities which often occur between the open water and upland margins is a further pattern which is dictated by the presence or absence of 19th

4

Trans. Norfolk Norwich Nat. Soc. 2014 47(1)

century shallow peat cuttings. These have, without exception, virtually completely terrestrialised and the fen surface is usually quite firm, but they support vegetation which is markedly different from that growing over either the former broad basins or on solid peat. Cladium mariscus is often abundant in these situations. Several National Vegetation Classification (NVC) types (Rod well 1995), including S24a and S24e vegetation are almost exclusively found over peat cuttings.

Other former management practices have also altered the fen vegetation to a greater or lesser extent. Traditionally, the open fen areas would have been managed for reed, sedge and marsh hay, with carr and scrub being cut for firewood. Drier areas would have been grazed by livestock. A use seems to have been found for most natural products of the fens and adjoining higher ground and during the 19th century strict bylaws often governed the management and use made of the marsh resources. Bird (1909) wrote of East Ruston Common " Not only did the rushes which grew there once form the wicks for the only candles then made , hut they afforded also the primitive carpets of the clay-floored cottages... The Furze hushes provided kindling for the peat fires , as well as material for effectually walling in the cattle sheds; and the upland turf supplied the place of the present day slates and tiles for roofing purposes ..."

The decline in the management of areas of fen for crops such as reed and sedge and marsh hay was associated with a move to tile and slate for roofing, the gradual replacement of horses by motor vehicles and the wide availability of straw for animal bedding, coupled with labour shortages in the post war periods. Some areas of fen are still cut for conservation purposes, but the decline in traditional fen management is largely responsible for the present extensive areas of scrub and carr in Broadland.

Many of the abandoned managed reedbeds still support strong populations

of Phragmites australis , and relict sedge beds usually remain dominated by -Cladium mariscus because of the ability of these species to resist invasion by woody vegetation to some extent, and to maintain species impoverished, sometimes pure stands. Examples of sedge beds of this type can be found at Brayden Marshes, where, despite an absence of commercial management over several decades, the former commercial sedge beds are still free from scrub and are often extremely species poor. Summer mowing for litter tended to encourage rushes and to suppress Phragmites australis and Cladium mariscus. Unlike the former Cladium beds, however, these litter fen sites have tended to succeed quite rapidly to carr. Juncus spp. dominated vegetation, both because of its structure, and the fact that it tends to grow in relatively drier situations, does not have the ability to resist scrub invasion to the same degree as Cladium mariscus.

The proportion of drained marshland relative to fen and carr is greatest in the lower reaches of the Yare and Waveney valleys. Historically, land drainage has been more successful in the areas which have extensive deposits of estuarine clay, which is not subject to the shrinking and wastage which usually occurs when peat fen is drained (Ellis 1965). In addition, the clay layer is often rich in minerals and therefore tends to produce better pasture.

In the upper and middle reaches of the river valley, the clay layer is absent or incomplete. Past attempts to drain fen sites upstream have generally ultimately failed due to shrinkage of the peat and the consequent progressive difficulty in draining the land. A proportion of this previously drained land has reverted to fen, and the vegetation is often of a quite different structure and floristic composition to that of the surrounding undrained fen. Reflooded former grazing marsh sites include Bargate and Surlingham Church Marshes on the Yare, Sedge Marsh on the Bure, and Stanley

Trans. Norfolk Norwich Nat. Soc. 2014 47(1)

5

and Alder Carrs on the Waveney. The extensive reedbeds at Ranworth Flood are the result of deliberately flooding an area of wet grazing land for reed culture.

The extent to which the fens are influenced by the chemistry of the river water which irrigates them varies from site to site and from valley to valley (George 1992). In general, sites closest to the main rivers, particularly to the Rivers Yare and Waveney are more nutrient rich and tend to be more alkaline than those closest to the upland margins. Sites which have been formerly drained for agricultural use are often more fertile than those which have been managed as fen, as a result of oxidation of the peat and consequent nutrient release.

The Broadland river system is strongly influenced by tidal fluctuation, both because of the proximity of the sea, and also the very gentle valley gradient, which averages just over 3 cm per kilometre (George 1992). The tidal influence extends for a considerable distance upstream, and even at Norwich, some 25 miles from the sea, the daily fluctuation is commonly between 30 and 60 cm. The tidal effect is much stronger in the Rivers Yare and Waveney than in the corresponding reaches of the Bure and its tributaries.

The influx of saline water into the river system is to some extent limited by fresh water flowing downstream, although in the areas closest to the sea, the salinity is sufficiently high to permit the growth of halophytes. Brackish water tolerant species such as Samolus valerandi, Schoenoplectus tabernaemontanii and Oenanthe lachenalii are found for a considerable distance upstream, although at some sites, their presence is due to salt water influence derived from non-fluvial sources, such as the presence of an underlying layer of saline estuarine (Breydon Formation) clay. The potential effects of nutrient enrichment are greatest in the case of fen vegetation which is contiguous with the river, or where an open dyke network allows nutrient rich river

water to percolate deep into the fen. This latter point is an important consideration in fen ditch management: whilst any adverse effects of nutrient enrichment as a result of increased water penetration from relatively nutrient poor rivers such as the Ant and Thurne into the fens are probably outweighed by the benefits of improved access and the creation of open water habitat in the fen, in the more eutrophic systems, such as the Yare, improved water circulation may result in the proliferation of rank vegetation in previously nutrient impoverished fen areas. In strongly tidal systems such as the Yare and Waveney, water circulation is much higher than in the non-tidal upper Ant. The installation of phosphate stripping equipment at wastewater treatment works in the upper valleys has substantially reduced the availability of this nutrient, however there is still some diffuse phosphate input, and nitrate levels in some rivers remain elevated.

The varying influence of the factors described above on fen vegetation has given rise to a very high degree of heterogeneity in the fen vegetation of The Broads, with each valley system exhibiting unique characteristics (Table 1).

The Ant Valley

The fens of the Ant valley floodplain have long been recognised as being of outstanding importance in Broadland (Wheeler 1978). They support the greatest diversity of fen and mire community types and are also notable for the high number of both nationally rare and scarce species which are found here, including some which are found nowhere else in the Broads.

The main Ant Valley fen sites include Broad Fen, Smallburgh Fen, Barton Fen, Sutton Fen, Sutton Broad, Catfield Fen, Reedham Marshes, Hulver Ground and Alderfen. Together, these sites represent one of the best remaining examples of lowland wet fen habitat, both in Great Britain and in

6

Trans. Norfolk Norwich Nat. Soc. 2014 47(1)

Table 1 . Distribution of open fen habitat types in the Broadland valleys (OHES 201 3).

	Total%	ANT%	BUR%	THU%	WAV%	YAR%
Mires and fen meadows	8.3	3.5	11.9	5.3	9.4	14.1
Mesotrophic tail-herb fens	40.7	63.1	60.7	32.4	6.1	18.4
Mesotrophic swamps	7.2	9.6	1.7	17.6	0.0	0.6
Total of 'high-value' fen habitats	56.2	76.2	74.3	55.3	15.5	33.1
Meso-eutrophic reedbeds and fens	13.7	13.6	12.9	16.1	13.2	14.9
Species-poor reedbeds	8.9	5.1	3.4	16.8	11.3	6.0
Eutrophic fens	13.5	3.1	8.4	5.7	18.1	37.8
Brackishfens	4.4	0.1	0.9	3.6	23.8	5.2
Saltmarsh	1.2	0	0	0.2	10.3	1.7
'Contact Communities'	2.1	1.8	0	2.2	8.0	1.4

western Europe (Barendregt et al. in prep.)

Several sites stand out as being of particular interest and value. One of these is Sutton Broad, where the fen vegetation demonstrates the hydroserai succession from open water to upland, through swamp, reedbed, tall herb fen, and 'small sedge' fen to marginal fen meadow and carr. Other sites of particular importance are Catfield Fen, which supports one of the three remaining populations of Liparis loeselii in addition to an abundance of Dryopteris crist ata, and Smallburgh Fen, which is of interest for its calcareous mire communities.

«

Reedham Marshes and Broad Fen are also of very great interest. On the valley scale, the Ant is probably the best example of a diverse floodplain fen in Broadland and almost all of its constituent sites are of exceptional quality, both in terms of species composition and community structure and distribution.

The course of the River Ant has been greatly altered during the medieval and post medieval periods, and formerly discharged into the River Thurne at Cold Harbour, rather than directly into the Bure. The course of the River Ant further upstream has also been modified by four

major diversions to facilitate navigation or to improve drainage of the catchment. The original early medieval course of the river is marked today by minor channels which follow the parish boundaries.

Much of the floodplain fen in the Ant valley is irrigated by the river, although a few sites in the northern part of the valley system, including Smallburgh Fen, are fed mainly by calcareous groundwater. In the lower part of the valley, south of Catfield, Breydon Formation clays overlie the Crag and these have an influence upon the vegetation, being both saline and rich in minerals (Pyne 2014).

The water quality in the Ant is generally good in comparison with some of the other main Broadland rivers. In the past, the nutrient discharge into the River Ant was much greater, but the effluent from the waste water treatment works upstream is now phosphate-stripped before discharge. Nitrate levels are comparable to those of the Bure. Nutrient levels generally are considerably lower than those found in the southern rivers.

The Ant valley is notable for a mosaic of closely juxtaposed vegetation community types, a large proportion of which are high quality fen, supporting a diversity

Trans. Norfolk Norwich Nat. Soc. 2014 47(1)

7

of uncommon species. The distribution of the fen communities is linked to a number of .factors, such as past and present management, past land use and the spatial position of the vegetation stands within the river valley, which dictates nutrient and salinity levels.

The largest remaining open fen areas in the Ant valley are at Catfield Fen, Sutton Fen, Barton Fen and Reedham Marshes. The majority of the fens in this valley are dominated by varying proportions of Cladium mariscus, Phragmites australis and Juncus subnodulosus. These fens support populations of almost all of the species which are considered to be nationally rare or scarce fen species: Thelypteris palustris , Thyselium (Peucedanum) palustre, Sium latifolium and Dryopteris cristata are particularly abundant, and Carex appropinquata and Cicuta virosa are also quite common. Several locally scarce fen species, however, for example Thalictrum flavum , are not found in the Ant fens, and the nationally scarce Lathyrus palustris and Sonchus palustris are only present at the extreme southern end of the system. Liparis loeselii has two out of its three Broadland stations in the Ant valley.

It is possible to distinguish two main groups of fen and mire communities within the Ant Valley; those of the floodplain, which derive a large proportion of their irrigating water from fluvial sources, and the valley mires, which are irrigated by water from springs and marginal seepage zones. Most of the Ant valley fens are of the former type; East Ruston Common, Broad Fen and Smallburgh Fen, however, are of the valley mire type. In addition, the marginal zones of many of the Ant floodplain fens close to the upland receive the majority of their irrigating water from seepage; the River Ant is only weakly tidal and water circulation at a distance from the river is minimal, particularly where the dykes to the river are choked with vegetation or in cases where an internal fen system exists, as at

Catfield. The vegetation type of these areas is dictated by the quality of the irrigating water, derived from the groundwater, rather than from a base rich fluvial supply. Calcicole communities are present in some locations, typically dominated by Molinia caerulea, often with extensive patches of Sphagnum spp. Erica tetralix also occurs in this habitat; other species found here include Calluna vulgaris, Cirsium dissectum , Succisa pratensis and Potentilla erecta. Wetter areas may support communities dominated by the locally scarce Carex rostrata, in a form of the M2 7 community. In some cases, where the irrigating water is most acidic, species assemblages most closely resembling M5 or M6 mire (Rodwell 1988) can be found.

The floodplain fens closer to the river are dominated by Phragmites australis and Cladium mariscus. Juncus subnodulosus is a common associate, particularly towards the margins of the fen areas on solid peat, where grazing and mowing for litter would have encouraged the spread of this species. Calamagrostis canescens is also very frequent in these areas on slightly drier ground. The Ant valley tall herb fen is notable for its wide range of associated forbs, including Lysimachia vulgaris, Lythrum salicaria, Angelica sylvestris, Thyselium palustre, Ranunculus lingua, Rumex hydrolapathum, Valeriana officinalis and Iris pseudacorus. Comarum palustre (Potentilla palustris), Galium palustre, Mentha aquatica, Thelypteris palustris, Hydrocotyle vulgaris and Scutellaria galericulata. Salix repens, Schoenus nigricans and Carex elata are often abundant in Cladium dominated vegetation where the substrate is of low nutrient status. In the unmanaged fen, scrub, particularly Salix cinerea, Myrica gale and at some sites, Betida pubescens, are very quick to colonise.

In parts of the valley, notably at Reedham Marshes and Catfield fen numerous oligotrophic nuclei have developed within the mesotrophic fen communities, marked by raised hummocks of Sphagnum, occasionally supporting species such as

8

Trans. Norfolk Norwich Nat. Soc. 2014 47(1)

Dryopteris cristata, or, where conditions are exceptionally base poor, Drosera spp. The W2b Betula pubescens-Dryopteris cristata community is more common in the Ant valley, and at Catfield Fen in particular, than anywhere else in Broadland. Similar calcifuge communities are found across the watershed, where the Crag lies adjacent to the fens west of FFickling Broad.

Recent studies at Catfield Fen indicate that the Sphagnum polsters have both increased in frequency and in area: in one small corner of Catfield Fen alone, an area of up to 4000 m2 is now dominated by sphagnum (Mason 2014). Moreover, some ' of the rich fen communities described by Giller & Wheeler (1986a, 1986b, 1988) are being replaced by vegetation dominated by the bog mosses Sphagnum fallax and S. squarrosum, and in the eastern part of the fen system only a few locations now do not support sphagnum (Pyne 2014). At the same time the frequency and abundance of characteristic and rare rich fen species such as Liparis loeselii and Potamogeton coloratus has reduced (Mason 2014, Parmenter 2014).

A large proportion of the fen area in the Ant valley was formerly worked for peat, either during the medieval period, thus creating Barton, Cromes, Dilham, Sutton, and Alderfen Broads, or during the subsequent phase of shallow excavations which produced the extensive turf ponds which underlie much of the fen area of Reedham Marshes, and Catfield and Sutton Fens. Almost all of the 18th and 19th century cuttings have now terrestrialised, although a few small pools remain in Catfield Fen. The most shallow of the medieval cuttings, for example Sutton Broad, have also become completely terrestrialised. Fen, carr and swamp vegetation have colonised the margins of Barton Broad, Alderfen Broad and Cromes Broad. The terrestrialised turf ponds and broads support a large number of locally and nationally rare and scarce species and communities.

The NVC S24 communities are particularly well represented in the Ant valley. The wide range of habitats found here are thought to have developed into their present diverse forms as a result of the complex pattern of management coupled with local variations in the substratum and hydrological regime.

The fen communities which overlie turbaries are often strikingly different to those which are found over solid peat. Cladium mariscus is almost invariably found growing over turf cuttings and several other species including Sium latifolium , Cicuta virosa and Dryopteris cristata either occur preferentially, or like Liparis loeselii , are exclusively found in this situation.

The Ant valley fens are unusual in Broadland for the presence of remnants of open water habitat which have persisted despite turf pond terrestrialisation. These pools and runnels are usually effectively isolated from the eutrophic river water and support a number of locally rare and scarce species including Utricularia spp., and the nationally scarce Potamogeton coloratus. Several of the broads, for example Alderfen and Barton Broad, support marginal reedswamp, Typha angustifolia swamp and S24a Carex paniculata tussock fen. These are communities which are much less common in Broadland today than formerly. Open tussock fen at Alderfen has been colonised by various Sphagnum spp. and the locally rare sedge Carex canescens (curta), which is probably more frequent in the Ant valley fen system than anywhere else in Broadland.

Several of the terrestrialised turbaries, for example at Broad Fen, Smallburgh Fen, Catfield Fen and Sutton Broad, have developed species-rich small sedge fen communities as a result of hydroserai succession. These areas support a large number of rare and scarce species, including the sedges Carex diandra, Carex appropinquata and Carex rostrata over a bryophyte carpet dominated by Calliergonella cuspidata and Campylium stellatum. Other species

Trans. Norfolk Norwich Nat. Soc. 2014 47(1)

9

which seem closely associated with this community type are Pedicularis palustris , Dactylorhiza traunsteineroid.es , Dactylorhiza incarnata and Liparis loeselii. Car ex limosa has also been recorded from this habitat, but not in recent years.

The most recently terrestrialised turf pond and broads support semi-swamp communities, with species such as Typha angustifolia, Cicuta virosa , Sium latifolium , Ranunculus lingua and Rumex hydrolapathum growing in abundance in Phragmites- dominated vegetation.

In the southern part of the Ant fen system, at Catfield and Reedham Marshes, the fens closest to the river are underlain by estuarine clay, which has been exposed in places by turbary. These area soften sup¬ port Typha angustifolia or reed-dominated communities, with a number of associated salt-tolerant species, including Schoenoplec- tus tabernaemontanii and Oenanthe lachenalii. Similar community types are also found in the lower Bure fens and also at Hickling, although their presence in the latter area is partly due to brackish irrigating water rather than solely to the presence of estua¬ rine clay.

The Ant valley is notable for its diversity of pteridophytes; Dryopteris dilatata is abundant throughout, particularly in scrub and woodland communities. Thelypteris palustris is also common, as is Dryopteris carthusiana. Osmunda regalis occurs quite frequently and is particularly abundant at Sutton and Catfield Fens. The nationally rare fern Dryopteris cristata is locally frequent in several of the Ant valley fens, most notably Catfield Fen, where it occurs in association with the acidophilous nuclei described above, alongside the hybrid x uliginosa.

Most of the traditional management practices are now no longer carried out, but reed and sedge are still cut commercially on a proportion of the open fen in this area, including Sutton Fen, Reedham Marshes and Catfield Fen.

The Thurne Valley

The fen sites of the Thurne valley occur in an area of low lying land which extends from Catfield Common east to Candle Dyke, and from Candle Dyke north and east to Horsey Mere and Martham Broad. The area is drained by the River Thurne, which has its source less than 2 km from the coast at Somerton and joins the River Bure at Thurne Mouth. There are two main tributaries, the largestof which, Waxham New Cut, drains Brograve Level and Hempstead Marshes in the north of the catchment.

An interesting feature of this catchment is that it is known to have historically drained into the North Sea via a former channel, the Hundred Stream which had an outfall between Winterton and Horsey. The dunes were breached at the site of this channel during the 1938 sea floods. The date of the reversal of flow which resulted in the present southerly course of the River Thurne is not known, but from cartographical evidence it appears that this reversal certainly occurred prior to the 1790s, when the survey work for Fadens 1797 map was carried out and in all likelihood pre-dated the records of the Sea Breach Commissions which were formed from 1609 onwards.

Much of the low-lying land in the catchment has been drained for agricultural purposes; the main fen sites occur in the vicinity of the broads, where drainage was either not attempted or was unsuccessful. The surrounding drained marshland supplies much of the water which enters this fen area and enrichment from fertiliser application, rather than from effluent, is the main source of nutrients in this system. The River Thurne is only weakly tidal upstream of Candle Dyke, but even so, the water levels at Catfield Staithe may rise and fall by around 15 cm under normal tidal conditions, although brackish water only infrequently penetrates the fen dykes in this area.

10

Trans. Norfolk Norwich Nat. Soc. 2014 47(1)

Much of the land in the Thurne catchment area is used for agriculture, namely arable land and improved grazing marsh. Open fen occupies around 600 ha, which is just over one-fifth of the total land area; the highest proportion of this vegetation type in Broadland. This is the largest area of near-continuous open fen in Broadland, with a relatively small proportion being wooded.

The broads in this area were cut from peat beds which were considerably shallower than at many other sites in Broadland and in places the bottom of Hickling Broad consists of Crag sands and gravels, rather than peat. The remaining uncut peat deposits beneath the fen surface are also very shallow and this may be partly reflected in the abundance of wet heath communities and wet acid grassland communities around the fen periphery, where a thin layer of peat overlies sand and gravel. Towards the south-eastern extreme of this complex of fens and broads, the layer of Romano-British (Breydon Formation) estuarine clay becomes thicker, and there is anecdotal evidence to suggest that part of Heigham Sound may have been dug for its clay resources rather than for peat George 1992).

This area contains a wide variety of fen habitats and a large number of nationally and locally rare and scarce species are found here, including Dryopteris cristata, which not infrequent around the margins of Hickling Broad and at Brayden Marshes, and Sonchus palustris, which is particularly abundant here growing on spoil by the margins of the broads and major channels as well as being found as an occasional component of reedbed communities. Cicuta virosa and Sium latifolium are uncommon over much of the catchment and may be intolerant of the often brackish conditions. Sium latifolium , is, however, quite abundant in the Martham Broad area, where the salinity is lower than elsewhere in the system. Thyselium palustre occurs throughout, and Lathyrus palustris is

also present. Thelypteris palustris and Carex appropinquata are scattered throughout the fens of this system, but are by no means frequent. Althaea officinalis can be found occasionally growing on dyke spoil. Schoenus nigricans is not uncommon in this area as a component of the managed sedge beds, but does appear to have declined from a former abundance, having once been so common that a site near Martham Broad, now commercial reedbed, is called Starch Grass; a corruption of Star Grass, the local name for Schoenus nigricans. Further extensive stands of this species are present as Schoenus nigricans swamp fen, which is not unknown elsewhere in the county, but has its only Broadland site at Blackfleet Broad, where it has been present for over a century.

Many of the vegetation communities which are found here differ considerably from those in the other valley systems, having a