Observations on the effects of the fire of 1973 upon the vegetation of Redgrave and Lopham Fens

OBSERVATIONS ON THE EFFECTS OF THE FIRE OF 1973 UPON THE VEGETATION OF REDGRAVE AND LOPHAM FENS M A L C O L M M A R K S AND R I C H A R D S W A N

Introduction F I R E S have always been an associated part of the fenland ecosystem whether started naturally or deliberately and it is probably true to say that until quite recently the fens were fired at least every few years. The investigation was carried out during the summer of 1974 to attempt to ascertain the effects of the fire of March 23rd, 1973, which was particularly extensive in burning 150-200 acres from the total of 314.

It began in the South-West corner of Redgrave Fen and spread rapidly across West Redgrave and Lopham Little Fen. From the former it went eastwards and jumped the River Waveney onto Lopham Middle Fen where it continued to the northern boundary before expanding to both the east and west. The burnt areas are illustrated on the map.

Methods The survey was initiated by dividing the fen into different regions based upon their major species component from which five distinct plant communities were easily recognisable. These communities were dominated by: 1. Mixed fen Vegetation.

2. Phragmites communis (Reed).

3. Cladium mariscus (Sedge).

4. Molinia caerulea (Purple moor-grass).

5. Fen carr (or bushes).

Mixed fen Vegetation is a term used to describe those areas, which due to topography lack any one dominant species but instead have small areas dominated by such species as Rubus fruticosus (Bramble), Urtica dioica (Stinging nettle), Juncus subnodulosus (Fenrush), Eupatorium cannabinum (Hemp agrimony) as well as those found to dominate their own communities.

For the purpose of this study, fen carr was excluded and the remaining four communities were subdivided on the basis of whether or not they had been burnt by the fire, the purpose being to compare burnt and unburnt areas of the same plant Community by both subjective and quantitative sampling techniques.

Within each of the eight areas, forty-five, randomly placed, one metre Square quadrats were sampled and each species present

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in them was given a value from the Braun-Blanquet scale of cover-abundance. Additionally, five of these quadrats from each area were cut to ground level in order to ascertain the Standing crop (dry weight of above ground plant material) of each species present.

EFFECTS OF T H E FIRE O N REDGRAVE A N D L O P H A M FENS

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To ensure valid results, three assumptions had to be made: firstly that unburnt (or control) areas have not changed since the fire, secondly that since the fire both burnt and unburnt areas of the same plant Community have received identical external factors exclusive of those produced or indirectly caused by the fire and thirdly that before the fire the areas now designated as burnt, were similar to their respective unburnt areas. To minimise the possibility of any one of these assumptions being drastically wrong, burnt and unburnt areas of each plant Community were sited as near as possible to each other.

Results Where possible, results were subjected to Statistical analysis to determine their significance. From the results, it will be shown that if the above assumptions were correct, the fire brought about several major changes in all four communities.

Little would be gained by illustrating the results fully,* however those of greatest importance to the Reserve, i.e. addition or exclusion of species from burnt areas, are given as species lists for both areas of the plant communities. These lists illustrate that an increase in species diversity (number of species in a given area) has occurred in all burnt areas ranging from 20% in Phragmites communis areas to over 50% in Molinia caerulea.

Large increases in Standing crop were found in all burnt areas ranging from 20-120% compared with their respective unburnt areas. These weights when broken down into the percentages contributed by the dominant species and those produced by their associated species, showed that in all burnt areas, the percentage of the latter group rose by a very large amount in comparison with their dominant species although this statistic should not hide the fact that in burnt areas, both dominant and associated species had greater amounts of dry plant material than those in unburnt areas. Thus the ratio of dominant to associated species feil after burning.

Species List 4 Angelica sylvestris Arrhenatherum elalius Betula pubescens Calamagrostis canescens Carex diandra C. elata C. panicea Centaurea nemoralis

B

U

x x

x

B

U

x

B

x

x

U

B

x

x x x

A copy of complete results can be obtained from the senior author.

U

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B u B Chamaenerioti angustifolium Chenopodium bonus-henricusXX Cirsium arvense X X C. dissectum X X C. palustre X Cladium mariscus X XX Crataegus monogyna X X Epilobium hirsutum E. palustre Equisetum palustre X X X Eupatorium cannabinum X Filipendula ulmaria X X Fraxinus excelsior X Galium aparine X X X G. palustre X X G. uliginosum X Holcus lanatus Hydrocotyle vulgaris X Iris pseudacorus Juncus subnodulosusXX XX X Lotus uliginosus X Lychnis flos-cuculi X Lycopus europaeus X X Lythrum salicaria X X Mentha aquatica X X Molinia caerulea X Myosoton aquaticum Oenanthe lachenalii X XX X Phalaris arundinacea X X X Phragmites communis Poa pratensis X P. trivialis X Polygonum bistorta X X Potentilla erecta X X P. palustris Quercus robur X Ranunculus repens X X Rosa sp.

X X Rubus fruticosus X Rumex sp.

X Salix atrocinerea X S. cinerea Schoenus nigricans X X Scrophularia aquatica X Solanum dulcamara X Sonchus arvensis X Sonchus oleraceus X Stachys palustris X Tamus communis X Thalictrum flavum X X Urtica dioica X X X Valeriana dioica X V. officinalis X X X Vicia cracca 29 23 35 T o t a l species present

2

3

u

B

u

X

X X X X

X X

X

X X

X

X X X

X X

X X X X

X

X

X

X X X

X X

X

X X

X

X

X X

X X X X

X X X

X X

X X X X

X

X

X

X

X X X X

X

X X X

X X X X

X X

X

X X X

27

32

22

X X X

B = burnt areas u = unburnt areas (for positions see map).

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Discussion In the field, the most notable differcnce between b u r n t and control areas was in the amount of plant litter present. In the latter, this had accumulated almost unhindered for many years while in burnt areas, almost all the litter had been lost to the fire.

T h e differences f o u n d between burnt and u n b u r n t areas could almost totally be accredited to this Observation resulting in the following effects: (a)

Nutrients held in the litter were normally returned to the soil at a constant rate but, b y the litter combustion and the associated increase in nutrient solubility, this Store was quickly depleted giving an initial enrichment to the soil but p r o d u c i n g a long-term deficit.

(b)

M a n y plants produce seeds w h i c h require the presence of light to encourage germination; the litter helped to exclude light and thus lower germination and prevent the establishment of n e w individuals. W i t h litter removal, these seeds were able to germinate on the exposed peat and b e c o m e established.

(c)

T h e effect was particularly noticeable w h e r e the c o m m u n i ties had formed upon h u m m o c k - h o l l o w complexes, possibly caused b y peat digging. T h e hollows became litter-filled restricting plants to the summits of the h u m mocks thus placing the plants at a greater distance from the ground water level.

B u r n i n g the litter made new g r o u n d available for colonisation so that a greater variety of plants became established; this effect was enhanced b y the increase in nutrient availability.

Conclusions T h e fire may at first sight seem to have had certain beneficial effects b y encouraging increases in both the species diversity and in the production of a greater bulk of plant material. T h e s e were, however, more than cancelled out b y the damage done to the true fen communities.

B y b u r n i n g the plant litter the nutrient stรถre was released at once thus robbing the plants of nutrients in future years, the fen reverted f r o m a closed Community with a unique species ensemble to an open Community into w h i c h ruderal species invaded. S u c h grasses as Holcus lanatus (Yorkshire Fog) and Arrhenatherum elatius (False Oatgrass) became established and may be to the detriment of some of the fenland species.

The fire had a third effect w h i c h proved to be double-sided in eradicating fire-tender species. Into this category can be put seedling trees and here fire m a y b e useful in holding back the spread of carr Vegetation w h i c h is a necessity if the fens are to be prevented from b e c o m i n g woodland.

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It would be both brave and foolish to say with absolute conviction that fire is either a good or bad factor. However, it must be realised that fire has probably been associated with the fenlands for longer than man and, as this Community still exists, it can be concluded that although uncontrolled fires are both menacing and damaging to the Vegetation and fauna, their effects will be rapidly healed by time providing that sufficient Vegetation remains to provide a reservoir of species from which recolonisation can occur.

Acknowledgements The authors would like to thank the Suffolk Naturalists' Society for a grant from the Charles Morley Bequest to cover the expenses of the fieldwork and to the Officers of the Society for placing it at their disposal. They would also like to thank Mr. P. J. Wanstall for his supervision and the Sharp family for their kind hospitality.

Malcolm Marks, B.Sc., Queen Mary College, University of London, Mile End Road, London, El 4NS.