D
– Blackrock DolomiteTHE MOULDING OF THE BIGWOOD
A BOTANICAL REVIEW OF THE ECOLOGICAL HISTORY OF WESTON BIGWOOD, GORDANO VALLEY, NORTH SOMERSET
Dr Bill Dixon
INTRO
Weston Bigwood (O.S. ST455750) is a 38 hectare woodland remnant situated in the county of North Somerset (previously Avon) and lies between the towns of Portishead and Clevedon.
The wood straddles the northern limestone ridge of the Gordano Valley between the village of Northweston at it’s easternmost point and is delimited westwards by Valley road, some half a mile before the village of Weston in Gordano
Weston Bigwood became one of the earlier reserves of the Avon Wildlife Trust in 1985, the Trust being given a fifteen year lease on the management rights of the wood by the owners, Amy Roadstone (now Hanson) who owned the western end of the wood to the first set of boundary stones and Bristol City Council who owned the area to the east of the stones. In 2000 the Trust managed to buy the wood using Heritage Lottery funding and local donations. The wood had already been designated as an S.S.S.I. by the then Nature Conservancy Council in 1971, primarily as a remnant of ancient woodland containing many classic ancient woodland indicators but especially because of the abundance of the relatively rare Small Leaved Lime (Tilia cordata) which is itself an ancient woodland indicator. The geology of the site is complex (see fig. 1), but is primarily of carboniferous limestone. The woodland was managed as coppice with standards until approximately the late 1940's. since which time the coppicing regime has been abandoned . The wood is classified as w8d Fraxinus-Acer campestre-Mercurialis woodland, Hedera helix subc.
FIGURE 1 – The geology of Weston Bigwood
KEY TO ROCK TYPES - North is towards top of page
LLS – Lower limestone Shale
BD
– Blackrock Dolomite
BL – Blackrock Limestone
GO – Gulley Oolite
KM – Keuper Marl
THE EVOLUTION
The embryo of Weston Bigwood was born under the seas in the Lower Carboniferous at about 200 million years ago as the calcarious shells of dead corals and similar forms formed layers of limestone at the bottom of the sea . At the end of the Carboniferous the Armorican uplifting created the Gordano ridges as a prescursor to the wood .
Earth movements had been going on during the Carboniferous and culminated in a couple of the supercontinents bumping into each other. These created some of the greatest movements that the earth has ever known. Great earth pressures elevated a system of mountain chains stretching across central Europe as far as Asia. Great ranges such as the Alps and Everest range were created thus. The parts of this elevation in southern Ireland, North West France and Southwest England are named the Armorican chain. The compression acted like ocean waves forcing rocks upwards from the south or southeast i.e. creating waves running roughly east to west. The ridges of Gordano were created in this way.
The Gordano ridges may have been much higher but have stood the test of time well. The hard limestone was probably overlaid with sediments and other softer rocks from time to time and would have been host to various flora regimes. Various species of flora present before the last ice age e.g. Norway Spruce (Picea abies) and Siver Fir (Abies alba) did not return to Britain. The current Weston Bigwood flora has been accrued since the last (Wurm ) glaciation about ten thousand years ago.
THE MOULDING
The current flora and their distribution within the wood have undoubtedly been moulded by five major episodes in it's recent history.
1. THE WURM TURNS
Dates used here are years ago (BP) and are approximate in that different authors quote slightly different timescales for the various time zones.
The last UK glaciation (the Wurm) ended at about 10,000 years ago. Although this latest ice episode did not bring ice to the south west of England it did create tundra conditins much like within the arctic circle in present times. This had the effect of dramatically replacing any previously existing flora with a typical tundra flora. Dense woodland as seen now in pockets must have formed later as conditions warmed. Evidence of this succession in the Gordano Valley (Weston Bigwood implied) may be obtained by looking at peat bore analysis in the Gordano Valley (Jefferies, 1955).
1. Peat analysis by Jefferies in 1955 - (Most tree pollen from Atlantic on is considered as coming from the hillsides of the valley - authors view). Pollen data clearly shows that the hillsides during the late glacial period (pre-boreal ,10000-9000) were a mixture of grasses and herbs with birch and a little pine. Birch, oak and pine were present in the valley during the boreal (9000-8000) and towards the late boreal replaced the herb rich flora. During the early boreal there is a rapid increase in hazel. There is a 1000 year gap unconformity here (between zones V and VI). By the Atlantic period (8000-5000) conditions became much wetter and most of the trees in the valley floor were swamped. Pollen counts suggest that Alder quickly colonised the valley during the early Atlantic, afterwards showing a gradual decline as the climate changed and Betula and Quercus replaced it. Throughout zone VII (Atlantic) there is little herbaceous pollen until at the end of the zone. This herbaceous pollen probably represents forest clearance. Pollen levels for Elm are similar to other parts of Britain at this time.
Translating this data for what Weston Bigwood could have been like - from 10000-9000 scattered birch and pine becoming more abundant and being joined by Hazel early and Oak later during the boreal (9000-8000) with Pine dying out, and becoming thick woodland by 8000. Other species must have colonised between 8000-5000 after which it appears that woodland clearance began in the valley.
THE RUSH TO GET ACROSS THE CHANNEL
As the climate warmed (atlantic period 8000-5000) early pioneers of Juniper,Birch and Pine were largely replaced as trees spread north and west to the UK from warmer refuges in South Europe predominately the Italian and Iberian peninsulars and South East Europe (the Balkans) where they had retreated during the iceage. Britain was still connected to the continent by a low bridge of land allowing uninterrupted march north of the most successful species. Oak, Elm, Hazel, Cherry, Wild Service and other Sorbus species spread from the south while the Balkans supplied Small leaved Lime and Beech at least. The Balkan species were later reaching Britain and some only just beat the opening of the English Channel at about 8500. The arrival of man and forest clearance began at about 4000 (increase of weed pollen ). After the ‘climatic maximum’of the Atlantic period, the sub-boreal (6000-2500) is quoted as warm after which the climate in the UK cooled significantly halting the northward colonisation of less hardy tree species and in particular the Small leaved Lime. By AD 1200 (3200) the modern landscape was pretty much formed.
Beech and Hornbeam never made it to Weston Bigwood but pretty much all of the others got there, notably Small Leaved Lime and Wild Service Tree. The latter two are stuck in a time warp for neither are found naturally outside ancient woodland having lost the ability to colonise new woodlands. In the case of Lime it is unable to set seed in the modern day cooler climate (pollen tube requires about 20 consecutive days of mid twenties centigrade to grow and fertilise the egg). Both species can survive by vegetative regeneration. Lime roots well from fallen branches and also when blown down some of the elastic roots remain in the soil, the tree remains alive, and shoots and roots form along the now prostrate trunk. Good examples of this are seen in Weston Bigwood. Wild Service suckers extremely well and sets seed but maybe the seeds are not transported or able to grow in new woodland.
2. 1066 AND ALL THAT
Medieval man's treatment of woodland, which included coppice with standards, partitioning, grazing animals, selective encouragement of certain species, and other activities have further moulded the flora and their distribution within Weston Bigwood.
In medieval times woodland was an important resource and used for building, fencing, burning and fodder amongst others. The mechanism for using these resources to their best effect was by coppicing. This involves cutting timber at size and time according to species and use, the cut stems near ground level quickly regenerating for a repeat crop at the designated timespan. The classic type is termed coppice with standards, the standards being Oak which being used e.g. for major boat and cathedral timbers would need to reach maturity, other trees would be cut at regular intervels forming the coppice. Coppicing would have had the effect of creating much lighter conditions within woodland than non-coppiced and therefore greater flora and fauna diversity.
The history of Weston Bigwood is not specifically known, but it must have been extensively used for many centuries. Evidence for some of the useage of the wood over the centuries may be gleaned from the artifacts found there. In particular there are several ditches and banks, and a number of boundary stones. Other artifacts are present as a series of ridges and bumps. In one area there is a semi-circle of stones possibly the remains of an animal pen or shelter, and there are reports of saw-pits and rough dwellings being present previously. As well as these artifacts there is a map of a survey of Portishead taken in 1741 (held by Bristol City Council Archive Office) which details the wood as consisting of many small partitions. Figure 2 is a representation of this map to show the compartments present in 1741. Many of the names associated with the compartments were well-to-do gentry, thus highlighting the importance of woodland at this time. The map below is aligned North (top) to South with Valley Road skirting the Western edge.
FIGURE 2 – Diagrammatic representation Weston Bigwood owners/compartments (from a survey of Portishead in 1741)
KEY
Dotted line – Wood boundary
Thick line – Path through the wood
Thinner lines – Individual owners delimits
Evidence that the wood was much more open when actively coppiced may be found in 'The Bristol Flora' written by John White in the early twentieth century. The following are quoted from his book.
Fly Orchid 'six plants together in the Big Wood'
Greater Butterfly Orchid 'Nightingale Valley and Weston Big Wood'
Cow Wheat 'Weston Big Wood'
Nettle Leaved Bellflower 'Woods along the ridge above Weston in Gordano'
Upright Cudweed 'in a wood between Portishead and Weston in Gordano' (Weston Bigwood ?)
Wood Vetch 'about the woodland extending to Weston in Gordano'
Hairy St. Johns Wort 'Between Portishead and Weston in Gordano' woodland inferred.
Columbine 'in the Big Wood, Weston in Gordano'
All of these plants require more open woodland than has been seen in recent decades in Weston Bigwood.
3. INACCESSABILITY
The inaccesable nature of the wood with steep sides both north and south and it's very shallow soil at least on the plateau area probably saved it from extinction by man during the agricultural revolution in the middle ages.
Woodland in England had reduced from 15% of the land (Domesday) to 5.4% by 1895 (Mathews, 1987). After the industrial revolution the use for timber as a crop reduced and further woodland was freed for conversion to field. The situation and shallow soil of Weston Bigwood certainly has contributed to its still being there at the fall of the 20th century.
A map of the wood construted after the 1741 survey of the area shows pretty much the same shape and size as current times as does earlier implied data from the Domesday Book.
4. ABANDONMENT
Research by Dixon (1998 unpublished PhD) performing incremental core analysis on the overgrown Small-leaved Lime coppice indicates that coppicing in the plateau area was abandoned at about the end of world war two. However coppicing in the southern (lower) part of the wood appears to have been abandoned much earlier. The effect as seen by the 1990’s was that the wood was generally very dark with overgrown coppice and little obvious ground flora. Many species of ground flora hitherto recorded had been lost or at least surpressed.
5. STORMY DAYS - The Burn’s day storm
On the 23rd January 1990 an ill-defined and shallow low pressure was developing off the Eastern seaboard of the USA. By midday on the 24th it had been joined by another system and the pressure had dropped to 992 mbars with satellite images indicating an explosive system developing. This particular cloud/front formation (called a baroclinic leaf) reliably predicts a storm of force 10 or greater. Early on the 25th the low tracked eastwards over northern Ireland with a remarkable further drop in pressure of 16mb in three hours down to 952 mbar. During the morning of the 25th the wood was battered from WSW with gusts (Bristol Channel data) of up to 67 km/hr. As the front rapidly moved north east during the afternoon an even faster rise in pressure resulted in a second wave of strong winds this time hitting the wood from the NWN direction. The strongest gust from this direction at 76 km/hr and evidence of tree fall direction and the areas affected strongly indicates that the greatest damage was done in this period.
Although this storm appears major it is important only in the context of the structure of the wood now. Similar great storms must have occurred in the past and had similar effects (prior to the advent of coppicing). This follows the classical theory of Patch Dynamics where patches regenerate potentially with different species to those areas undamaged. Further major storms create their own patches which are likely to be different to the younger previous patches. Once the wood was coppiced such storms may have had only a limited effect. About 20 areas of damage large enough to be considered as patches were created during the storm, felled 500 to 600 boles (of probably about 300 to 400 root plates) and caused about 10% canopy loss.
In the map below where North is directly towards the right of the page it can be seen that the windfall throughout most of the wood is from the North West whereas some fall to the southwest part is much more westerly
THE CURRENT FLORA
The combination of the above factors have left the wood with a rich and diverse flora
The wood comprises standards of Pedunculate Oak (Quercus robur) and a few Sessile Oak (Quercus petraea) or hybrids between the two species, with old coppice mainly of Small Leaved Lime (Tilia cordata) and Ash (Fraxinus excelsior). Survey data from the late 1980's showed the main canopy constituents as Pedunculate Oak (Quercus robur), Small Leaved Lime (Tilia cordata) and Ash (Fraxinus excelsior), all of about 30% each of the canopy. In addition there are smaller numbers of Whitebeam (Sorbus aria), Cherry (Prunus avium), Crab Apple (Malus sylvestris), Field Maple (Acer campestre) and Wild Service Tree (Sorbus torminalis), most of which are old coppice as well as quite a lot of Wych Elm (Ulmus glabra) most of which is at present part of the subcanopy. Distribution surveys have been conducted on most species of tree in the wood.
Other trees recorded from the wood include a few Sycamore (Acer pseudoplatanus) and a single Beech (Fagus sylvatica) which was presumably planted. A few Turkey Oak (Quercus cerris), and a single Holm Oak (Quercus ilex) at the edge of the old quarry. Ratcliffe (1977) quotes Common Lime (Tilia x vulgaris), English Elm (Ulmus procera) both of which do not seem to be present, 'the hybrid between Sorbus aria and Sorbus torminalis' and 'a Sorbus close to Sorbus rupicola' as also being present in the wood. Reference to Sorbus spp. in Nature in Avon (Proceedings of the Bristol Naturalist's Society 1991) is as follows; Sorbus x vagensis (Wilmott) - 3 trees in Weston Bigwood were lost in the 1990 storm but should regenerate by suckers; Sorbus eminens (E. F. Warb.) - present in the old quarried area (Willis, 1991). Interestingly a small specimen of Sorbus eminens was discovered at the edge of the ride near where it is joined by the valley road path in about 2008. In a later edition of the Nature in Avon some doubt was expressed to the actual species of Whitebeam present in the wood and this needs to be clarified. In the vicinity of the ride there are a some Goat Willow (Salix cinerea) and the introduced Buddleja davidii also thrives here.
The shrub layer is typically Hazel (Corylus avellana), Hawthorn (Crataegus monogyna) with some Spindle (Euonymous europaeus), Holly (Ilex aquifolium), Wayfaring Tree (Viburnum lantana), Yew (Taxus baccata) and in the more open areas Dogwood (Cornus sanguinea) and Guelder Rose (Viburnum opulus). Subcanopy Maple, Ash, Wych Elm and Crab also form part of the shrub layer while waiting for a canopy gap to appear at which time they may grow to join the canopy. Holly, Hawthorn and Hazel also occasionally reach the canopy in the wood.
The ground flora is variously dominated by Wood Anemone (Anemone nemorosa), Dog's Mercury (Mercurialis perennis), Bramble (Rubus fruticosus agg.), Bluebell (Hyacinthoides non scripta), Ivy (Hedera helix) and includes rarer species such as Herb Paris (Paris quadrifolia), Birds Nest Orchid (Neottia nidus avis) and local species such as Goldilocks (Ranunculus auricomus) and the limestone thicket specialist Purple Gromwell (Lithospermum purpurocaeruleum). Other typical ancient woodland indicator species include Sweet Woodruff (Galium odoratum), Madder (Rubia peregrina), Wood Sphurge (Euphorbia amygdaloides) Yellow Archangel (Lamiastrum galeobdolon) Pignut (Conopodium majus) Sanicle (Saniculum vulgare) Barren Strawberry (Potentilla sterilis) Hairy Woodrush (Luzula pilosa) Wood Melick (Melica uniflora), Wood Millet (Milium effusum) and Wood sedge (Carex sylvatica).Other sedges indicitive of ancient woodland but which prefer moister soils are absent. Weston Bigwood is a dry wood being well drained by the limestone on which much of the wood is situated.
THE STRUCTURE
Looking at the wood it is easy to see that it is classic but overgrown coppice with standards, the standard trees being almost entirely of Oak but with one or two Ash., the coppice trees have now joined the canopy but can still be identified by being multi-trunked. In agreement with other historical evidence the coppicing regime was abandoned at about the end of world war two in the wood, although in the southern part of the wood coppicing was abandoned earlier. Here some of the large limes form ‘fairy rings’ with the original central trunk now rotted away and each coppice pole forming a discreet tree. Increment cores taken from some of the Limes in 1995 gave an age of the coppice boles of about 40-50 years. As the abandoned coppice grew much of the wood became very dark with the overgrown coppice casting heavy shade and thus reducing any flora underneath. The cutting of a ride through three quarters of the wood in 1987, and numerous gaps (patches) created by the 1990 storm have created a much more open woodland in places, with much dead and lying timber and a more diverse ground flora possibly reflecting times when the wood was actively coppiced. Recent management has seen the creation of four coppice coups in the south west area of the wood continuing the ‘light’ diversity within the wood as the canopy gaps created by the storm close. These events have produced a more varied ground flora including the reappearance of some of White's ‘lost’ species. In particular Nettle Leaved Bellflower, Hairy St Johns Wort and Columbine have reappeared in the ride after an absence of more than ninety years, and Wood Vetch has appeared along the path on the plateau. There have been up to three Greater Butterfly Orchid in the ride (but not still there). Some woodland species not quoted by White have appeared in recent years. These include Tutsan which is a native of ancient woodland. Some species such as Cow Wheat and Fly Orchid have not (yet) returned. And of course these open areas have attracted lots of ‘non wood’ flora usually transient on recently disturbed areas such as scrapes where timber has been moved, tractor tyres marks and bare areas under recently removed Bramble. Some of the more interesting include Hollyhock and Bleeding Heart.
It is probable that historic management of the wood is responsible for the surprising distribution of some of the trees described below under ‘Main wood and Plateau’ and may at least in part be due to it's medieval compartment structure. The main ditch and bank delimits the medieval eastern end of the wood which is still recognisable by the trees and ground flora present on each side. Thus most of the central plateau was kept free from grazing animals and is coppice with standards. Outside this ditch and bank to the east of the wood there is a classic 'Woodland Pasture' structure indicating past grazing in this area (this is described in more detail later).
Detailed analysis of the distribution of canopy trees within the wood reveals more about the wood's true character and past. In particular the Small Leaved Lime occurs in discreet discontinuous stands, often dominant where it occurs, and near the north west edge the wood it is the only coppice tree present. It is absent from part of the plateau and much of the eastern part of the wood. Although the delimitation of the Lime does not match the 1741 compartments, the strikingly linear edge of it's distribution in the north west of the wood is strong evidence of human intervention. In the south eastern part of the wood at the base of the plateau the Limes are much bigger and here coppicing appears to have been abandoned considerably earlier. There are standard Oak more or less throughout the wood and also a few standard Ash. In one area roughly matching a plot on the 1741 map of an area of 40 to 50 metres square consists almost entirely of coppice Oak and this may have been used by an Oak tanner. Some other plots must have favoured Lime which may have been used to stoke surrounding lime kilns or used as fodder or ropes. Some plots by inference from their names may have been maintained as 'field' areas and others as uncoppiced woodland, for example 'wood at Sparley's Splott'. The distribution of Wild Service Tree is interesting in that it is scattered throughout the wood except the eastern end and does not occur beyond the major ditch and bank system that delimits the 'wood pasture' .
The Wood Pasture
The Eastern end of the medieval wood is delimited by a major ditch and bank system and comprises mainly of standards with little old coppice. Here the understorey has a much larger proportion of Hawthorn compared to Hazel than the rest of the wood and also contains a few large Holly trees.This appears to have been wood pasture in which only the standards would have survived and only prickly or unpalatable ungrazed understory could grow. No Lime or Wild Service Trees occur in this area but there are a few Whitebeam. Of the ground flora a little Herb Paris occurs in the wood pasture but less commonly than in most other parts of the wood. In contrast the area to the west of this ditch and bank system is more traditional coppice with standards with understorey predominately of Hazel but with several other subcanopy species present. The inference is that the area to the East of the main ditch and bank is secondary woodland not present in medieval times.
Other
The woodland flora also shows a discontinuity of distribution for some species. Rather surprisingly there is only one small patch of Early Purple Orchid (Orchis mascula) just at the north western edge of the plateau. Similarly Town Hall Clock (Adoxa moschatellina) only occured around one of the main badger setts although it appears to have died out there. The Goldilocks Buttercup (Ranunculus auricomus) is only found on the eastern part of the plateau to the east of the main ditch and bank and then almost entirely along the edges of paths. Other species such as Twayblade (Listera ovata), Herb Paris, Sphurge Laurel (Daphne laureola), and most of the other ancient woodland indicator species are distributed in most parts of the wood both on the Limestone and over more acidic rocks. A few species characteristic of more acidic soils do occur at the eastern end of the wood, namely Ransoms (Allium ursinum) and Sorrel (Oxalis acetosella).
The ride which is maintained annually to preserve a low sward with intermediate scrub grading into the canopy (alternate sides are cut in alternate years) has become floristically rich with a number of limestone grassland species colonising as well as open woodland species. Meadow species that have appeared here include Yellow Wort (Blackstonia perfoliata), Centaury (Centaurium erythraea), Vervain (Verbena officinalis), Red Bartsia (Odontites verna), Cowslip (Primula veris) etc. with over 200 species of plant in total recorded from the ride. The majority of the canopy gaps created by the 1990 storm and the ride became rapidly covered with a dense cover of Bramble, suppressing the woodland flora. As the canopy closes over these gaps the Bramble dies back and the woodland herbs are reappearing (but not necessarily the same in species and distribution). The ride need annual maintenance to keep it free from Brambles etc.
MANAGEMENT
Since the Avon Wildlife took over the wood in 1985 there has been a management plan and policy. This was initially done by (transient) full time AWT personnel often on short-term contract which meant the plan being rewritten frequently with no co-ordination. When I became voluntary warden I was able to give stability to the management and since then we as a voluntary team have had considerable input into the policies. Woodlands are pretty resilient and management needed is often minimal. In Weston Bigwood there are three main acivities.
In 1987 a 10 metre wide ride was cut into the south west half of the wood to about two thirds through following the route of a path noted in medieval times. This concentrated where people would walk and allowed more light for flora and butterflies including the now declining Silver-washed Fritillary. Weston Bigwood is a ‘Bramble’ wood and the opened up ride becomes a Bramble patch in about two years. Therefore the ride needs to be cut every couple of years. A diverse flora is maintined by cutting different parts of the ride in two or three year rotation.
Over the last several years a section of the south west part of the wood including mainly areas severely damaged in the Burn’s day storm have been coppiced in a four coup rotation, the latest being formed in February 2012. We have been monitoring the coppice coups with two metre quadrats each Spring to see if the regeneration flora is any different to that of the the precoppice.
Other management includes keeping paths open, maintaining the steps and reporting fallen trees.
MONITORING
In the late 1980’s I formed a team of voluntry wardens. It seemed a good idea to involve them in scientific survey and monitoring and this became of greater interest after the Burn’s day storm in 1990 as the structure of the wood was changed dramatically. The storm also triggered a seven year study of how ancient woodland regenerates leading to the award of my PhD in 1998.
Original surveys included a Breeding Bird census which was carried out first by Pete Evans and then Simon Wilson and currently by Geoff Harris. Results over the years have shown that changes in the woodland structure have not dramatically affected bird populations with the exception of Marsh Tit which has not bred in the wood for several years after previously being logged at two or three pairs every year.
An intriguing and novel method of tree surveying was devised by the late Noel Moreton. In order to simplify area sampling of trees he proposed a circular quadrat composed of a rope radiating from a central point usually a tree. The sampler would complete a circle using the rope as radius counting the trees encountered within. Over a couple of years over 60 such quadrats were done allowing us to accurately estimate the canopy composition which was pretty much a third each of Lime, Ash and Oak
Other tree surveys concentrated on actual numbers of low density Sorbus species. Wild Service and the adventives Norway Maple and Sycamore were treated thus.
Fixed Point photographs were taken for several years as the canopy gaps closed
Rare and specialised flora was monitored, the methods depending on the type of distribution and plant numbers.
For example early surveys of Herb Paris revealed that this species is abundant in the wood. Therefore we concentrated on two or three colonies (plants - as this species generally forms discreet colonies whch are really one body connected by underground rhizomes).
Goldilocks and Purple Gromwell occur at low density and in dense colonies so a measure of the length, and breadth of each is taken.
Plants with low numbers are counted e.g. Birds Nest Orchid, Early Purple Orchid.
Event-driven work included myself and Linda Gregory mapping the fallen trees as best we could just after the storm. It involved a lot of jumping over fallen trees as more than 70 trees were blocking the main paths through the wood.
Detailed analysis techniques and results
Coppice quadrat analysis
Two metre square areas within each of the coppice coups during each spring, three per coup. Flora present is estimated by % cover and/or number of plants present. Results indicate a rapid build up of bramble, followed by a return to woodland flora as the coppice grows thus eliminating the Bramble. Although the results show what is expected nevertheless they provide a good measure that we are pursuing correct strategy. The data shows the expected trend of rapid colonisation by Bramble followed by an equally rapid decline as the coppiced trees regenerate and a return of the previous woodland flora.
Herb Paris
Two or 3 long standing colonies have been recorded one for nearly 20 years. Results show that there are is a greater % of flowering to non-flowering spikes with perceived increase in light (not measured). One patch has moved some two metres (over >10 years) to follow a light differentialal as the canopy closed over. With generally less light this colony has diminished by two thirds. Another small colony of 30 spikes had ‘died out’ completely by early 21st century but one or two plants have since reappeared. It will be interesting to see what would happen if this site becomes lighter as it seems that the colony is still existing.
The data below for the first study colony shows that this colony has declined in numbers of spikes with time. Although not recorded it is visually obvious that the canopy has closed in around this site giving darker conditions (but note that Herb Paris flowers before the canopy leaf break). It is interesting to note that the number of flowering spikes does not appear to vary as much as the total number.
Goldilocks
Colonies mapped and measured for length and maximum width.
Early Purple Orchid
The number of flowering spikes are counted. Currently we know of 1 colony just off the northern path plus a few by the extension of the ride towards the southwest entrance gates
Purple Gromwell
Every 2 years or so the roadside colony is measured for length and maximum width. Visual recollection before measuring was started indicates that this patch is increasing in length.
Broad-leaved Helleborine
Each year the plants are counted and mapped
Birds-nest Orchid
This is done as an ad hoc survey – If you see one log it on the map.
Wild Service Tree.
A count of mature trees and saplings was done in the 1980’s.
Whitebeams
These have never been fully counted. These are sufficiently few in the wood to do a full count of them. Then perhaps we can get a Whitebeam expert to have a look at some of the ‘interesting’ variations.
Ride plant survey
This is done yearly as a one off headcount of all species present in the ride. Usually late May is best to get most species.
Butterfly monitoring
Butterfly Transect
Individual species monitoring e.g. Silver Washed Fritillary
THE FUTURE
Now that the wood has reverted to natural processes of regeneration there is the possibility of change in the composition of the canopy. Evidence of an increase in the amount of Ash and a decrease in Oak and Birch can be seen from study of two older gaps (30 and 10 years before the 1990 storm) In the 30 year old gap Birch and Oak are being replaced with Ash, while in a ten year old gap a large Oak is being replaced by Ash and Elm. The amount of Oak present in the wood in the past may have been maintained at an artificially high level due to Crown requirement for Oak standards.
What of the future? The wood as a S.S.S.I. is likely to be preserved. At present the Avon Wildlife Trust is allowing natural processes of woodland regeneration to take place. This consists of leaving dead timber and trees to rot naturally and allowing gaps to regenerate to canopy themselves.The regeneration of gaps (patches) demonstrates the classic example of patch dynamics. The 20 or so gaps formed by the 1990 storm will possibly fill with a different canopy mix than was present prior to the storm. The next great storm will create another series of gaps which will probably involve parts of the wood not affected by the 1990 storm. Thus a mosaic of patches in different stages of regeneration and with different canopy trees may be visible in a hundred or so years.
Overgrown coppice of course is not the same as 'natural' woodland and the multi-stooled trunks cause denser shade and may become unstable and split outwards. Regeneration of gaps formed by the 1990 and future storms will gradually redress this balance.
There is a further consideration - For centuries woods such as Weston Bigwood have been artificially maintained as more open due to coppicing with a corresponding distinct and evolving flora and fauna. There are some that feel that some sort of coppicing regime should be maintained, although there is an increasing popularity in restoring some woods to high forest status comparable to that prior to mans colonisation of Britain. To maintain light diversity within the wood, over the last several years four coppice coups have been cut in in the south west area of the wood and will be recut on a rotational basis, thus maintaining both types of woodland. The coppice coups occupy about a tenth of the wood.
In conclusion Weston Bigwood is a vitally important woodland both as ancient woodland but also as a model for scientific study.