Wood anemone (Anemone nemerosa) at Hembury Woods SSSI/SAC.A (fairly) faithful indicator of ancient woodland across the country. Conservationists have long known that the older a habitat, the richer its wildlife is. Thus the more species it supports the more important the site is in terms of nature conservation.
This importance of continuity of habitat is most vividly illustrated in woodland, and, more specifically, in ancient semi-natural woodland (ASNW).
All species of ASNW can in effect be used as 'indicators', or as 'barometers' of the health of a habitat. But how can you properly find out how many invertebrates or fungi are present without years of detailed survey? Birds are too mobile and mammals too elusive. Lichens and bryophytes are excellant indicators but are extremely complex organisms and require great expertise to properly identify. Moreover, many coppice woods are known to be of ancient origin but this system of management eliminates larger, lichen bearing trees and allows for large fluctuations of light which is sometimes not conducive to lichens. Air pollution exacerbates the problem in using them reliably in areas such as the south-east of the country.
This leaves the vascular plants, that is, the flowering plants, trees, ferns and native conifers. Easy to identify and locate. But of course, as with everything, there are limitations.
There is now a tendency for certain woodland plants to be used as proof that a wood is ancient. This was never the reason for the creation of the 'indicator' lists. They were originally to 'indicate' the richest woods, and, coincidentally, happened to 'indicate' that such woods were often those of ancient origin.
We will come back to these limitations after a brief trip back to the 1980's when the Nature Conservancy Council (NCC) set out to identify and map the nations ancient woodland for the now famous Ancient Woodland Inventory. The ultimate objective of this inventory was to secure appropriate management of the nations remaining ASNW by first identifying and mapping it. This was the practical aim of the NCC, and whilst their surveyors became well versed in the documentation and clues as to a woods use in the
past (clues discussed in the blogs
'Finding Ancient Woodland' and
'Reading the Wood') they needed biological clues, a way of evaluating the richness of ASNW as a habitat in the
present.
It has long been known that many woodland vascular plants are restricted to old woods and that they are slow and ineffective colonisers of new woods and plantations. This has been exacerbated by the isolation of many old woods, and thus the further isolation of many ancient woodland plants.
Following on from studies by George Peterken in the native limewoods of Lincolnshire, and from Oliver Rackhams work on Oxlips in Hayley Wood in Cambridgeshire (both in the 1970's), Francis Rose and Richard Hornby set about compiling lists of ancient woodland vascular plants for the NCC. They needed to restrict their lists to species which showed a strong affinity for ancient woodland and were typical members of botanically rich woods. Refinements occured as work progressed, and as Rose illustrates in
Indicators of ancient woodland: The use of vascular plants in evaluating ancient woods for nature conservation in
British Wildlife (Vol 10, No 4, 1999), by 1978 they had refined the lists so that the species they identified :
' a) seldom occur outside woodland, or
b) even if they do, are nonetheless indicative of long continuity of woodland cover, or
c) seem to be reliable indicators in at least part of the region. '
Extemely rare plants were transferred to a separate list, and different lists were drawn up for different regions, so that each region of southern Britain had a list of 100 Ancient Woodland Vascular Plants (AWVPs) and a list of 52 Rare Woodland Vascular Plants (RWVPs), spread across these regions. These RWVPs were defined as either occuring in very few sites (ie fewer than 10) or were confined to a very localised area.
Over time these lists have been extended to the rest of Britain with minor adjustments here and there dependant upon locality and improved field data, but the general consensus on which plants constitute as reliable 'indicators' (broadly) remains.
Limitations of the AWVP listsAs Rose notes in the aforementioned article 'While the wide use of AWVP lists as a surveyor's tool is obviously gratifying to its originators, their sometimes apparently uncritical use leads us to remind users that they should be regarded only as a tool, and not as an infallible guide. The lists should be used intelligently and in combination with other information.'
The 'other information' is that such as covered in earlier blogs,
'Finding Ancient Woodland' and
'Reading The Wood', that is, looking at old maps and documents, and seeking other clues within the wood itself as to its antiquity. Only then can the presence of certain AWVPs contribute to the overall picture as to the
possibility of a wood being of ancient origin.
George Peterken in
Identifying ancient woodland using vascular plant indicators in
British Wildlife (
Vol 11, No 3, 2000) highlights the risk of reading too much into lists of plants through an analysis of the data he collected in Lincolnshire in the 1970s.
Between 1972 and 1981, aided by several local botanists, he collected plant lists for 362 woods of which, through documentary evidence, he knew 89 were ancient woods and 273 were secondary woods.
Through these lists he attempted every variable to actually test the reliability of AWVPs as reliable 'indicators' of ancient woodland.
First of he tried using single species that were strictly confined to the ancient woods in Lincolnshire (the commonest of which were Common Cow-wheat, Bird's-nest Orchid, Pendulous Sedge and Herb-Paris), but found that they were absent from the greater majority of ancient woods.
He then considered possible indicators collectively, by tallying the four most frequent species confined to ASNW in Lincolnshire (those listed above), and the four most frequent species that were
almost confined to ancient woodland (Pale Sedge, Great Woodrush, Sweet Woodruff and Greater Butterfly Orchid), and found that '62 ancient woods would have been correctly identified as ancient, but 27 ancient woods would not have been identified, and 6 secondary woods would have been incorrectly identified as ancient.'
In his usual meticulous manner he tries every variable, including trying to identify the species that most closely conforms to a pattern that would be shown by the perfect ancient woodland indicator. In central Lincolnshire such a species would thus be present in all 89 ancient woods and absent from all 273 secondary woods. The nearest species was Wood anemone, found in 81 ancient woods but also in 14 secondary woods. This meant that it would have misled in 22 of the 362 woods. Other species would have misled even more frequently.
As Rose points out ' We do not claim that all, or indeed any, of the AWVPs are strictly faithful to ancient woodland. Where secondary woodland abuts directly onto older woodland, it will acquire species much more rapidly than do isolated woods'.
Peterken's studies backed this up, with the majority of secondary woods containing AWVPs in close proximity to ASNW, though, it must be stressed, not all.
Many woodland plants behave differently from one soil type to another (hence why an indicator plant in the south-east may not be so in the south-west) or it may be confined to ancient woodland (such as Oxlip) because other habitats it frequented (such as wet meadows) have been ploughed up and destroyed.
Futhermore, many AWVPs adorn our hedgerows leading some to question the validity of that plant as an 'indicator'. Such hedges may be 'ghost hedges', that is, all that remains of an area that was formerly woodland but is now pasture, or it may simply link up to an ancient woodland.
Equally, many ancient grazed wood pasture sites which are known to be sites of great antiquity are not renowned for their AWVPs, but are for their 'Old Forest Lichens' (an index of which similar to AWVPs Francis Rose has produced). Conversley, coppice woods renowned for their rich displays of AWVPs are largely bereft of 'old forest lichens' as mentioned earlier.
Some wild flowers are also grown in gardened landscapes and may be escapees, so unless they occur well within a wood they should be discounted.
The key here, as Rose notes, is that 'the sum of the lists of AWVPs is more valid than their constituent species'. That the more AWVPs can be found in a wood is more important than individual species as of themselves. As he goes on to say:
'Notwithstanding these limitations, which need to be understood when evaluating sites for nature conservation purposes, we believe that the regional lists of AWVPs do provide a broad framework in which one can place individual sites. For the identification and evaluation of diverse woods, rich in vascular plants, they should be perfectly adequate. To establish a wood's probable age and origin, however, they should be used with due care, and preferably in combination with historical information.
Our basic thesis is that any one (or even several) AWVP species may have little or no significance. The whole concept is concerned with statistical probability. We believe that as the number of AWVP species that occurs in a site increases so the statistical probability of the wood being ancient also increases. Our AWVP species form a continuum from the very rare (too rare to use in the tables) that are certainly characteristic of, and faithful to, ancient woods, to those that are widespread in old woods, but not always confined to such woods.'
Regional variationIf we accept the limitations noted, we have an invaluable tool (alongside others) in determining the
likelihood of a wood being of ancient origin, but, even more importantly, of assessing its value for nature conservation, and of being able to prioritise our activities around these flora rich woods.
The great value of British woodlands, as hinted at in previous '
Woodlands' blogs, is its great variety due to topography and past management. Thus we have regional 'Types' of woodland which further enables us to focus our woodland conservation management. AWVPs aid us in this focus and also reflect this great variety.
The south and south-east of Britain contains more ASNW than anywhere else in the country. It also maintained traditional woodland management practices (such as coppicing) for longer than elsewhere in the country (thus allowing in the light many AWVPs need to flourish), and then reinstated these practices quicker than anyone else. Such lowland woods have thus been less modified by the ravages of forestry (though not untouched) and are generally seen as the priority in terms of woodland conservation. Consequently the AWVP scores for woods in this part of the country are generally the highest. Much of this is also down to geology.
As Rose notes:
'The richest woods are usually those on varied topography, often on inclines, which combine acid and lime-rich soils, with moist, heavy soils on the lower ground. Without this necessary diversity, even the most historically stable, undisturbed wood is unlikely to have a AWVP score exceeding 30.'
Such points as have just been made are very important to bear in mind for when we turn our attention to a National Trust wood on Dartmoor such as Hembury Woods. This will be a blog of its own with a list of its AWVPs and other woodland vascular plants.
To put all this into some context, some of the richest woods in Hampshire and Sussex have AWVP scores of 74, with the average being around 44 to 53.
In the south-west, for the woods known, the highest score for a wood in Somerset is 41, with the average being around 33.
In Cornwall they seem to score from around 23 to 34.
In understanding these scores in light of the limitations noted, it has to be stressed that these are scores for known ASNW, and the context i am trying to illustrate is of the richness of ASNW in the south and south-east of the country as compared to western woods (which are largely on acid soils). Conservationists have long known this, and, in effect, the scores for AWVPS 'indicate' this to be so.
Here, of course, it must be noted that western woods are fantastically rich in lichens and bryophytes. This is where their value lies. Thus, i suppose, a blog on woodland epiphytes must follow.
To sum up, from all of the above, it must be clear now that AWVPs are a useful aide to ascertaining the
possibility of a wood being of ancient origin as long as it is in conjunction with other information.
The key to remember is that AWVPs indicate the great floral value of ASNW due to its age, but they are not proof of its age.
It's real value to woodland conservationists goes beyond this and is ably summed up by George Peterken who i am more than happy to give the last word to.
' Ancient woodland indicators' should really be used in reverse: rather than deduce history from plants, we should use our knowledge of woodland history to learn about the responses of species to habitat change, and thus their likely responses to future management.'
Notes1) The Nature Conservancy Council (NCC) morphed into English Nature who then morphed into Natural England.
2) The accepted AWVP lists for most of Britain is that collated by K.Kirby in 2004, which can be found in
The Wild Flower Key by Francis Rose (Revised Edition 2006).
3) Other AWVP lists, for Wales and Scotland, and some northern counties of England, are accesible from the Countryside Agencies or in specialist journals. A full list of the counties and where their relevant AWVP lists can be found is in
Field Surveys for Ancient Woodlands: Issues and Approaches - A Report to the Woodland Trust (2009).
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