I know the area well, and it's pretty well mapped, although there's plenty of scope for refinement because the bulk of the mapping was done purely by GPS before any aerial imagery was available. I was not planning any mapping activities at all, but as it turned out I realised this is a near perfect time for getting to grips with some of the complexities of mapping woodlands.
|Wood Anemones, Bisham Woods|
Bisham WoodsThe Bisham Woods (now owned and managed by The Woodland Trust) are a classic example of Ancient Woodland (woods which have had continuous tree cover since 1600) in Britain. This term effectively corresponds (particularly ecologically) with the North American term old-growth or the German term Urwälder. In the British context the focus has been on establishing documentary and ecological evidence that the woods are likely to have existed since trees returned to Britain after the last Ice Age.
The earliest map showing these woods to hand is a draughtsman's drawing from 1809 for the first series of Ordnance Survey maps. This is part of a collection recently digitised and share by the British Library through Wikimedia Commons.
|Pinkneys Green, Cook Dean Common and Bisham Park Wood|
excerpt from 1809 Ordnance Survey drawing
Source: British Library via Wikimedia Commons,
© Crown Copyright under Open Government Licence v1.0
|Ditch marking compartment boundary in Bisham Woods|
Bluebell leaves can be seen carpeting the ground in the rear compartment
|Rhododendron under-storey in Bisham Woods|
Geological ContextBisham Woods mainly lies on a bluff of chalk above the River Thames. The chalk is part of a detached section of the Chilterns which lies South of the Thames, with the chalk being covered by newer strata close to the lines of the M4 Motorway, A4 road and the Great Western Railway about 5 km further South. Windsor Castle is built on another one of these outliers. A naive impression would be that the Thames may have originally flowed S of this block of chalk and more recently moved to cut it off from the remainder of the Chilterns. However, the curious change of direction which the Thames makes to travel N to Henley, then East to Marlow and finally South again at Cookham has a rather fascinating explanation: the Thames used to flow NE from Marlow!
The Thames changed course several times as various glaciations blocked this route. Additionally the land surface was different: at least some changes have occurred post-glaciation due to changes in isostay. Whatever the complexities of the full picture (and I completely fail to grasp them all), the Thames laid down river gravels some of which are now virtually at the top of the surrounding hills. Isolated patches of these gravels sit on top of the chalk or clay creating a different soil types: in Bisham Woods they belong to the Gerrards Cross Gravels.
A thin capping of newer 'rocks' overlay the chalk in places: mainly in the Southern extension of the wood and the farmland to the East. These are the Lambeth Group. At least on Sunday their clayey nature was fairly clear where horses and cyclists had been through the wood.
Thus through some interesting changes in the areas paleogeography the woods have several different soil types. It is abundantly clear that something affects which trees grow where and this is a major factor in these woods.
If you want to explore the geology more throughly the Geological Survey have a nice viewer (unfortunately not quite getting OSM attribution right).
Goulding's WoodThe southernmost part of the Bisham Woods complex is Goulding's Wood, it forms a gore between the A309 and arable farmland and so is quite narrow at it's southern end. I did nearly all my exploring in this part of the wood.
|Oak Woodland with a ground layer of Bluebells in Goulding's Wood|
(should look superb in 6 weeks time)
Note the Hazel coppice stools.
|Sycamore dominated woodland in the Northern part of Goulding's Wood|
Contrast this with the appearance of the same wood a couple of hundred metres away (above)
Elsewhere returning along a separate path I came across Oak woodland with a dense field layer of Bramble, and another, larger conifer plantation. There were also scattered (planted) larches near the top end of Goulding's Wood. I didn't have time to walk to the parts of the wood on chalk which are mainly Beech woodland (although there is an admixture of ornamental and other tree oddities as well).
Old Brick WorksThe last wood I visited is a bit different. This is the Old Brick Works at Pinkneys Green. It's a example of how varied the surface geology is here: the brick works used a patch of London Clay to make fancy roof tiles, many used in local housing.
Within the woods one still comes across remnants of the works: bits of old tramway, rusting machinery, slabs of concrete. The brick works closed in 1964 and the area was rapidly colonised by shrubs and trees. Much of the ground is very wet so Willows and Birches have done particularly well.
|Birches with Willow catkins in background|
Old Brick Works, Pinkneys Green
(note orange alga Trentepohlia on RH Birch trunk)
|Scarlet Elf Cup Sarcoscypha sp.|
This is a characteristic late winter species of Willow woodland. It seems to need frosts to appear and grows on old willow twigs in the litter of the woodland floor. (Just one illustration of why knowing what sort of woodland may be useful on a map).
The Brick Works is a classic example of how vegetation takes over places once they are no longer in use. I'm not sure exactly how to describe this plant community, but it is very similar to the willow regeneration which occurs on old Gravel Pits, with lots of Grey Willow (Salix atrocinerea) and masses of Bramble in the understorey, so probably close to the Salicion cinereae although probably lacking some indicator species in the ground layer.
ConculusionsThis sunny spring day was great to notice the differences between this different types of woods. Good sunlight helped, as did the absence of leaves in the canopy. Together these really make the colours and structure of the woodland floor much more obvious. The density of standard trees and their variety are also more obvious. In Britain it is not too difficult to identify most important woodland trees even without their leaves (and even then in many cases there are old leaves in the litter). One thing which is not so easy is recognising significant plants in the ground layer at this time of year, but this didn't apply in these woods.
I would encourage anyone interested in mapping (or just enjoying) woodland to find sunny days over the next few weeks and try and photo the salient features of woods you know. I think it's much easier in many cases than trying to interpret a wood in high summer. That being said, I hope we might be able to spend the Sunday morning after State of the Map in Karlsruhe doing just that. Watch this space.