The structure of woodlands

Woodlands are made up of layers  It is fairly obvious, when you visit any woodland, that there is tall stuff and short stuff and stuff  in between.

Being a scientist/ecologist I like to firm this up a bit. When I taught ecology to ‘A’ level students at the East Anglian Field Study Centre we inevitably had a day on woodlands.  On some occasions we went to a nice coppice with standard woodland which was called Honeypot Wood and on other occasions we went to Thetford Forest, a Forestry Commission plantation, but even there I managed to find some interesting ‘stuff’ and some layers.

I used to teach the students that woodlands are ‘stratified’ and that in a natural British woodland there are potentially 5 strata.  The important phrase there is ‘natural British woodland’, because that probably does not exist. There are areas described as ancient woodland which simply means that a woodland has existed on that site since 1650.  There is no guarantee that  the areas have always supported a woodland, since the last ice age, and definitely no guarantee that it has not been modified in some way by man so therefore it is not strictly natural.  Probably there is no natural woodland anywhere in the UK.  Possibly there is some in Poland and Ukraine but not here. Anyway let’s get on to the strata.

There are five layers or strata.  Starting from the top they are the first tree layer, the second tree layer, the shrub layer, the field layer and finally the ground layer.

Woodland strata diagram by Alan Waterman

This is a diagram which I have drawn to illustrate the strata  ( Sorry only got a B- for Art).  I tried to find one via google to save having to draw it myself and discovered that there are all sorts of different interpretations of the layers of woods, with layers such as emergent, canopy and understory,  so be aware that other terminology exists. This is an example of what I found.

However I think that my subdivisions are logical and it is what I taught to thousands of students in the past, so I am sticking with it.

Let’s look at each layer in more detail. None of this is rocket science – it is all fairly straightforward.

Magnificent Oak TreeThe First Tree Layer; This is the fully mature large trees which will reach a height of 40 metres, the Oaks and the Beeches of our natural woodland. It will include some other trees such as Ash and Lime and if you include introduced species then there are several conifers like Larch, and some deciduous trees such as Sweet Chestnut. This is a nice example of a  tree which would qualify for the First Tree Layer, it is an Oak and is growing at the RSPB Nag’s Head reserve in Gloucestershire. Probably 250 years old, however you could find out roughly its age… have a look at my article on How to age trees.

The Second Tree layer; Now this is a bit more of a mixture. This is made up of smaller trees growing roughly to 20 metres in height, but with a large degree of variation. These layers blend one into another, so do not expect to go to a wood and see sharply defined bands. The second tree layer will contain trees of the same species as those in the first tree layer, i.e little Oaks and little Beeches waiting their turn to possibly develop into  big trees.. If at some stage the large Oak towering above them dies or is struck by lightning or the woodman’s axe, then the smaller Oak will be able to replace it. Likely as not this will not happen and the little Oak will remain small all its life.  We also have in this layer a selection of trees which are simply little trees and will remain so all their life no matter how much light they are given. These include trees like Silver Birch, Hornbeam, Yew, Crab Apple, Bird Cherry, Spindle and many others. Some like the Silver Birch can get quite tall and almost push their way into the first division of trees, a bit like the Watfords and Crystal Palace’s of this world, but others like the Spindel and the Crab apple will always be the minnows, the Accrington Stanleys of the tree world.

Wind blown HawthornShrub Layer;  Shrubs are woody and have multiple stems, so  they include bushes like Hazel, Elder and Sallow. There are shrubs which can, over time, become almost tree like, – I am thinking of Hawthorn and Holly. Then there are really low bushes like Bramble and Gorse so this layer covers a multitude of species, some which might reach 6/7 meters high whilst others  rarely get above 2 meters.  In this category we also get the climbing plants such as Ivy and Honeysuckle.

The photo to the right is of a Hawthorn growing at Holme Dunes in Norfolk, quite windy there sometimes! But it is almost like a little tree.

Field layer; This is the layer in which we get all the typical woodland wild flowers, so it includes the Bluebells,  Foxgloves and  Wood Anemones. However it also includes all  other flowering plants such as the grasses sedges and rushes and all the non flowering ferns.

Ground layer; As the name suggests these are the plants which grow right on the floor of the woodland, so this consists of the mosses and a related but more  primitive group called liverworts.

Fungi do not come into these categories as they are not green plants and in modern classification they are not even plants. Lichens are also excluded as they can occur anywhere from the highest branches to the lowest levels of the woodland growing, for example, on a fallen bit of bark which is really in the ground layer.

So this is what I understand as the stratification of woodlands. I think it is logical. It makes sense to me and I hope it makes sense to you.   When you next visit a woodland take a few minutes to stand back and examine it and look and see and appreciate the different strata. They may not all be there but I would be surprised if you can’t find at least three.

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