Vegetation Layers in an English Woodland

The different layers within a woodland are one of those classic illustrations from ecology textbooks – usually a cartoon graphic showing how the structure changes as you move from the ground up to the canopy. The distinctions are indicative – there is often plenty of overlap between the layers – but these are a useful way of thinking about the woodland in terms of its ecological function beyond what simply meets the eye. Helping you to see the wood for the trees, if you will!

We undertake tree climb and inspect surveys for bats in a variety of settings – from individual trees in gardens to well-spaced parklands and dense woodlands. Each of these gives a different insight into vertical variation but a recent climb in an oak and ash woodland gave a nice opportunity to illustrate how this changes in a typical ancient woodland habitat.

English Oak Woodland Structure
Photographs taken at 2m (bottom), 6m (middle) and 12m (top) in an English oak and ash woodland to illustrate the variation in spatial structure.

The number of ‘layers’ and their distinctions vary between sources and across countries – I’ve seen three defined layers, nine defined layers and every number in between delineated in various graphics. I’m going to keep this simple and focus on the three broad categories which are shown in these photos:

Field Layer – taken at 2m height

Ground Layer
Photograph taken at 2m showing the field or ground layer in an English oak and ash woodland


This photograph encapsulates what could be considered the field layer, the ground layer or the forest floor depending on which divisions you use. Broadly, this is the view from the ground – the most apparent vista for most visitors to the woodlands but the scene varies greatly as the year progresses. In secondary woodland or more botanically diminished sites, this can be a mass of ruderals such as nettles, cow parsley and bramble whilst some plantation woodlands can be head-high in bracken. In ancient woodland – such as this – springtime sees a flush of ancient woodland species which time their flowering early in the season before the canopy closes overhead. As May arrives and the trees and shrubs come into leaf, the main event on the forest floor is already coming to an end. Some species flourish later in the season but in September, the vegetation has largely died back often leaving a relatively bare floor. The dominant vegetation remaining is therefore the trunks of the trees and the shrubs which rise above the field layer to leaf and flower higher.

Scrub Layer – taken at 6m height

Shrub Layer
Photograph taken at 6m showing the hazel-dominated shrub layer in an English oak and ash woodland

This could be described as the under-canopy, the scrub layer, the shrub layer or the under-storey layer. This is the level at which the shrubs flourish – those smaller woody species which often include hawthorn, blackthorn, hazel, holly, dogwood and elder, amongst others. These shrubs can often form such a dense canopy that the view to the treetops above can be quite obscured – something of an issue when you’re searching for bat roosts! In this instance, hazel and hawthorn dominate and as the photograph shows, they form a dense and highly cluttered environment some 6m above the ground.

Canopy Layer – taken at 12m height

Canopy Layer
Photograph taken at 12m showing the space between the shrub layer and the woodland canopy in an English oak and ash woodland

This is up at the top of the trees, where the taller trees branch and close their canopies to claim first use of the sunlight available. I’m not right in the canopy in this photograph – the trees did not require us to climb to the peaks – but this nicely illustrates the shrub layer below with the high canopy of the oak and ash above. In some forests, the species composition includes a sub-canopy of smaller trees – such as rowan, silver birch and field maple – which form a layer between the lower shrubs and the taller climax species. This photograph nicely illustrates how much of a space can open up between distinct layers within the higher reaches of the forest structure.

Why does it matter?

An appreciation of how the character and conditions of a woodland change on a vertical plane are just as important, although a little harder to appreciate, than the variation on the ground. The illustrations above are for just one section of just one woodland and each type of species composition but they help to give a visual idea of how this works.

One example of how this type of variation is important is when considering how bats might use a habitat. Different species of bat have different hunting characteristics and habitat preferences, and this often relates to how open or cluttered a habitat is. Species such as brown long-eared are highly maneuverable and will glean insects from leaves so a reasonably cluttered woodland environment suits them perfectly. At the other end of the spectrum, a noctule hawks in open space, often flying over fields and catching its prey on the wing. Species such as pipistrelle are typically characterised as edge specialists which forage along woodland edges, hedgerows and other similar environments.

Pipistrelle bats are typically described as ‘edge’ species which forage along the edges of vertical features, such as a woodland edge

The field layer of the woodland looks ideal for a species such as brown long-eared but you might think that it would be too cluttered and enclosed for other species. However an insight into the structure above the shrub layer reveals a mosaic of open space and leaf cover which provides perfect ‘edge’ habitat for a hunting pipistrelle, and the voids between the trees would be perfectly navigable for a noctule in search of a roost. This was the subject of a talk by Ian Davidson-Watts at a recent BCT Bats and Woodland conference which you can read more about in this blog post here.

If you’re looking for tree climb and inspect surveys for bats, do check out this page for further details!


What do you need to climb-and-inspect trees for bats?

I have put this post together as a brief guide for anybody wanting to know more about what is required to climb and inspect trees for bats. When I was looking into the requirements, I couldn’t find anything which simply set out the steps so I hope this will be useful!

Climber in an oak treeTrees can be difficult to assess for bat roosts. Sometimes you can spot a feature – a woodpecker hole, split or fissure for example – which you can identify with reasonable confidence as a bat roost, perhaps by signs such as scratch marks, fur rubbing or droppings. But, in my experience, these types of features are the exception. You are much more likely to spot, what Henry Andrews would call, PRF’s or Potential Roost Features – see his website here for perhaps the best resource I’ve come across dealing with the use of tree roosts by bats.

You might want to be have more confidence in your assessment for a range of reasons; perhaps as a local bat group simply gathering data on local roosts, perhaps as part of research into bat distribution or movement, perhaps to protect the bats for example if the tree is dangerous and must be removed.

There are two approaches that can then be taken. Firstly, you could carry out a dusk/dawn emergence survey to watch for bats leaving or returning to a roost. This is quite labour intensive and has a number of limitations, such as the difficulty in being confident that a bat appeared from a particular location and the transient use of many roosts providing little confidence in a negative result. A second option is to inspect the feature closer up, using an endoscope perhaps to look within and see whether there are any bats residing within and whether the feature is actually as suitable as it may appear from the ground.

In order to carry out this “climb and inspect”, you will need two things:

Firstly, a licence to disturb bats with a handling endorsement. This is because bats are legally protected under both domestic and European legislation and it is an offence to intentionally or recklessly disturb them in their place of rest. See the BCT website for more info. This licence requires a large amount of specialist knowledge and experience of bats and their roosts. If you are interested in gaining the experience and, ultimately the licence, getting in touch with your local bat group is the best place to start. If you are already working in a professional capacity, I can highly recommend the Bat Licence Training Course run by Richard Crompton and Sandie Sowler.

Secondly, you need the competence and equipment to safely ascend to the feature. The basic qualification for this is the City and Guilds CS38 – Tree Climbing and Aerial Rescue Operations course. This not only teaches you the basics of climbing but also how to rescue somebody from a tree – if an accident does occur, it is critical that you know how to safely bring somebody down to the ground where they can be treated. This is usually a week long course which takes you through the principals, the theory, the legislation and the best practise for tree climbing before teaching you the practical techniques of climbing. At the end of the week, there is usually a separate assessment taking several hours where your competence will be assessed. As well as the climbing technique and principal, there is a requirement to identify a range of common tree species so this is something to brush up on beforehand. Another hint – you will probably not need to spike up trees for climb-and-inspect work, this is damaging to the trees and is only to be done if you are an arborist who is going to take the tree in question down (or in an emergence rescue situation).

Once you are qualified to climb, there are a few more things you will need before you can start:

1) The climbing equipment. The basic kit consists of a harness with leg-loops, rope, prussock cords, caribinas and a secondary support strop. These must all conform to the minimum legal requirements.
2) The safety equipment – this includes a helmet with a chin-strap, a knife with a retractable blade, a first aid kit and sturdy (steel toe) boots. High visibility jackets are also useful, especially for a groundsman. See point 3…
3) A groundsman! If something happens in the tree, you must have somebody with you who is qualified to climb and perform an aerial rescue. This might be required is for example there is a problem with your equipment meaning you can’t descend upon it, or if you were to hit your head and become unconscious. You should only ever climb if you have this second qualified climber on the ground the entire time that you are in the tree.
4) The kit to inspect the feature – an endoscope is either an eyepiece or, commonly now, a screen attached to a fibre-optic snake which allows you to see around corners and in deep, dark cavities of trees where bats might be hiding. This has the potential to cause disturbance to bats which is why the relevent licence and experience are critical. Other pieces of kit which might be useful include binoculars, a torch and a mirror!

If you were climbing for your own pleasure on your own land, this may be all that you require. However, if this is being undertaken in any kind of professional capacity, you would also need the appropriate insurance in terms both of personal safety and public liability.

For more information on the assessment and CS38 qualification, you can see the assessment criteria provided by NPTC here.

I work for a local Ecological Consultancy based in Grantham and we do offer professional climb and inspect services in the East Midlands area and beyond – if you would like more info then drop me a message and I will get in touch with you!

Potential Roost Feature (PRF) worthy of closer inspection!
Potential Roost Feature (PRF) worthy of closer inspection!