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
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!

 

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Green hairstreaks at Barnack Hills and Holes

On the way back from a site survey in Cambridgeshire, I had the opportunity to call by and take my lunch break in Barnack Hills and Holes in the sunshine. Having seen a photograph posted by Nottinghamshire Wildlife Trust the previous day, I knew that green hairstreaks were on the wing so I went off in hunt of these iridescent little jewels!

Green hairstreak (Callophrys rubi) can be hard to track down for a few reasons; firstly, they are wonderfully camouflaged against the leaves – seeing one at a flower can be almost like noticing that one of the leaves is drinking! Secondly, their populations can be small – some colonies are only a few individuals strong and so tracking them down on a large site could be time consuming.

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Spot the hairstreak drinking from the hawthorn flower!
Luckily, after a pleasant half an hour wandering through the cowslips and orchids, I came across several on a hawthorn shrub. It was a windy but sunny afternoon and this hawthorn bush was more sheltered than many others. When in flight, the butterflies reveal the brown uppers of their wings making them easy to spot on the wing – but they always rest with their wings held together and blend beautifully into the greenery.

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Green hairstreak amongst the hawthorn leaves
I watched two males take up sentry posts, settling on hawthorn buds or leaves, poised to leap into action and defend their territory against intruders. Other males were the most targets likely to be chased, along with other insects and butterflies of other species, but they also use this vantage point to spot females on the wing. When resting and waiting, these males tilted their wings towards or away from the sun, micro-managing their temperature with these movements.

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Male green hairstreak poised on a hawthorn bud ready to intercept intruders
Green hairstreaks are widely but locally distributed in the UK – this means there is likely to be a colony somewhere near you but they have particular habitat requirements so you might need to go to just the right place to find them. They occur in a range of habitat types including dry heathlands, grasslands, old railway embankments, woodland and quarries. The key habitat requirement seems to be open grassland with a good scrub component. Adult food plants include trefoils, vetches, hawthorn, gorse and spring flowers such as cowslip and bluebell. The larval food plants are similarly diverse including trefoils, dogwood, vetches, gorse and broom.

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Green hairstreak on a hawthorn flower – one of the favoured food sources for the imago (adult butterfly)
These butterflies are on the wing early in the year – often in April and early May through to early-June or perhaps later in the north. Barnack is a great place to spot them but this site can help you find a location near you!

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Green hairstreak standing guard over his territory on a hawthorn flower