In the first of his weekly series, Mike’s Easter Monday message focuses on our wonderful oak trees.
In addition, each week he will be sharing a glimpse of how nature is changing as we move through the seasons:
This week: Holly blue and orange-tip butterflies made a welcome appearance. Blackthorn blossom is fading as cherry edges toward centre-stage; broom flower is joining gorse, hawthorn is just about at bud burst and far earlier than usual (whatever usual is these days), our oaks are in leaf.
OAKS ON THE COMMON
English Oak (Quercus robur) The only native oak on the Common, the English oak rightly holds a special place in our culture, history, and hearts. It supports more life than any other native tree species in the UK. Here on the Common, nearly half our 4,000 trees of size are naturally seeded oaks.
Holm Oak (Q ilex) Introduced to UK in the 1500s. Not as adapted as our native oaks, it supports plenty of our wildlife and is the genus’ only evergreen species. On the Common they are now almost invasive, having ‘escaped’ from the Old Cemetery where some were planted.
Turkey Oak (Quercus cerris): Introduced to the UK in the 1735, it is now impacting our native oak populations. Less valuable to wildlife, but much faster growing and a host of the knopper oak gall wasp.
American Red Oak (Q rubra): Just when this fast-growing North American native was introduced to the UK and by whom is a bit of a mystery. However, most authorities seem to agree that it was first cultivated in the UK by Philip Miller at the Chelsea Physic Garden in 1739 with an earlier date of 1724 given although it is unclear where information came from for this. It certainly appears in seed box lists from the famous partnership between early American plant collector John Bartram, Peter Miller and Peter Collinson from as far back as 1744.
Not as valuable to wildlife as our native oaks, but popular with nesting birds, pollinators – and autumnal photographers! The ones you see on the Common and at Vine Road were all planted originally and there is also evidence of self-seeded saplings.
How old are our oaks? Although we estimate our oldest oaks at 150-200 years old, which coincides broadly the time grazing ceased, most are less than 75 years old. Our oldest trees grew up in the open. Paintings and early photos confirm the Common was almost bare of trees until well into the 20thCentury.
How can we identify oaks?
The bark is brown, rough and especially on the trunk near the ground, deeply rutted. The outline shape is typically ‘square’, with outer branches reaching up almost as high as the central lead branch, but in denser woods the shape will have a tall trunk, often with dead side branches. This relates to how the trees have been planted and how they compete for light.
Like all plants, light from the sun is the food and energy source and oaks will do all they can to reach for it. In dense woodland, trunks grow straight and tall, making them good for timber. Places like the New Forest were created and managed to produce timber for ship building. More recently, following the fire at York Minster, these were the only trees in the UK large enough to replace the ancient beams. Restoration had to wait for the timber to be cut and to dry naturally.
However, when there is no need to fight for light the tree branches low and spreading – why waste energy? The result is lovely trees for climbing in and we have about six of these. Generations of children have grown up with their favourite climbing tree – in some cases now into second and third generations. The Oak and the Dragon is a wonderful interactive story based around our oldest oak, written by Sharon Morgan and read by Niamh Cusack as part of the OSO Bedtime in Barnes project
In Spring, the oak’s distinctive lobed leaf is one of the first young children learn to recognise. Later in the year come the familiar acorns (nowadays many may be deformed into wrinkly ‘knopper galls’ caused by a small solitary wasp). You may also see other galls. One of our most common – the marble gall – is particularly obvious, as it dries into a round brown ‘marble’ which hangs on the branch a long time. These galls used to be much prized as a source of dye for ink and are not harmful to the tree.
The value of our oaks: A healthy mature oak may have as many as 250,000 leaves that, through the wonder of photosynthesis, use the energy in sunlight to convert carbon dioxide and water to sugar and oxygen. Sequestration, the continual process of storing carbon, is a natural product of tree growth. Young trees absorb carbon more quickly, but older trees have greater storage capacity as well as being host to a greater number of species, so there is huge value in having mixed ages in a woodland. The economic value of trees is still being assessed but runs into tens of thousands of pounds each. In urban settings, where trees help mitigate the increasing impacts of climate change by improving air quality, absorbing harmful particulate matter and lowering temperatures, the value is likely to be far higher.
Doing so much good makes it hard to realise that trees can also be ‘weeds’ and may find themselves simply in the wrong place. Indeed, a tree is no more welcome in the acid grassland than a dandelion in a Wimbledon tennis court. Find out more about how and why we manage trees on the Common in this conservation update from Will.
Symbiosis and biodiversity: The English or Common oak is host to more invertebrates than any other tree species in the UK. In June/July there may be as many as 110 different species of butterfly and moth living on oaks – as caterpillars or hatched – including purple hairstreak butterflies, tortrix moth (with little green caterpillars that hang on silk threads to avoid predators), as well as oak beauty, brindled beauty, mottled umber, oak hook tip and many more.
A hazardous and unwelcome visitor is the oak processionary moth. Current recommended guidelines and information from the council are available here: Oak Processionary Moth Warning
Oaks grow easily from acorns – but there will have been many thousands of acorns for each single tree that reaches maturity. Did you know that squirrels take a nibble out of acorns before burying them which is certainly not helpful for germination? A lesser-known and interesting partnership is with Jays. These birds love to stash acorns to feed on in winter. They often do this out in the open meadows, some distance from the parent tree. Through winter, they do not always find their stash and if mice and other animals also miss them, then they will germinate, and you may get a sapling up to half a mile away from its parent. Indeed, this is how a forest can ‘walk’ over the centuries, notably in response to climate changes. It is evolution and adaptation at work, with the trees in the best locations doing better than those that find the going tough.
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References and Further Reading:
Bridgeman Karen; Painshill Park; pers comm 17.04.2020
British Oaks and their History
Woodland Trust A – Z of British Trees
Wohlleben, P, Flannery T, Billinghurst, J, 2016. The Hidden Life of Trees. Vancouver Canada: David Suzuki Institute
Mike Hildesley 13th April 2020