Know Your Oaks — WildArt Photographer of the Year

Know Your Oaks

by

Ian Parsons

This week, BBC Radio 4 ran a piece on the poor show of acorns this year on many of our Oak trees. This follows a bumper year in 2020, the differing spring weather patterns between the two years no doubt being the main reason for these polar extremes, although a reduced crop following a bumper year is not unusual. Acorn production is not only important for the ongoing succession of one of our most revered trees, but is an important food source for many other species, the Grey Squirrel and Jay being the ones that first spring to mind. This snippet reminded me of the fantastic series on trees Ian Parsons wrote last year and I thought I would share his article on identifying the two species native to the UK, the Pedunculate and Sessile Oak.

Rob Read.

It may come as a surprise to know that we have two species of Oak that are native to Britain and both of these species are very common. They do look alike, especially in the winter months when separating the two is very difficult indeed, but in leaf they are actually quite easy to tell apart. The two species that are native to Britain are the Pendunculate Oak (Quercus robur) and the Sessile Oak (Quercus petraea), with the common names referring to the main differences between the species.

Pedunculate is a biological term meaning growing on a stalk; sessile is also a biological term that is the complete opposite of pedunculate, it means that there is no stalk. So, pedunculate equals stalked, sessile equals no stalk. The stalks (or lack of) that are referred to in the common names do not relate to the leaves - instead they are in relate to the fruits of the tree, the acorns. The Pedunculate Oak has its acorns on a stalk, while the acorns of the Sessile Oak have no stalk. However, in what seems like a deliberate arboreal ploy to confuse matters, the leaves of the Pedunculate Oak have no stalks (well, very, very short ones) while the leaves of the Sessile Oak have very obvious long stalks!

The shapes of the leaves also differ slightly, both have the typical oak shape we are all familiar with, but the leaf of the Sessile Oak tapers down to its long stalk, whilst the leaf of the Pedunculate Oak ends in two lobes either side of the very short stalk - often completely obscuring it. With these small differences, and the possible confusion with unstalked leaves and stalked acorns etc., you can see why we tend to generically refer to the trees as just Oaks; but as it happens, our two species aren’t typical Oak trees at all.

Above: A classic ‘hanging’ Sessile Oak woodland in Wales. ©Rob Read.

There are around 600 species of Oak in the world; the majority of these species are actually evergreen, i.e. they don’t shed their leaves in the autumn like our two native species do. We also tend to think of Oaks as being rather English, but with only the two species we are very ‘Oak poor’ when it comes to the number of species we have - especially when compared to Mexico which has 109 endemic species of Oak that are found nowhere else in the world.

We do use the label English for the Pedunculate Oak, but it is a bit of misnomer as both this and the Sessile have large natural ranges that encompass most of Europe and parts of Asia. It is possible to be walking in natural ‘English’ Oak woods in countries as far apart as Portugal and Iran.

Apart from the stalks and the leaf shapes, the two Oaks in Britain can also be separated by their preferences. Trees are just big plants and, as any gardener knows, many plants prefer different types of soil and climate over others. Broadly speaking, the Pedunculate Oak is a tree of the British lowlands, preferring life from sea level up to 300m, particularly favouring deeper, richer soils. The Sessile Oak is a tree that prefers the uplands, 300m above sea level and above, areas of high rainfall and doesn’t mind shallow acidic soils. The western half of the British Isles offers ideal conditions, and it flourishes in this part of the UK in particular.

One of my favourite woods is Wistman’s Wood in the centre of Dartmoor in Devon. It is a place of legend and myth, a place the where the Wisht Hounds supposedly run free… Wistman’s Wood is an oak woodland, but if you ever visit it don’t expect towering straight stemmed trees. Growing at 400 metres above sea level on the high moors of the south west means that the woodland is very exposed to strong winds and lots of rain. As a result, there are no majestic oaks here; instead you have a motley collection of twisted and contorted stems, often growing horizontally between large granite boulders - these stems are covered in mosses, lichens and ferns. The trees only manage heights of between four and five metres, mere dwarves when compared to the oaks we often picture in our minds. Majestic they may not be, but they are truly beautiful. It is a stunning place that shows you the extremes that trees can survive in.

As I have already mentioned, our two oaks differ in their preferences, the books tell us that the Sessile is the oak of our western uplands and the Pedunculate is the oak of lower, drier altitudes. With Wistman’s being so high up, in an area of high rainfall and in a landscape of acidic soils, the oaks that make up the woodland have to be Sessile Oaks. Except, of course, they are not. Proving beyond all doubt that trees don’t read books, especially the Oaks of Wistman’s Wood as they are all the lowland loving Pedunculate Oak!

Both of our species of Oak are known for their longevity and many of our veteran trees in Britain are examples of them, great hollow hulks of trees that have long captured our imaginations. You have the Royal Oak, a Pedunculate Oak in Shropshire in which the future Charles the Second hid, then there is the hugely squat Sessile Oak in West Sussex called the Queen Elizabeth oak under which Queen Elizabeth the First waited with her bow to hunt deer that were being driven towards her (apparently she missed!). Stretching our historic imagination a bit further is the Major Oak of Sherwood Forest, supposed home to that most enduring of outlaws, Robin Hood. Featured in one of the accompanying photographs is the Big Belly Oak situated alongside the A346 as it runs through Savernake Forest near Marlborough in Wiltshire. This tree is thought to be around 1,100 years old, which means that it would have first taken root around the time of William the Conqueror – a humbling thought.

Above: The Big Bellied Oak in Savernake Forest is over 1,000 years old. ©Rob Read.

Both of our oaks can form large trees; Sessiles can be slightly taller than Pedunculates, but both can easily attain around 40 metres in height (130 feet) when growing in good conditions. Their shape is easily recognisable even at a distance, the large rounded dome like canopy of the mature oak is a feature of many of our landscapes, but they are not the only oaks within it. The Turkey Oak (Quercus cerris) is a tree that was introduced to Britain in 1735 and has subsequently naturalised and can now be found growing throughout the country. It has the typical oak-leaf format, but the leaves are more leathery in appearance with a shiny upper surface. Generally, they are very slender leaves and more deeply lobed than our two natives, but they can also be highly variable. One thing that isn’t variable though are the tree’s buds which have long bracts around them giving them a hairy appearance, a ‘hairiness’ that also applies to the acorn cups which are covered in hair-like protuberances. From a distance a Turkey Oak can be told from both of our natives by its pyramidal shape, the leading shoot is dominant and instead of forming a rounded dome it looks more like a triangle on the skyline.

Oaks are a major part of the environment and the natural heritage of Britain, and you generally don’t have far to go to find one. So, the next time you see one, have a good look at the stalks and see if you can spot your Sessiles from your Pedunculates!

Ian Parsons.