Have you ever wondered what the difference is between a tree and a shrub? The answer does not always seem clear. Is an ancient oak, growing small and stunted on a mountainside classed as a shrub or a tree? In this article you’re going to learn the difference between trees and shrubs. It’s a question that many of the students on my FREE Tree Identification Course have asked me, so I thought I’d tackle it here.
When we think of a tree, usually the image of a majestic oak comes to mind. Standing tall, proud and ancient with a wide crown of leaves. But a low-growing mountain-side juniper with it’s spreading habit is just as much a tree as the mighty oak. Let’s find out why…
WHAT MAKES IT A TREE?
Firstly, what differentiates trees from other plants? Well, a tree puts on woody growth which is permanent, where as with annual or perennial plants the shoots or whole plant dies back each year. Many of our wildflowers have to start from scratch, reproducing from seed each Spring. Whereas our trees will sit out the winter waiting for the warm weather to return.
It is the cambium layer, which lies under the bark of a tree which produces new permanent cells. As well as performing a bunch of important jobs for the tree these cells also form the woody trunk and structure of the tree. With all trees the thickness of their stems increases year on year (when healthy) which means that the diameter of the trunk and branches gradually increases. It is this annual growth which gives a tree its rings by which we can count its age.
WHAT IS A SHRUB?
The word “shrub” isn’t a botanical classification, but rather catch-all term for a plant which has more than one main stem, is generally less than 5 metres tall, and grows with a spreading habit.
So, the term “shrub” can refer just as much to a perennial garden bush like a hydrangea as it can to a woodland tree such as a hazel.
One of the most common examples of trees in shrub form is the hedgerow. Here hawthorn, blackthorn, hazel, holly and others are clipped back annually to form the dense, linear hedges we’re so familiar with. Hidden amongst all this there can often be species which we think of as trees such as oak, ash and sycamore, which too have been clipped back. So, does this make them shrubs or trees? I’ll let you ponder that one.
Here’s a great quote from the Reader’s Digest Field Guide to Tree & Shrubs of Britain which sums it all up;
“The difference between trees and shrubs is simple. Trees have a single woody stem, from which branches grow to form a crown. The branches of shrubs arise at ground level, forming a crown without a stem.”
But what’s truly going on here? To understand this we need to look deeper at the natural and human forces affecting whether a woody plant grows as a tree or shrub.
HOW SHRUBS THRIVE IN THE WOODS
In the woodland, the smaller trees which live happily in the shadow of their taller brothers (the oaks and ash of the forest) occupy the understorey. This is sometimes referred to as the ‘shrub layer’ of the woodland. These trees typically won’t grow taller than 5 metres, having found their niche in the habitat here.
Think of a woodland understorey made up of holly. Being an evergreen it sits very happily in the shade of taller trees through the summer. This is because once the leaves of oak and beech have fallen the holly can photosynthesise to its hearts content and take advantage of all that available light.
Meanwhile many shrubby trees come into flower or leaf earlier than the trees species in the high canopy. Blackthorn puts out its blossom before any other tree, ensuring that the pollinating insects give it their full attention before there’s too much competition. Meanwhile, the tiny red flowers of hazel can appear as early as December. The elder is our first native tree to come into leaf, as early as January, catching that winter sun. It’s amazing how our woods have evolved so that everyone gets their moment in the sun. There’s a beautiful interconnected poetry to it.
WHEN A SHRUB BECOMES A TREE & VICE-VERSA
So, can a shrub become a tree? Well, usually if a tree has been continually cut back then the result is a whole lot of stems. These will continue to grow in this shrubby form, though there will be some stems which die back naturally as the healthiest ones grow on, but it will always be a shrub.
In a woodland setting a hawthorn will often be in shrub form as part of the understorey. However, out in a field, where it has seeded with sufficient light and space it can grow with just one or two main stems and be very much a tree.
There are only a couple of ways a shrub can become a tree. First, when it is either deliberately pruned by us to take back its tree form, though it will always want to put out new shoots from the stool. The other route to true tree-dom is if you have a tall tree species such as oak, sycamore or beech which have been cut back in the past but then neglected for many years. These will usually form into trees, however they often still have more than one main stem. We see this in neglected coppice woodland and in over-stood hedgerows.
On the flip side through traditional woodland management like coppicing, some tree species are encouraged to grow multiple stems growing from their base (known as the stool). The regrowth of rods put out by the tree can then be harvested on an ongoing cycle. These can be made into coppice products such as charcoal, wooden furniture, firewood and turnery.
Even normally tall-growing trees like oak, ash and sweet chestnut respond well to coppicing. They can be encouraged into this shrub-like form so their multiple stems can be harvested on a 15-30 year cycle.
SHRUBS: A DEFINITIVE LIST
Here’s a list of our common native trees that can often be seen growing in shrub form, many of which are found in hedgerows and waysides across the UK;
hazel, elder, juniper, sea buckthorn, crab apple, hawthorn, blackthorn, buckthorn, alder buckthorn, box, dogwood, guelder-rose, field maple, spindle, holly, rowan, wayfaring tree, wild privet and any of the willows.
It can be so interesting to really look in to the details of our native trees and notice the changes that they undergo throughout the four seasons. That’s just what I’ve created for my FREE introductory online course called Kickstart Your Tree ID Skills. Here you will find a whole host of resources to take you from clueless to confident on your way to really knowing your trees.
When you sign up to this free mini-course you’ll be identifying common trees with video tutorials and photo galleries at your fingertips. Start your journey to becoming a fully fledged Tree Expert today. The course includes Tree ID Cheat Sheets which you can download to your device and take outdoors with you.
“I’ve been frustrated for so long trying to learn my trees myself and haven’t gotten far. This course answered everything and has seriously upped my game.” Dr. Patrick Alexander
GET TO KNOW YOUR TREES
I’ve also created some videos on how to recognise one of our most common shrubby trees, the hazel. You can follow me in either winter or summer and learn the key features you should be looking out for so that you can recognise this species whatever the time of year. Check out the videos below.
Happy tree hunting folks.