Recognising….open trees

By Megan Gimber

2020-09-14

Picture a tree in a woodland

What does it look like? I am going to guess that you are picturing a tall tree, with very few branches until the top canopy where it reaches the light.

Now picture a tree in a field

I bet the tree you are picturing has branches lower down the trunk, it has a full billowing crown, and maybe, just maybe, a thicker trunk.

If I was right, your imagination has already shown you many of the differences between trees grown at high density and those grown in open habitats. Good work!

Put simply, if you grow trees from two identical acorns, one in a woodland and the other in a field, they will result in two very different looking trees. And this is because as trees grow from little saplings, they grow in response to the light conditions around them.

For your acorn grown in a woodland*, the best sunlight is gathered all the way up in the canopy, so trees grow straight up to reach it. Side branches are lost as the tree gets taller as they are no longer very useful for harvesting light. The result is a tall, thin tree with few side branches.

For your acorn grown in a field, the story is different. Those first branches it grows when it is just small continue to collect light, and so continue to be useful and to grow. As the tree gets taller it sends out more branches, all around the trunk to collect light from all angles. The result will be a tree with many branches, in all directions, starting low down towards the base of the trunk all the way towards its natural height.

Oaks grown in a woodland compared to one of a comparable age grown in the open. Woodland trees grow tall, with few branches, stretching for the light at the canopy. Those grown in open habitats spread their branches in all directions to capture sunlight 

So why is this important?

With well-founded concern for both the environment and the state of nature, we are looking at increased interest in tree planting across the UK. And this is to be warmly welcomed! Trees are amazing, we need more of them and so does the atmosphere and nature. But we need to work out what kind of trees we want, and the density at which they are grown is an important factor in this (it is one of many important considerations, to be sure, but I see this as a biggie).

Open grown oak, Stodmarsh Reserve, Kent

Young oak at Stodmarsh Reserve, Kent, showing the classic structure of a tree that has grown its whole life in the open; branches radiating in all directions, all the way down the trunk, and forming the lovely, even, cushion-like crown

Now I'd like you think back to the thicker trunk of the open-grown field tree. There is a reason your clever imagination pictured this. Trees grown in open habitats tend to live longer than those grown under the high canopy of woodland, thus more likely to develop those thick trunks indicative of age. Only open grown trees generally get the opportunity to fulfill their natural lifespan, and grow into post-maturity to become veteran and ancient trees. And this is important due to the frankly incomprehensible array of wildlife that an ancient tree will support.

Again, one of the main reasons for the difference is light; it is said that an oak** takes 300 years to grow, 300 years to mature, and 300 years to die.

And in the last stages, they go through a process called retrenchment; when the tree no longer has the strength to maintain its full canopy it re-grows a smaller, lower one and loses the high branches. This is why many ancient trees often seem so short – they weren't always so!

Now fast forward your imaginary oak seedlings, and picture your woodland tree going through retrenchment– as soon as it lowers its canopy it will be outcompeted by taller trees, shaded out and will quite quickly decline and die. This is why it is rare to see ancient trees in closed canopy woodland – even ancient woodland, and when you do, it is often an indicator that the area used to be more open. This story isn't just true of our shade intolerant oaks - all of our native trees become less able to cope with shade as they pass through their mature years into their ancient ones.

And for your tree going through the same process in open pasture - no problem! It still gets enough light to survive, and it can gracefully decline, sometimes for hundreds of years in this new form.

Unfortunately, we have already seen what happens to ancient trees when we plant plantations around them, and it's not worth repeating. A lack of recognition of the value of the open habitat surrounding ancient trees is dangerous in a time when we are scrambling to find space to plant. And in a similar vein, if we think it's important that we have ancient trees in the future, we must make sure we allow space for the establishment of trees in open habitats as well as planting woodlands.

The value of ancient trees

There have been whole books written on this topic, so I can't do it justice here. In short, inherent in their structure is a myriad of different micro-habitats that in turn are home to a diverse range of wildlife. Hollows for nesting birds, bees and bats; sap runs; bark flaps; water pools and snags. These old trees are also perfect for mosses and lichens, which can be quite fussy about where they grow; only the large, horizontal branches, with dappled sunlight and sheltered, humid conditions that the lower limbs of a large old tree can offer are good enough for some lichen species. It is not difficult to see why many are now so rare.

But needless to say, the dead and decaying wood found within the ancient trees is really the star of the show; this increasingly rare habitat is home to some of our most endangered species. Decaying wood such as in the centre of a hollowing tree supports over 2000 species in the UK alone. Even this decay goes through several successional life stages, so the longer the tree survives in this decaying condition the more valuable the habitat becomes.

Not only do we need to recognise the difference in open grown trees to closed canopy ones, we need to recognise their value. Of course, woodland habitats have a lot to offer too, and can offer things that individual open grown trees cannot. I am certainly not saying we should value one over the other, but whilst ancient woodland gets quite a lot of attention (again, quite rightly so!), the habitats that our ancient trees live in are often overlooked, undervalued and misunderstood. We must start amending this by taking another look, and recognising open grown trees for what they are: powerhouses of nature, and our future ancients.

*Oaks may be a poor example here, as they actually aren't good at establishing under heavily shaded conditions in the first place (they are what we call shade intolerant trees).

**Here I go again with the oak examples (other tree species are available).

An old open-grown oak tree in Richmond park, wider than it is tall; it has already gone through retrenchment. With no trees around to compete for light, its new shorter structure won't hold this tree back from living hundreds more years. 