……in more ways than one. The first groups are about to start their block week training, and we have the remains of two hurricanes heading for us this week. When I started writing this, we had severe weather warnings in place, but it appears now that it has been downgraded- phew!
When we get severe weather warnings, they usually tell people to stay put, and not to travel unless absolutely necessary, but as far as trees are concerned, they can’t move or hide, and staying put is the only option they have.
So how do you cope with extremes of weather if your only option is to ‘stick it out’?
The leaves look like this; oak, on the left, has what is called a simple leaf, and ash, on the right has a compound leaf. Essentially a compound leaf is lots of simple leaves arranged along a central stalk (properly called a rachis).
If you look at the trees in the main picture together, you will see that the ash leaves look very silvery. This is because they are bending away from the wind, and the leaflets are folding back, showing their lighter coloured undersides. The effect is a bit like a dog sticking its head out of a car window, and its ears blowing back! The oak’s leaves don’t do this quite as obviously, they tend to twist and move around individually. This dissipates the energy of the wind impacting on the tree. Both of these reactions are effective in reducing the wind resistance of the tree.
If you saw this as a video, you would see that the upper parts of the trees are all moving around as the wind blows them, with leaves and small branches swaying, but by the time you get to the main branches and the stem, there is virtually no movement at all. This is how trees survive extreme weather events, they absorb the energy, and then dissipate it through the structure.
In fact the Beaufort scale I mentioned above, (you know, the one that goes from 1 up to hurricane force 12+, and you hear on the shipping forecast?) which is used to simply measure the strength of wind, can be approximately measured by observing the behaviour of trees. Around a force 3-4, you will see the behaviour above, small branches moving, and leaves sometime getting blown off, especially in autumn. By force 5, small branches are moving quite vigorously and by force 6, the larger branches are joining in. It’s around about this point that bits start breaking off. If you go out after wind of around 30 mph (around force 6), you will see lots of small branches and leaves all over the road. Once you get above this, we start to lose whole branches and eventually as wind increases in strength, whole trees snap, or come up by the roots.
The thing is though, trees have several other tricks up their sleeve, before they reach the point of falling over or snapping. As small branches break off, what does this do for the crown of the tree? Well, it makes it smaller, less dense, allows more air through and reduces wind resistance, so trees are really designed to allow small bits to break off first, to reduce the loading on the whole tree. The last bits a tree wants to lose, is the stem and root system, but it can afford to lose some branches, and regrow them later. So it will pretty much sacrifice a lot of the crown, to keep the stem upright and those roots in the ground. You will see this in tropical areas prone to hurricanes. If you look at some of the tragic pictures of countries hit by Hurricanes Irma, Jose and Maria, a lot of the trees look like they have been killed, they are just stalks, but actually they have only lost the leaves and part of the crown. Palm trees, for instance, have large compound leaves that get torn off, but left to their own devices, they will regrow a new crown fairly quickly. After the storm in ’87 and subsequent ones, many trees were felled, or chopped up prematurely because people thought they wouldn’t survive. In fact even if a tree has blown over, and as long as a good proportion of the roots (around 30%) are in the ground, they can still carry on, albeit horizontally for a while! These are quite often referred to as ‘Phoenix’ trees, who ‘rise from the ashes’ of the old tree
In addition to the crowns, tree stems are flexible, they aren’t like concrete blocks set in the ground, they move and sway with the wind, again to help dissipate energy. If you watch a whole tree swaying, you will see that the crown, branches and stem are all moving at different speeds. This helps to break up the motion, it’s called damping.
It was once common for marching soldiers to be asked to break step when crossing things like bridges. This is because the repetitive rhythm of the soldier’s steps, could induce swaying in the bridge structure, which could lead to failure. It would be similar if all parts of the tree swayed in harmony in the wind, it would actually help to amplify the motion rather than reducing it, and be more likely to lead to tree failure.
So trees employ the same strategy as marching soldiers, by breaking the rhythm of the branch movement, the overall motion of the whole tree structure is reduced.
It’s a fact that people worry about trees falling in the wind and harming them and their property, and as an Arborist I have heard people expressing these views a lot. It is very difficult to convince people that only very, very, very rarely are people seriously injured or killed by trees. The odds of being killed by a tree have been calculated to be around 1:10000000, about the same as being struck by lightning, so there is no need for people to worry unduly when the wind is blowing, and there is a tree near your house. Whilst undoubtedly accidents do happen and it is tragic for the people involved, it is worth considering that despite the huge numbers of trees we have in this country just how rare an event it is, and not to think that trees are either inherently weak, dangerous or necessarily an immediate threat.
Tragic events, however, do attract publicity, and unfortunately then, we tend to get kneejerk reactions and lose trees that should not necessarily have been removed. It’s our job as Arborists to educate people to appreciate that the benefits of trees massively outweigh any potential risks, and that it is worth talking to a professional tree person about your concerns, and getting advice, rather than wading in with a chainsaw and taking the tree out.