• Fanny - Nature Spells

The story of how trees grow

Do you know how trees grow? Besides the growth of the root system, they have two types of growth: vertical and in diameter. Is it the base that pushes the top ever higher? Every centimeter high that stretches every year? If that were the case, when a bird lands on top of a tree and accidentally breaks its tip, then that broken tip would be on top of the tree its entire life. If we hung a sign on a tree, it would go up, little by little... You could almost believe it by taking a look at the picture below.

How did that band end up up there? My canadian colleague reminded me that we had come a few months earlier to put it at shoulder height, when the ground was covered with a *thick* mantle of snow...

In fact, when it comes to height growth, the tree adds an extra shoot to its top and a new crown of branches every year, like a child builds a tower by placing cubes on top of each other. Counting the crowns of branches is therefore a fairly reliable way to determine the age of a tree, give or take a few years. It is particularly easy with conifers, especially pines (beware of firs, they have more than one trick up their sleeve and their size can be very misleading: we'll talk about it another time...). By watching the various growth heights, you can also notice the more difficult years (drought...).

Can you count the crowns of branches on these Pines? They are between 10 and 15 years old! You can also find out how long a branch has existed on a tree the same way by counting the number of sections on a single branch. The highest crown is composed of branches of one section, the branches of the second crown have 2 sections, etc.

At the high end of the trees there is always a bud, called the "terminal bud": it ensures this height growth. The "axillary" buds, located nearby, grow into the branches. However, if the terminal bud suffers damage (our friend the bird came to perch, the frost, a hungry deer, a negligent human with a young tree...), the axillary buds will take over and the tree will often develop several new stems at the top. Generally, one of them will quickly take the dominance over the others and will restore a straight profile to the tree, or else we will be dealing with what we call "split tops". This is the case with the tree below, several hundred years old and left alone by the settlers in Eastern Canada: at a certain height, it splits into two trunks because neither of the two branches which took over from the damaged terminal bud gave up in the race for light.

Split tops trees grow with the leafy area of ​​almost two trees and therefore have a boosted diameter growth compared to their neighbors who have never lost their tip. How is this growth in diameter possible? Is it the center of the tree that makes cells and the bark that spreads? In reality, the center of the tree is made up of dead cells (but not rotten): it is called the "heartwood". This is where the CO2 accumulated by the tree is stored throughout its life and it is the part used in carpentry and woodworking for example. Around is a part of living wood through which the sap passes and which is sensitive to insects and diseases: it is the "sapwood". Between the sapwood and the protection formed by the bark, there is a thin layer of cells capable of dividing and of taking on various functions: it is called the "cambium". To keep things simple, some of these cells created will become sapwood inwards and ensure this growth in thickness of one ring per year; others, towards the outside, will become bark.

If we wanted to summarize, we could compare the growth of trees to cones that pile up on top of each other every year!

Consequently, if we cut the tree several meters high, we will inevitably count fewer rings than if we cut it at its base.

Next time you come across a hollow tree, try to see if you can find the birth of its branches inside as in the photo below: this is a phenomenon that I really enjoy observing and a reminder of how the tree grew...


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