I love this phrase, it's one that crops up a lot in the Tree ID courses which I run for the FSC (Field Studies Council). It's interchangeable with the botanical term 'marcescence', which is even better!
So what is it, in real life?
Brace yourself for some botany: abscission is the process by which deciduous trees lose their leaves.
Firstly, the tree pulls all the sap, nutrients, and useful things such as chlorophyll out of the leaf, and back into to the twig, ie the main body of the tree.
Next, the petiole, or leaf stalk, forms a "scab" where it joins the twig, and this layer of cells is properly called an abscission layer. This abscission layer seals off the junction, where the leaf joined the twig, and then the leaf can be shed, without risk to the tree.
Most deciduous trees go through this procedure en masse in autumn: and the removal of the expensive and useful chlorophyll is the reason that leaves go brown in autumn.
Incidentally, the reason we get the beautiful autumn leaf colours is because many leaves are not 'just' green, they have other colours in them as well, but the chlorophyll is sucked out first, so all the green-ness goes, leaving red, orange, yellow etc, for a brief period, until those chemicals are also absorbed, abscission occurs, and the leaf - usually a dull brown colour by now - falls, discarded, to the ground.
Right, now we know what abscission is - it's the process by which trees safely shed their leaves without loss or damage to the tree.
So delayed abscission (also known as marcescence!) is where the leaves don't fall once the chlorophyll has been reabsorbed by the tree.
Instead, they hang around on the tree, all brown and rustly.
Oddly enough, this phenomenon only occurs in juvenile trees, and in our daily lives, we can see this for ourselves, when we look at clipped Beech and Hornbeam hedges.
I'm sure you've seen hedges filled with dead brown leaves, in your local area: here's one, from one of "my" gardens a few years back:
I'm sure you'll have seem something similar in your area - a tightly clipped hedge, full of apparently dead brown leaves.
It's most likely to be Beech: if the leaf coverage is a bit sparse, it might be Hornbeam.
This is where the 'juvenile leaves' bit becomes relevant: as far as the trees are concerned, by constantly trimming them down to less than about 6-8' (approx 2m in new money), they are fooled into thinking that they are still teenagers, so they hang on to their leaves over the winter.
Please don't ask me why the age of the tree should affect the abscission, because I don't have an answer to that. *runs off to do some quick research* OK, there are several theories, none of which are very solid, but the most likely - in my opinion - is that younger trees hang onto the dead brown leaves to deter grazing: the brown leaves are unpalatable, and allow the buds for the next year's leaves to develop, without them being eaten.
You can see this phenomenon again in youngish trees: the lower branches will retain the dead leaves, while the upper ones are bare.
Here's a great local example - a thin line of young Beech trees, planted alongside a rural road: presumably to act as a physical divider, and to maybe be a bit of a windbreak?
You can actually see in this photo, how the upwind side of the tree is being shaped: the branches on the right - the direction from which the wind blows - are being held in a less horizontal, more upright position, and they are considerably shorter than those on the downwind side, to the left.
But today, the interesting point is that the lower section of the tree is full of dead leaves, while the upper growth is not.
This is repeated all the way down the line of young trees, and you can go out and look for this yourself, now that you know what you are looking for.
So which trees exhibit this behaviour?
The main ones are Beech:to a lesser degree Hornbeam: and Oaks do it, too.
And how is it useful? If you want a hedge for privacy and screening, but you don't want heavyweight conifers, then go for Beech, because if you keep it clipped to below about 7-8' in height, it will hang on to those leaves, and give you some degree of privacy pretty much all the way through the winter, until the new leaves arrive in spring.
Mind you, you will then have to rake up the dead leaves in spring, when everyone else finished doing theirs the previous autumn!
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