WHY DO TREES DROP THEIR LEAVES IN THE AUTUMN FALL

Every year, the seasonal progression from summer to autumn is marked by the changing colours of deciduous plants. Most notably - and by far the most famous - are the displays witnessed in the ancient forests of New England. Unfortunately for those of us who appreciate such things, just as the leaves are at their most spectacular - they fall off, making an untidy mess on the floor.

But why do trees (and of course shrubs) drop their leaves in winter? Surely this is terrible waste of resources as the plant then has to replace them all – plus a few extra – during the following spring? But before that question get answered it is important to understand what is it a leaf does?

WHAT DO LEAVES DO?

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Each individual leaf is a miracle of evolution – without which, plants as we know them would not exist. Every one of them is a tiny factory which uses energy from the sun (in the form of light) to convert carbon dioxide and water into energy rich sugars – this is the metabolic process known as photosynthesis. Once produced, these sugars are transported throughout the plant - via its vascular system – where the energy is used to power growth and all other metabolic processes. Without these sugars plants would be unable to survive.

However - to photosynthesise effectively - leaves need to be able to collect as much light as possible, and their size and shape are vital in this. Nevertheless, compromises have to be made between the strength, size and weight of the leaf. Too heavy and the plant may not be able to support or direct the leaf to the optimum position for maximum light collection. Too small and the leaf may be unable to produce enough energy to sustain the plant. And is the leaf is not strong enough, it will become damaged in adverse weather and may end up not working at all!

SO WHY DO TREES DROP THEIR LEAVES?

The winter season is always guaranteed to bring two things. The first is far lower light levels while the second is terrible weather.

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With the lower light levels leaves will become increasingly unproductive, but with a drop in temperatures the plants metabolic rate is also reduced and so photosynthesis can effectively stop.

With regards to poor weather conditions, a combination of strong winds, snowfall and freezing temperatures would provide any large broad leaved tree a serious risk from damage if they kept their leaves in place. Firstly, heavy snowfall would remain in the canopy placing huge stress on the branch framework, and if you combined that with strong winds you'll definitely have a recipe for disaster. Of course, the leaves of deciduous plants are particularly sensitive to freezing temperatures anyway as internal cells are easily ruptured when exposed to large enough ice crystals.

The lesser of two evils is to absorb as much of the available and usable nutrients that are within the leaf structures as possible and then lose the remaining ‘leaf husk’ before snow appears. Of course – as with many things in nature – nothing is wasted as the following leave litter is broken down further by bacterial activity to create a humus rich mulch.

HOW DO TREES KNOW WHEN TO DROP THEIR LEAVES?

As winter approaches, days will become progressively shorter and cooler, and it is this small yet crucial day by day change that acts as a trigger for the trees winter dormancy mechanism. As mentioned before, this environmental trigger begins the absorption of leaf nutrients and carbohydrates back into the stems, but it also starts an irreversible phase of leaf drop.

A membrane of specialised cells known as the abscission layer, will begin to develop at the base of the leaf’s stem. As the membrane grows, it increasingly restricts the flow of sugars and water between the leaf and the rest of the tree. Incidentally, this change also helps to promote the breakdown of chlorophyll pigments for absorption back into the stems. When this layer is completely formed, it is then dissolved causing the physical separation of the leaf from the tree.

For related articles click onto the following links:
HOW TO GROW A CONKER TREE FROM A CONKER
WHY DO LEAVES CHANGE COLOUR IN THE AUTUMN FALL

Why do trees shed their leaves in the fall?