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.

WHY DO LEAVES CHANGE COLOUR?

Besides the highly specialised green chlorophyll pigment, there are two other major pigments found within the leaf - carotenes and anthocyanins. Carotenes are yellow coloured pigments while anthocyanins are red coloured pigments and along with the chlorophyll they will occur in differing ratios depending on the plant species and variety, and sometime can depend on the uniqueness of the individual plant. During the growing season the chlorophyll pigments mask the other two pigments in green leaved plants. As the chlorophyll is absorbed back into the plant the carotenes and anthocyanins remain and it is this which gives autumn leaves their colour. The intensity of the colour will also depend on the concentration of remaining stored sugars.

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.

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9 comments:

Michelle said...

Thank you for this posting of "why do trees drop their leaves in the autumn fall" - I had forgotten the process of how it all happens. The Autuman/Fall season is my favourite, next to Spring, with its vibrant colours. Wonderful post! Michelle

Nell Jean said...

Day length must figure heavily in leaf drop here, as we get little cold weather before oak leaves are turning brown and falling off with little fall color. Our best fall color comes late in November.

Laura Livengood Schaub said...

Thank you Simon, I really learned something! Contrary to what some people think, we have gorgeous fall color in California that lasts well into December; nice to know what's REALLY going on!

Kiki said...

I absolutely loved reading on this topic, you are a fantastic writer...one of my favorite topics..nature..and anything having to do with trees...thanks for writing on this fascinatng process.
Kiki~

Mary Delle said...

Simon, I am impressed by your credentials and your posts. You have a lot to offer the gardening community!

Pam's English Garden said...

Fall in Pennsylvania is unbelievably spectacular! Simon, it's great to be reminded what makes the miracle occur each year. Thank you. Superb post!

James Missier said...

hi Simon, Just drop by to check your lovely blog. Didn't realise about the fall of leaves, doesn't take place in my place & so this is new to me.
Have a nice day.

Joanne said...

What an interesting post and blog, although I do enjoy pretty pictures of plants and flowers I also like to learn more about gardening and there is always more to learn I will certainly enjoy looking out for more of your posts.

Bay Area Tendrils Garden Travel said...

Simon,

Congrats on the new web site. So much work, I can only imagine!

Fascinating to read about the process of falling leaves,
Cheers,
Alice
aka Bay Area Tendrils Garden Travel