November 13 UK - How the leaves change

Dull day today and a few leaves are still hanging on, though there are none on the highest bit of the tree on the right now.

Here is a series of photos that shows how oak leaves change. In fact it was a scan (taking photos with a scanner is a good way to get close up of small things provided they are reasonably flat.)

The colour change is due mostly to the loss of the green chlorophyll in the leaves. The production of it stops as the days get shorter, it gets used up in producing sugars from the air and sunlight and it gradually disappears from everywhere except the veins. (See the second picture.) In summer the chlorophylls in the leaf are the main colours and mask everything else. When the chlorophylls go other colours start to show. At first they mix with the chlorophylls changing the tone of the green and producing yellows and then they appear as the main colour. Finally as the leaves dry the brown colour of the leaf appears.

Oak leaves turn to brown quite quickly (beech leaves stay yellow much longer). This as far as I can discover is because there is a lot of tannin in oak leaves. The yellow orange colours are caused by carotenoids in the leaves. No English native trees have the spectacular reds and oranges of American maples. These colours are caused by a process that turns the glucose in the leaves into another colourant known as anthocyanin. This process is driven by sunlight and the most spectacular colours occur in sunny autumns.

(This is all a regurgitation of the books and online stuff I have read - anyone who knows more please add your knowledge.) Meanwhile you can learn more in Wikipedia or, if you like your science simple, try Science made Simple.
Or try about.com which is great for all sorts of things.

Leaf fall seems to be caused by a completely separate process. Though it too is triggered by the shortening days. There are special cells called abscission cells at the base of the leaf and these swell in autumn and cause the leaf to break off levering it away from the twig.

I picked up a series of beech leaves here they are. It is the yellow of the beech combined with the browny orange of the oak that makes the New Forest in autumn look as beautiful as it is.
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Click on the big photo just below this and you will be able to see the oak and beech woods at their best. I love them.