The shedding of leaves is of course, amongst other things, an excretory mechanism. A number of abscissional cells develop to form a weak point at the petiole base, allowing the leaf to fall without damaging the tree. As the leaf falls it leaves a scar and in the case of the Horse Chestnut it is very distinctive. The scar resembles a horseshoe, complete with nails, and it is widely believed that this feature has given the tree its common name. For clarity (?) I have shown the scar upside down.
Horse Chestnut twig. Stefen Leys Pocket Park, Daventry.
10 October, 2016
As the leaves fall they take with them various substances such as tannins which would otherwise be harmful. These unwanted substances can include toxic metals and much interest is being shown in using trees not only to remove these metals from contaminated soils, but as a way of 'harvesting' these potentially valuable materials.
Oak leaves turning gold. Aston le Walls Pocket Park.
9 October, 2016
So the leaf, in falling, takes with it unwanted and harmful chemicals. But there is another very good, obvious, yet often overlooked reason why a tree will shed its leaves. As summer advances the leaves will begin to experience many kinds of attack from other organisms to a point where a leaf is rendered almost useless.
One source of trouble can be galls. The photograph shows a dozen or so galls of Neuroterus quercusbaccarum, the Common Spangle Gall. These are overwhelmingly on the under side of an oak leaf and a group like this will probably not have much effect.
Oak leaf with Spangle Galls. Aston le Walls Pocket Park.
9 October, 2016
The upper side of the galled leaf.
Caterpillar damage is also an obvious problem. In my own garden my Rose 'Canary Bird' has had some of the leaves skeletonised by the larvae of the Rose Slug Sawfly, Endelomyia aethiops.
Damage caused by the Rose Slug Sawfly, Stefen Hill,
Daventry. 19 October, 2016