During Autumn less daylight is reaching the Earth and consequently it gets colder. For the trees this means that the sap flowing inside is put to an hold. The function of the leafs is to vaporate water, if this should continue during the colder period of the year the tree would simply dry out. Therefore, as the leafs are getting less light, cork is formed in the stalk so the leaf will fall off. But a couple of weeks before this happens, the leaf-green or so called chlorophyll (giving the leaf a green color) is breaking down and also some waste material is injected, changing it into a colorful appearance like yellow, orange, red, brown, etc.  Usually this takes places between the months of September and November. Normally at the end of November almost all trees have lost their leafs, but the last couple of years, with no (night)frost or storms, they even lasted till mid December.  

Taken at the end of October, showing a colorful row of Plane-trees.