Leaf fall is well underway as I write in early November. This is triggered in deciduous plants by the reduction of daylight as the days get shorter. The process is called abscission which is an active and deliberate process. Abscission is caused by the cells of the stem, from which the leaf is growing, pushing the leaf stem (petiole) off when it is no longer needed. Nutrients from the breakdown of green chlorophyll will have already been reabsorbed by this time to feed the tree or other plant; this is why the leaves change colour before they are shed revealing the showering yellows and reds of autumn.
Have you noticed how the different species of tree lose their leaves at different rates? Currently, the black poplar trees in Hempstead Meadows Nature Reserve have lost almost 100% of their leaves. Similarly, ash trees have lost about 70% of theirs, but some of this may be due to ash die-back disease, which is spreading rapidly through the trees of Sussex. There will be individuals that are resistant to the disease and survive to repopulate in due course. This is called natural selection. However, the oaks have only defoliated about 8% of their leaves. Indeed, some species of oak are actually (semi-) evergreen, such as the holm oak, which is not native to Britain, but lives naturally in Southern Europe. Holm oaks only renew about 30% of their leaves each year. Mean-while, back to the current state of the deciduous trees in Uckfield, sycamore trees have lost about 50% of their leaves, horse chestnut about 30%, beech about 20%, the limes of Lime Tree Avenue about 30%, birch about 40%, but this tree is very variable. Finally, the sweet chestnut trees of Hempstead Lane have only lost about 30% of their leaves.
Another thing I have noticed about the trees is that they shed their seeds before they let their leaves go. This is probably a deliberate strategy to maximise the generation of nutrients from the leaves to bulk out the seeds, which can be quite large, such as the horse and sweet chestnuts, acorns and beech nuts. Other seeds such as the tiny birch seeds are light and more plentiful and can be carried long distances by the wind.