As summer winds down and fall approaches, our nights become cooler and longer and the days shorter. This is known as photoperiodism, and is the largest factor in why leaves change color in autumn. As the nights get longer, the process of senescence becomes apparent through color change and the falling of leaves, leading the tree into winter dormancy.
As the photoperiod decreases, a plants’ ability to synthesize chlorophyll, which is responsible for the green color of leaves through spring and summer, becomes reduced, and yellow and orange carotinoids and xanthophylls, which are always present within the leaf, begin to show. In some plants, the leaf cells produce red pigments, the anthocyanins. In these leaves, once the chlorophyll has decomposed, the anthocyanins mask the caretenoids, thus turning the leaves red. Some species, particularly the oaks, contain high quantities of tannins in the leaves which are responsible for brown colors.
The color change and dropping of leaves are a plants’ way of avoiding freezing. During freezing, the water within leaf cells turn to ice, causing disruption of the tissues. Damaged tissues become inviting sites for fungi and bacteria to invade the plant. In order to prevent this fungal invasion, plants drop their leaves and form a protective seal over the areas where the leaves were once attached before the freeze damage can occur.
So even if your plants look dead and lifeless through the winter, rest assured they are monitoring their environment, keeping track on temperature and day length until spring!
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