This year in Portland, Ore., the official first frost occurred on October 25. Leaf color depends on weather, so were conditions this year better than many for producing the stunning fall color I’ve seen around my neighborhood? Barney, my adorable golden retriever, was very patient with me while I snapped photo after photo in the past few weeks of the brilliant color of the maples and oaks.
Interestingly, one website suggests that fall color is an exception to the rule that most things in nature are the way they are for a purpose. Brilliant fall leaf color, on the other hand, doesn’t help a plant to survive; it’s merely “part of an orderly shutting-down of plants for the winter. The compounds that remain in leaves and create the attractive colours are the ‘left-overs’ that the plant could not resorb or recycle. Unlike the familiar animal signs of autumn, such as squirrels burying nuts or flocks of geese heading south, which are clearly beneficial to [the survival of] those species, the spectacular scenery afforded by the death of leaves appears not to have any benefit to the plants. The answer to the question of why fall colours are so beautiful lies more in the eyes of the beholders, us, than in some grand scheme of nature. Fall colours are a wondrous coincidence!”
While I adore and appreciate the color, I often forget the fascinating science behind it. Read on for the scientific version of fall color (thanks to information found on www.naturenorth.com):
Deciduous plants, those that drop their leaves for winter, allow their leaves to die in an orderly fashion, a process called ‘leaf senescence’. These photosynthetic factories are shut down and much of the water and nutrients within are reclaimed by the plant, and transported to the root system for storage.
It is during leaf senescence, the shutting-down and recycling of the leaf's contents, that we witness fall colours. One of the first events in senescence is that production of photosynthetic pigments stops. As the existing chlorophyll in the leaf breaks down, it is not replaced. As a result, the green colouring begins to fade. The always-present carotenoid pigments, which break down more slowly, are now revealed. The green of chlorophyll gives way to the yellow of carotenoids. For plants whose leaves turn yellow in the fall, the explanation of fall colours is just that simple.
For plants that produce darker orange, reddish or purplish colours the story is more complex. During senescence these plants are producing anthocyanins, a red pigment, inside the leaves. Sugars, which continue to be produced until all the chlorophyll breaks down, can become trapped in the leaves as the transport system that would normally move them out of the leaves shuts down. In the presence of sugars a group of already-present, colourless compounds called flavonols are converted to anthocyanins by the action of sunlight. Leaves which contain roughly equal amounts of yellow carotenoids and red anthocyanins can appear bright orange. If the anthocyanins predominate the leaf will appear bright red. Because sunlight must hit a leaf in order to produce anthocyanins and there is little movement of this pigment within the leaf, shaded or partially shaded leaves may develop unique patterns.
As senescence progresses further even the carotenoids and anthocyanins decay and their colour fades. By the time the leaf falls, or shortly thereafter, the once vibrant colours will have faded to a dull tan or light brown. As with the other colours there are various compounds in leaves that act as brown pigments. One of the most common is tannin. Tannin and other such compounds are among the slowest to decay, giving colour to leaves even after they have fallen. Plants whose leaves turn brown before they fall often contain large concentrations of such compounds.
Cool nights with sunny days in early autumn makes for good anthocyanin production. Low, but not freezing temperatures, help slow the movement of sugars out of the leaves at night. Sunny days help produce sugars initially, then drive the process that converts sugars and flavonols into anthocyanins. A cloudy autumn with warm nights will not produce good fall colours, at least not for plants that develop red colouring.
Weather can affect fall colours in other ways, too. An early frost can help to break down chlorophyll more rapidly and bring about a more coordinated fall colour display. If the frost affects all the plants in a given region they will tend to become more synchronized in the development of their colours.