Autumn is a good time to comment on leaf colour change. Contrary to popular belief, frost does not cause the colour changes of autumn. In fact, early frosts can kill leaves, leaving them shrivelled and brown. As the days grow shorter and cooler towards the end of summer, broad-leaved trees develop a corky layer of cells across the base of the leaf triggered by a chemical process within the leaf which measures the length of day. As water and minerals are lost, chlorophyll breaks down and with its disappearance, the green colour fades, revealing underlying colours. The yellows and oranges come from carotenes and xanthophylls and the brown and rusty colours from tannins. On the other hand, the crimson, red and purple pigments are anthocyanins, which were not present in the leaf earlier, but result from chemical changes in the dying leaves. The colour can vary from tree to tree depending on inheritance, soil conditions, disease and the weather. A longer article is included below. For a more Technical discussion see Nature’s Paint Brush Part Two. For further reading I have attached an article by by Steve Gahbauer of the Rouge Valley Foundation.
Fall is not the end it may seem to be, but a stage in the continual renewal of life. An article in “American Forests” (Sept/Oct ‘94) by L. A. DeCoster, describes the leaves as tree engines with lots of fuel. A 60 year old maple may shed 200,000 leaves (55 kg. or 120 lbs.) An acre of 30 year old mixed forest will shed ten million leaves weighing 1400 kg. (3000 lbs.) We’ve enjoyed the colours; we can now see through the crown. Is there a broken branch that could lead to decay? Are there cavities & woodpecker holes? Are branches rubbing against one another or against the house eves? The article points out several things that happen:
- Before they dropped of they left the beginnings of next year’s engines in the form of millions of buds.
- The soil is getting moister because fallen leaves are no longer in the hot sun pulling tons of water through the trees. That 60 year old maple may have transpired 3,400 kg. (7,500 lbs.) of water.
- They are no longer producing oxygen.
- The leaf litter is nature’s blue box. The 30 year old mixed forest will have taken 160 kg. (350 lbs.) of nutrients from the soil; about 110 kg. (240 lbs.) is returned in leaves and twigs.
The city picks up the leaves on the streets and takes them to a compost site. Here are seven things you can do with your dead leaves:
- If they do not interfere with you, leave them alone.
- If they are in your eves troughs, remove them.
- If they are on your lawn, mulch them.
- If they are on your street, help yourself to free fertilizer.
- If your neighbours blow them onto your yard, thank them.
- Stabilize steep slopes by putting leaves against the upper side of logs, branches etc. laid across the slope.
- If you have lots of leaves, build a box to compost them.
In a short while we will be presented with one of nature’s great spectacles. The summer green of our trees will be replaced with a symphony of colours, ranging from yellows and rusty browns to reds and purples. The change to fall colours will have started early in the northern forest. Cool weather comes in late August or early September. The hills covered with Birch and Poplar become golden while the lowlands largely Spruce remain dark green. If the summer has been dry, Birch growing on thin rocky soil may turn yellow early in August. Later, in October, after the broadleaf trees have shed their leaves, bright candles of yellow Tamarack appear in the swamps.
The change to bright fall colours, which starts in the northern forest, moves south as days shorten and the air cools. While Kapuskasing, in the northern forest, reaches the peak of colour in mid September, Huntsville, in the Great Lakes St. Lawrence Forest, is surrounded by hills of red Sugar Maple at Thanksgiving.
Here along The Don, at the northern edge of the Carolinian zone, full colour comes even later. White Ash is one of the earliest trees to change, showing colour by mid September and completing leaf fall early in October. By that time the colour change of other trees will be well along the way. Along city streets, our native Sugar Maple turns earlier than the introduced Norway Maple. Oaks tend to be later. While the peak colour may be as late as the third week of October, by early November nearly all trees have shed their leaves except for the odd Willow and a few Oaks that hold on to brown leaves till spring.
Contrary to popular belief frost does not cause the colour changes of autumn. Early frosts can kill leaves, leaving them shrivelled and brown. As the days grow shorter and cooler towards the end of summer, broad-leaved trees develop a corky layer of cells across the base of the leaf that cuts off nourishment and makes it easy for the dead leaf to break of and fall to the ground. The leaves actually measure the length of day. This is accomplished by a pigment within the leaf that has two forms, changing from one to the other depending on the amount and type of light received. During the day the change is to one form and during the night the change reverses. As the days grow shorter the relative proportion of the dark form increases and so, when the amount of the dark form reaches a specific level, it starts a process that results in the growth of the cork cells that eventually cut off the leaf. As water and minerals are lost, chlorophyll breaks down and with its disappearance, the green colour fades, revealing underlying colours. The yellows and oranges come from carotenes and xanthophylls and the brown and rusty colours from tannins.
On the other hand, the crimson, red and purple pigments are anthocyanins, which were not present in the leaf earlier. Maples and a handful of other trees produce this chemical as a sunscreen. Low autumn temperatures diminish the leaf’s ability to use light effectively and also trees are pulling precious nutrients from the leaves back into the stem for storage over winter. The leaves become fragile and easily damaged by ultraviolet light. The leaves need sunblock, and anthocyanins do the job. Some recent research suggests another reason why maple trees spend valuable resources to manufacture anthocyanin in the fall. The chemical may be a natural herbicide. It appears that anthocyanins leach from the leaves into the soil and in the following spring, protect seedlings and saplings from competition. This is an explanation of why dry sunny days and cold nights bring out the best fall colours.
The change varies from tree to tree depending on inheritance, soil conditions, disease and the weather. Trees on dry hills especially at the end of a long dry summer turn early. Early and severe frosts can destroy the multicoloured effect by killing the green tissue outright. The leaves may then turn nearly black or an uninteresting brown. Dark and rainy days can reduce the brilliance of the foliage and shorten the period of leaf fall. On the other hand there are dark day in the fall when the leaves can look especially saturated with colour.
Before the leaves drop off they leave buds that hold next years leaves in miniature. They are no longer producing oxygen or holding up some of the rainfall as they did in the summer, so air pollution starts to climb and the soil gets wetter. When a leaf falls in the forest that is not the end it may seem, but rather step in the renewal of life. In the forest the fallen leaves add organic material to the soil and return the nutrients that the tree took up during the summer. We in the city want to keep things tidy and therefor gather up the leafs so the city can pick them up and take them to a compost site. What are we doing with our leaves? If you have abundant leaves, why not build a box to compost them? This way you help yourself to free fertilizer.