Log Cabin Chronicles

Those Autumn Leaves

Beth Girdler


What happens to the leaves in the fall?

They flutter to the ground, blow around, become part of a squirrel's winter shelter, become part of the forest floor or get raked into piles, put into compost heaps (hopefully) and eventually decompose. I have several years' worth rotting nicely around the base of large and healthy trees.

As a child, I always volunteered to rake the leaves. I loved their colors, their smell, and the rustle of them when I surrounded myself with a pile and pretended to be a bird in a nest.

In my parents' hometown of Lindsay, Ontario, people put their leaves into clear plastic garbage bags. The bags go out to the curb where, for four Saturdays every fall, they are picked up by the town and taken to the local conservation area. The bags are then emptied, the contents shredded, and later each household can return for a bag full of luscious leaf mulch.

What a great system! Why don't all municipalities do this?

I hate to see all that organic material going to the dump. Stuffed into bags, the leaves do decompose, but the bags they are in still add to our ever-growing garbage problem.

Back to my original question. -- what I really meant to ask was, what happens to leaves before they fall in the fall? Why do they change color? Why do they detach?

Cooler shorter days signal the beginning of the end for leaves. This gradual death is known as leaf senescence.

Most tree leaves are green in the summer because of the presence of a pigment known as chlorophyll. This pigment converts sunlight into food for the tree.

Since sunlight is needed to make chlorophyll, its production decreases as the daylight hours grow shorter.

Cooler temperatures also effect chlorophyll production, which eventually stops altogether. The green pigment then breaks down and disappears.

At the same time, nutrients such as nitrogen and phosphorus are drawn from the leaves and stored in the twigs and branches for the winter.

There are other pigments in leaves, those that break down more slowly than chlorophyll, that are only revealed at this time of year.

The yellow carotenoids (the same compound that colors carrots or corn) can now be seen in poplar, birch, and beech leaves.

The red anthocyanins (which colors apples and strawberries) become apparent in the leaves of sumac, red oak, and red maple.

Leaves left with equal amounts of the two will look bright orange. Since sunlight affects the amount of pigment in each leaf, partially shaded leaves may develop varying concentrations resulting in interesting color patterns.

Eventually, most leaves turn brown. Tannin is a brown pigment and one of the slowest to break down. Trees whose leaves turn from green to brown in the fall contain high concentrations of tannin.

Another process initiated by the seasonal decrease in light and temperature and by decreases in certain hormones (yes, trees have hormones too!) is known as abscission. At the base of each leaf stalk, cork cells develop that seal off the leaf from the twig.

These cells protect the twig from water- and nutrient-loss after the leaf falls. Once the seal or abscission layer is made, the cells on the leaf side die and wind, rain, or an early snow snaps the leaf from the tree.

When a leaf separates from its twig, it leaves a scar. Take a close look at one and you will see tiny dots. These dots are the truncated ends of veins that used to carry nutrients to and from the leaf. Each dot is called a bundle scar.

Above the leaf scar, you will see small buds containing next year's leaves and flowers. The size and placement of these buds and bundle scars, along with the shape of the leaf scar that is left, is distinct to the species of tree from which the leaf fell. It is thereby possible to identify trees in the winter.

All you need is a good book (Native Trees of Canada by R.C. Hosie is my favorite) and a magnifying lens. Like any form of plant or animal identification, naming trees without their leaves is a challenge that gets you outside and develops your powers of observation.

Don't panic if your pine, spruce, or cedar tree drops a pile of needles once in a while. It isn't sick. The leaves of evergreen trees also change color and are shed, but at a much slower rate, sometimes taking over two years to fall. Remember, needles also make great compost.

There are few things as energizing as an afternoon walk on a cool, blustery autumn day along a woodland path strewn with freshly fallen leaves. As the wind whips leaves from branches and sends them eddying around your feet, the fresh sweet smell and crunching sounds combine with stunning displays of red, yellow, and orange to set ones senses soaring.

Please, when they fall, rake your leaves, enjoy them (I still jump in them with my son), compost, and recycle them on yours or your neighbor's garden. Treat them like the gold that they are.

The naturalist Beth Girdler writes from Ayer's Cliff, Quebec.

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