Autumn Glory: colour displays forecasted in the Rockies
by Ward Cameron
The onset of autumn has provided the impetus for untold poems and art pieces. There's something about the change of colours that seems to bring on a change in attitudes. The same old panoramas suddenly explode with blazing yellows and oranges. Have you ever wondered what triggered the leaves of the aspen and poplar to turn golden and then drop?
Like biologists are want to do, they've separated the process into two processes with two unique names. Colour change is part of a process known as “senescence” and the dropping of leaves is referred to as “abscission”. Essentially, senescence describes the process of the leaf becoming dormant and abscission describes the process by which the plant essentially self-amputates the dormant leaf. Enough terminology, let's take a look at what's happening within the tree itself.
Autumn is brought on by shortening days and cooling nights. Plants are very much in tune with these changes and, once a certain threshold is passed, begin to move into a state of dormancy. The onset of colour change and leaf fall help to prepare the trees for the long winter ahead.
The first thing we notice is colour change, the rate of which varies with just how cold the weather has been. During years with a hard frost early in the season, you may get an explosion of colour that is relatively short-lived. Years with a slower onset of cool temperatures see the colours lingering longer, but the show is less dramatic. At the time that I’m writing this, there is already a frosting of snow on the summits and winter is in the air. We migh have a fantastic show this year.
When we look at leaves in the summer and notice the green colour, what we're actually seeing is the green pigment of chlorophyll. Chlorophyll is produced by the leaves and provides food energy for the plant. With the onset of autumn, it's no longer produced and slowly dissipates, allowing some of the other pigments in the leaves become visible.
These were always present, but were masked by the dominant green colour. The yellows of the aspens and poplars are caused by a group of yellow and orange pigments called carotenoids. In some trees, other pigments may also begin to show through. Blazing reds and fiery purples are visible in some of the smaller shrubs and flowers. These add a splash of colour to the golden autumn aspen groves.
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These Larch trees in Burstall Pass flaunt the fall colours that speckle the Rockies at this time of year.
Leaf fall has been with us a long time. It first appeared some 350 million years ago and has taken place ever since. No wonder the word "fall" has become synonymous with autumn. Leaf fall occurs in two stages. The first is the actual removal of the leaf and the second the protection of the scar left behind.
Before a leaf falls, it forms a specialized layer cells near the base of the leaf. Often the cells making up this layer are smaller than surrounding cells, leaving an area of weakness. When the leaf prepares to fall, rapid growth in this cell group is accompanied by metabolic changes that actively dissolve some of the cell walls. In this way the layers of cells become separated and the weight of the leaf eventually severs its connection with the branch. Following the severing of the leaf from the branch, a thin layer of cork then grows across the stump left behind.
Once the leaves are on the ground the only thing left to do is to rake them into a large pile and jump in them. They also make an ideal addition to those compost heaps you've been planning.
Ward Cameron is a Canmore-based naturalist, photographer and author. His Web site, www.mountainnature.com, is an on-line guide to the nature of the Rocky Mountains. His popular guide book, Canmore & Kananaskis: Best Hikes, Best Activities, includes hiking trails, popular attractions, historical vignettes and superb colour photography.
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