Autumn is one of my favourite seasons. I do appreciate the seasonal change and the beauty that autumn brings to us. Only a few places in the world have the combination of tree types and climatic conditions necessary for vivid autumn foliage. It is the deciduous trees and shrubs that display the most noticeable autumn colour. Evergreens also develop autumn colouration, but the colour changes are more gradual. Many homeowners become unnecessarily alarmed when in the autumn the interior needles of their pines, spruce and fir change colour. They mistakenly interpret this as a disease or insect problem, not realizing that even evergreens occasionally shed their leaves.
The variety of leaf colour that we enjoy is due to the physics of sunlight striking the pigments in the leaves. Cholorphylls, carotenoids, anthocyanins and tannins are the four broad categories of pigments found in leaves. These same pigments are also responsible for the array of colours in flowers. During spring and summer, the leaves serve as the principle site for the photosynthetic process in which carbon dioxide and water with the aid of light are manufactured into carbohydrates or food for the plants. This process takes place in the cells containing chlorophyll. This chlorophyll also gives the leaf its green colour. As the days shorten and temperatures cool, there is a decline in the rate of photosynthesis as well as the synthesis of new chlorophyll. As the green pigment decreases, other pigments present in the leaf become more evident.
Other pigments present in the leaves include yellow or orange carotenoid pigments. This is the same pigment that gives the carrot its familiar colour. Carotenoid pigments in the leaves account for the yellow and golden leaf colour we see in the fall. The yellows of horse chestnut, poplar, birch and ash are lovely examples of the carotenoid pigmentation. The presence of tannins along with the yellow carotenoid will give a more golden yellow pigmentation to the leaf.
Anthocyanin pigments are responsible for the pink, red and purple colours in the leaves. Maple, some oaks, viburnum and others show a reaction between various sugars and the anthocayanin pigment. That beautiful bright orange colour seen in some of our maples is the result of red anthocyanin pigment mixed with yellow carotenoid.
Many ask why is the colouration some years so brilliant? The answer is simply perfect climatic conditions. The most important environmental factors that influence the degree of colours are light intensity, temperature and water supply. Bright light favours brilliant red colouration. Every fall we get those autumn shades of yellow and brown but the brilliant hues that make the autumn landscape truly spectacular are a direct result of the weather. Warm, sunny days followed by cool nights with temperatures below 10 C, but not freezing will produce the best colour. Rainy or cloudy days without much light that occur near the time of peak colouration will decrease the autumn colouration. Heavy rains and high winds can sweep the leaves off the trees early. Freezing temperatures and very heavy frost will essentially stop all processes occurring within the leaf tissue prior to the pigments reaching maximum development.
So enjoy the colours of autumn. Go for a walk, take a picture or two and enjoy this beautiful season to the fullest.
— Hanbidge is a horticulturist with the Saskatoon School of Horticulture and can be reached at 306-931-GROW(4769); by email at growyourfuture@gmail.com or check out our website at saskhort.com.
