Autumn is a time of reflection. I love autumn as it is both a time of awe-inspiring colours and a foreboding of winter soon to come, but the former somehow overshadows the latter for me.
We are fortunate to live in a part of the world where our four seasons are distinct and where vast tracts of natural foliage persist for us to see and admire vistas of colour in September and October.
Everywhere hues of orange, red, purple, yellow and brown intermingle with shades of green. Although this is an expected annual event, we still watch in wonder as the trees change colour and the leaves eventually fall. But what is really going on here?
Let’s explore this a bit. Plants are full of water, a medium which is critical to the survival of the trees, as fluids carry food through the plant and help flush waste from them. Water freezes in winter, so food and water transport become impossible as the veins in the plants become clogged with ice crystals. While a bit simplistic, this captures the essence of why it’s better not to try to make food during winter if you’re a tree.
So how do summer’s green leaves change colour? Well, surprisingly, they don’t! The autumn colours are there all the time but are masked by chlorophyll; the green pigment most plants have, used to produce their food. In the autumn, chlorophyll production gradually declines and eventually stops entirely. At this point, the green colour of the leaves fades, and the underlying colours emerge.
Shades of red, orange and yellow predominate and are derived from different pigments. Carotenoids produce the orange, xanthophyll the yellow and tannins the brown colours, while anthocyanins produce the reds and purples.
Interesting, but why are some trees red, others brown and still others yellow or orange? It’s not really clear, and it may have something to do with the type and quantity of sugars produced in the leaves or how much pigment and in what combination exists in the leaves in the autumn. Suffice to say, it is complicated, but the outcome is clearly gorgeous, whatever the reason.
The lengthening nights seem to be the overriding factor stimulating trees to change colour, but other factors come into play. The intensity of the colours which develop in any particular year is related to weather conditions, occuring before and during the time the chlorophyll in the leaves is declining, with temperature and moisture the main influences.
A succession of warm, sunny days and cool, crisp (but not freezing) nights seems to bring about the most spectacular displays. Under these conditions, high volumes of sugars are produced in the leaves, but the gradual closing of the veins in the leaves traps these sugars in the leaf cells. These conditions encourage the production of anthocyanin, which tints the leaves with the more brilliant red, crimson and bright orange colours overshadowing the greens of summer. In conditions where we don’t have the sunny days mixed with the cool nights, the anthocyanins are not produced at the same rate, colours are more subdued, and the yellows and browns dominate. This fall seems like one of those spectacular autumns where the colours are intense and unforgettable!
But why don’t evergreens (conifers) lose their leaves (needles)? Well, they do, just not in the same way deciduous trees do. Essentially, conifers’ leaves are tougher than their deciduous cousins and often have a waxy surface which allows them to continue to function throughout the year. They don’t become damaged by freezing and thawing during the long-dormant periods of winter. Most do lose their leaves but gradually over a prolonged period of months or years. Some species, like the White Pine, drop masses of needles in the autumn, usually from the inner branches, while others, like the Larch (Tamarack), lose all their needles and go dormant like the deciduous trees. Natural processes are complicated but fascinating, as you can see. So as we bid farewell to summer’s beauty, let’s welcome the spectacle of the autumn colours!
Geoff Carpentier is a published author, expedition guide and environmental consultant. Visit Geoff online at www.avocetnatureservices.com and on Facebook.