Autumn leaves, a dazzling phenomenon

Photo of a sugar maple (Acer saccharum) with orange leaves

When the green of North American trees turns to yellow, orange or bright red, there is no doubt autumn has arrived. This fascinating show, put on for us every year, is renowned worldwide. Tourists come from far away to see "our" colors, and with good reason: this autumn show of colors in our deciduous forests is unique! But why do our trees change color?

It has often been said that the fall colorization of leaves is frost-related. Although frost can indeed influence their intensity, the blaze of colors is more likely caused by the shortening period of daylight (photoperiod). Cold nights and short days stimulate the production of a hormone responsible for leaf drop, ethylene (Gaz that triggers some maturation processes in plants.).

Photo of a yellowish fall Quebec landscape

In autumn, in anticipation of leaf drop and in response to the signal sent by ethylene production, trees produce a layer of cork cells at the junction of the leaf and the branch. Little by little, this cork blocks the veins which transport water and minerals to the leaf. The leaf ends up falling, but the tree, which has already sealed the freshly exposed branch cells, is not injured. This process is called abscission.

In spite of lacking nourishment, the leaf survives for a time by relying on its reserves. When essential minerals are no longer available to regenerate chlorophyll (the green pigment responsible for photosynthesis), it degrades. The disappearance of green, the dominant color, allows yellow pigments,carotenoids (Yellow or red pigment produced by a plant.) and xanthophylls (Yellow pigment.), which are normally masked, to appear.

Sugars and colored chemical compounds

In maples, the layer of cork does not completely prevent raw sap from penetrating into the leaves. As a result, sugars are forcefully accumulated in the leaf tissues, causing colored chemical compounds to form, the phenols (Benzene derivative.) and anthocyanins (Pigment plant in shades of red or blue.). The latter, brilliant red, change color according to the acidity of the environment: scarlet red in acidic soil, violet purple in alkaline soil. The formation of this pigment is enhanced by cool nights followed by sunny days.

Photo of a fall landscape, with colours from green to bright red

During the process of abscission, the old leaves experience a major stress and are much more sensitive to attack by oxygen free radicals (oxidants). By accumulating anthocyanins (antioxidants), maples help their leaves to survive while their remaining nutrients are absorbed. Fore example, the trees attempt to recycle nitrogen, an essential nutrient, by breaking down the chlorophyll contained in these dying leaves.

Not all tree leaves turn red in autumn, since, for most trees, the production of anthocyanins would require a greater expenditure of nitrogen than they could recover from their old leaves. Other trees simply have not developed the evolutionary strategy used by maples.

Temperature

Temperature also influences the show of colors. A period of drought at summer's end can hasten the change of colors in the foliage of some trees. As well, in eastern North America, a frost-free fall usually leads to less spectacular shows of color, whereas an early frost can cause the leaves to turn brown or even die.

Before the inauguration of the Biodôme de Montréal, a unique museum that recreates four of the most beautiful ecosystems of the Americas, researchers attempted to verify how the deciduous trees in the future "Laurentian forest" would react to simulated fall and winter conditions in this new 21^st century zoo. The challenge was that engineering constraints required that temperatures never dip below 5°C. Simulations of these conditions showed that although it was possible to provoke leaf drop, the show of colors would never be as flamboyant as nature's own. This confirmed the importance of frost in creating the fall colors that characterize our forests.

Photo of autumn landscape with many colours

A multitude of colors

The decreasing percentage of chlorophyll, which reveals the carotene, as well as the accumulation of anthocyanins, are progressive changes. This explains why, to our great delight, we can see maple leaves that are simultaneously colored green, yellow and red. Further, the diversity of species present in our forests creates a particularly rich mix of colors, ranging from the purple characteristic of the American ash, to the fiery yellow of sugar maples.

Photo of the purple leaves of an American ash, Fraxinus pennsylvanica

In Western Europe, for example in France, the more limited diversity of forest species and milder autumn temperatures mean that our cousins are deprived of the fall spectacle we enjoy. While the grass may often be greener on the other side of the fence, in this case, we have no grounds to complain!

Text by Michel Labrecque, curator of the Montréal Botanical Garden. Adapted from Quatre-Temps magazine by Mathieu Lanteigne-Cauvier.

Researcher fact sheet: Michel Labrecque

Autumn leaves, a dazzling phenomenon

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