Autumn brings a brilliance of colour, rich and warm. Russets, golds, ambers, ochers, burgundies, scarlets, a fleeting pageant presaging the decay and dormancy of winter. The transient period from summer greens to glowing yellows, oranges and reds has an intense beauty, moderated by the weather, changes in temperature, the lowering levels of light and chemical processes effecting the pigmentation of the leaves. It vividly signifies the shifting cycle of seasons, a tree’s transformation from fruitfulness to quiescence.
Leaves are coloured by pigments, chlorophyll, carotenes and anthocyanins. Variants in colour are due to differing amounts of each pigment in the leaf. During the warmer months, with longer daylight hours, chlorophyll dominates, absorbing sunlight and carbon dioxide from the atmosphere and water from the soil then converting them into sugars, used in respiration or stored as starches, providing the energy trees use to grow. Other shades are present throughout the year but are masked by the chlorophyll. The carotenes and xanthophyll modulate the light energy taken in by the tree, helping to screen leaves from scorching and withering in the summer heat.
During autumn, light levels decrease, temperatures drop and plants prepare for winter. Nutrients are salvaged from the leaves and stored in the roots for use the following spring. Chlorophyll production slows and radiant yellows and oranges of the carotenes develop. Hormonal changes cause elongated cells to develop across the base of the leaf stem, preventing vascular movement and concentrating sugars in the leaves, these are then converted into the rich reds, pinks and purples of anthocyanins.
The variability of sunlight, rain, wind and temperature of each year, the aspect and microclimate surrounding individual plants and the chemical processes these induce will affect the intensity and levels of autumn leaf colour. Cold nights stop chlorophyll production, greens quickly fade and yellows emerge. Bright sunlit days increase sugar concentration in the leaves and reds become stronger. Dull, wet days produce more mellow, muted colours. Individual trees reach ‘full tint’ when there is no green left but both heavy frosts and strong winds can cause leaves to drop before full colouration is reached. Greatest colours will be seen in years when autumn days are dry and sunny with nights that are cool but above freezing.
The final leaf drop is caused by a reduction in the hormone auxin in the leaves. The cells along the abscission layer at the base of the leaf stem elongate, preventing the movement of nutrients and causing a rupture in the bond between leaf and tree. Allowing moisture to remain in the branches and trunk, preventing the tree from becoming brittle and protecting it from storms, reducing nutrient needs and allowing trees to remain dormant until light levels and temperatures increase again the following spring. Gradually the tree sheds its leaves, wind and gravity release them, floating, spiralling, quivering, wavering, drifting into loose patterns, leaving a fleeting coloured tapestry on the ground and the naked winter tree.