Sugar maple
Acer saccharum, one of about 100 maple tree species worldwide, is a native of eastern North American forests and is noted for its displays of autumn colour. As days shorten and temperatures drop in the world's temperate regions, the dog days of the northern summer give way to short-lived autumnal colours. The once green leaves of deciduous trees and shrubs take on shades of brown through golden yellow to vivid red and purple. Despite such spectacular colour changes having intrigued us for centuries and stimulated commercial exploitation of the most flamboyant displays (predominantly in north eastern North America), our scientific understanding of the phenomenon is patchy; the chemistry of the process is reasonably clear yet the function of autumn colour is far less apparent.
Autumn colours are due mainly to two classes of pigments: carotenoids (yellow-orange) and anthocyanins (red-purple). Chlorophyll, the green, photosynthetic pigment of healthy leaves, masks the colour of many carotenoids that are present all year around. As chlorophyll is naturally broken down with the onset of autumn, the colours of carotenoid pigments gradually become visible. In contrast, anthocyanins are actively synthesised just before leaves are shed. Leaf nutrient levels (especially phosphates) rise during the summer but at the onset of autumn these nutrients move back into the tree's stem, triggering anthrocyanin synthesis. The most spectacular anthrocyanin colours are produced when autumn days are bright and cool, and nights chilly.
Explanations for the adaptive significance of red autumn leaves include, protecting plants against the harmful effects of light at low temperatures (photoprotection hypothesis) and as a warning signal to reduce insect attack (coevolution hypothesis). Under the coevolution hypothesis, the intensity of red colouration indicates the concentration of defensive chemicals in the plant. Consequently, overwintering insects that avoid red leaves will be fitter than those that do not, and red-leaved plants will have reduced parasite load.
Sugar maple is also a source of timber and maple syrup. Overwinter, sugars are stored as starch in the roots and trunk, and are then reconverted back to sugars as the sap rises in the spring. Sugar-loaded sap is extracted by tapping into phloem tissue in the bark. The extracted sap is boiled to concentrate the sugars. Maple syrup, prepared from sugar maple for centuries before Europeans discovered the Americas, is today an industrial process, dominated by Canadian producers, that supplies a global market with more than 45 million litres of syrup annually.
Further reading
Archetti M et al. 2011. Unravelling the evolution of autumn colours: an interdisciplinary approach. Trends in Ecology & Evolution 24: 166-173.
Lee D 2007. Nature's palette. The science of plant color. The University of Chicago Press, pp. 270-295.
Stephen Harris