Why do leaves change colour in autumn?
Every autumn, we are greeted by a spectacular array of coloured leaves. But why do leaves change their colours before they are shed for winter?
Autumn…the year’s last, loveliest smile.
― William Cullen Bryant (1794–1878)
As the long, lazy days of summer depart, they do so in blazing glory: deciduous trees and shrubs shed their leaves in preparation for winter. But before their leaves fall, they lose their characteristic green colour and adopt a variety of hues; scarlet, orange, yellow, bronze, red, gold, brown, purple.
To celebrate, I am sharing a video, but I also thought that it would be fun to share a few scientific ideas about the biochemistry, physiology and evolutionary reasons that support and explain the changing colours of autumnal leaves.
Why do leaves change their colours in autumn?
Leaves act as small factories that produce food for their tree. To do this, they contain special pigments that capture energy from sunlight. These pigments, known as chlorophylls, are organised into layers within small organelles located inside leaf cells. Chlorophylls use the energy from sunlight to fuel a series of biochemical reactions that combine carbon dioxide and water to produce oxygen gas and sugars. This process is known as photosynthesis.
Sunlight is white light, which comprises all the colours in the rainbow, but chlorophylls can capture only specific narrow portions of white light, the red and blue light found at each end of the visible spectrum. There are two distinct types of chlorophylls, P680 and P700, so named for that part of the spectrum where each type is most efficient at absorbing red light (in nanometers). Since chlorophylls absorb red and blue light and reflect the green part of the spectrum, this is the reason that leaves appear green.
In autumn, the decreasing daylength causes trees to stop producing several plant hormones that are necessary to trigger the replacement of ageing chlorophylls. When these plant hormones disappear, leaves undergo a process known as senescence — they get old. During leaf senescence, several events occur: first, the tree actively reclaims important nutrients, particularly nitrogen and phosphorus, by moving them out of the leaf. Second, chlorophylls are broken down into colourless molecules and thus, leaves lose their characteristic green colour. (These molecules, the tetrapyrroles, are not photosynthetic.)
Interestingly, increasing carbon dioxide concentrations associated with global warming postpone the development of autumnal colouring in tree leaves, so even though daylength changes predictably every year, autumn leaf colours are arriving later every year (ref).
Plant leaves also contain other types of light-sensitive pigments — the carotenoids (orange colours) and xanthophylls (yellow colours). Normally, the much more abundant chlorophylls mask these other pigments in a sea of green, but when chlorophyll is broken down, the carotenoids and xanthophylls become visible, giving leaves their autumnal blaze of colour.
Tree species endemic to some geographic regions — most famously, many that are native to the northeastern coast of North America — can produce anthocyanins (blue, purple, scarlet and red colours) in the intracellular sap of their leaves at the end of summer. The ability to produce these pigments is genetic — requiring a number of special enzymes — thus, not all plants can manufacture them. The anthocyanins (which change their colour from blue to red depending upon pH), are only produced under special conditions: first, the tree must be exposed to bright light and second, its phosphate levels must be low. So for example, even though light is bright during summer, leaves have high phosphate levels, so anthocyanin production is inhibited. But because the tree is moving nutrients out of its leaves in preparation to shed its leaves in late summer and autumn, the amount of phosphate decreases while the light is still bright. This reduced phosphate level alters sugar metabolism in the leaf — thereby triggering production of anthocyanins, which give leaves their red colouring.
Why do some tree leaves become red in autumn?
On the eastern seaboard of North America, roughly 70 percent of deciduous trees produce anthocyanins in autumn. This is in stark contrast to deciduous trees endemic to Europe, which rarely produce red pigments. Since anthocyanin production is expensive — requiring both the resources and the enzymes necessary to synthesise these molecules at a time when the tree is shutting down for the winter — people have wondered why they do this.
In 2009, an interesting paper reported that red leaves are an honest signal to aphids of the tree’s quality as a host (ref). This signalling hypothesis resulted from a series of experiments that found that aphids, Dysaphis plantaginea, avoid apple trees with red leaves in autumn and that aphids found on these same trees in springtime have lower fitness. Lending support to this hypothesis is the observation that wild apple trees commonly produce red leaves in autumn whereas domesticated apple trees, which are protected from the ravages of insect predators, do not. Additionally, apple tree varieties that produce red leaves are more susceptible to insect-borne diseases. Further, red-leafed trees produce smaller fruits. Taken together, these findings suggest there may be an energetic trade off between fruit size, leaf colour and insect resistance.
Thus, red leaves — and perhaps any autumnal leaf colouring — may serve as an honest warning signal to insects and other herbivores. However, there are a number of other hypotheses that may also explain the evolutionary origins of leaf colour change (ref & ref).
Why do trees lose their leaves in autumn?
Not all trees lose their leaves on a seasonal basis, but those that do are known as deciduous trees. The deciduous trees that most people are familiar with live in northern temperate zones, and these trees lose their leaves in preparation for cold weather. However, there are deciduous trees in the tropics and subtropics that shed their leaves too. These trees drop their leaves prior to the annual dry season.
Basically, leaf shedding evolved because the energetic costs and resource investment necessary to maintain leaves are greater than the benefits to the tree from continued photosynthesis when light is poor, the environment is dry and temperatures are severe (ref). Thus, by shedding its leaves before entering a metabolically dormant phase, the tree reduces water and nutrient loss and increases its chances of long-term survival.
And now, here’s that video that inspired this train of thought
Thank you, Doctor Fred, for inspiring me to delve into a literature that, apart from the biochemistry, I’ve never really examined before. And you, dear reader, are invited to read more about UK biodiversity.
Thomas H. & Stoddart J.L. (1980). Leaf Senescence, Annual Review of Plant Physiology, 31 83–111 | doi:10.1146/annurev.pp.31.060180.000503
Hendry G.A.F., Houghton J.D. & Brown S.B. (1987). The degradation of chlorophyll — a biological enigma, New Phytologist, 107 (2) 255–302 | doi:10.1111/j.1469–8137.1987.tb00181.x
Hamilton W.D. & Brown S.P. (2001). Autumn tree colours as a handicap signal, Proceedings of the Royal Society B: Biological Sciences, 268 (1475) 1489–1493 | doi:10.1098/rspb.2001.1672
Lee D. & Gould K. (2002). Why Leaves Turn Red, American Scientist, 90 (6) 524 | doi:10.1511/2002.6.524
Archetti M., Döring T.F., Hagen S.B., Hughes N.M., Leather S.R., Lee D.W., Lev-Yadun S., Manetas Y., Ougham H.J. & Schaberg P.G., et al. (2009). Unravelling the evolution of autumn colours: an interdisciplinary approach, Trends in Ecology & Evolution, 24 (3) 166–173 | doi:10.1016/j.tree.2008.10.006
Archetti M. (2009). Evidence from the domestication of apple for the maintenance of autumn colours by coevolution, Proceedings of the Royal Society B: Biological Sciences, 276 (1667) 2575–2580 | doi:10.1098/rspb.2009.0355
Taylor G., Tallis M.J., Giardina C.P., Percy K.E., Miglietta F., Gupta P.S., Gioli B., Calfipietra C., Gielen B., Kubiske M.E. & Scarasia-Mugnozza G.E. (2008). Future atmospheric CO2 leads to delayed autumnal senescence, Global Change Biology, 14 (2) 264–275 | doi:10.1111/j.1365–2486.2007.01473.x
Originally published at The Guardian on 31 October 2013.