Why Do Leaves Change Colour in Autumn?

Leaves are expensive to run. In winter there is too little light for photosynthesis to pay for itself, and broad leaves lose water and can be damaged by frost and snow load.

So deciduous trees shut them down deliberately. Before dropping a leaf, the tree reclaims valuable nutrients from it, particularly nitrogen, some of it locked up in chlorophyll, and phosphorus. The colour change is a visible side effect of that recycling process.

Four groups of pigments produce the display:

• •Chlorophyll, green, which dominates all summer and breaks down first
• •Carotenoids, yellow and orange, present all along but masked by the green
• •Anthocyanins, red and purple, mostly produced fresh in autumn
• •Tannins, brown, left behind once everything else has gone

Yellows and oranges are simply revealed. Reds are made on purpose, which raises an obvious question: why would a tree spend energy manufacturing pigment in a leaf it is about to discard?

The leading explanations are that anthocyanins act as sunscreen, protecting the leaf from light damage while the tree is still busy withdrawing nutrients, and that they may deter insects from laying eggs. Neither is fully settled, and the fact that trees invest in this at all suggests the benefit is real.

The main signal is shortening day length, which is reliable from year to year in a way temperature is not. As nights lengthen, a layer of cells forms at the base of the leaf stalk and begins cutting off the flow of water and sugars.

Temperature then modifies the display. The best colour comes from warm sunny days followed by cool but not freezing nights, which lets sugars accumulate in the leaf and boosts anthocyanin production. An early hard frost or a drought can cut the show short by making leaves drop while still green or brown.

Conifers take a different strategy. Their needles have a small surface area, a waxy coating and natural antifreeze compounds, so they can survive winter and keep photosynthesising whenever conditions allow.

The trade off is that needles are less efficient at capturing light than broad leaves. Each approach wins in different conditions, which is why deciduous forests dominate temperate regions with distinct seasons while conifers dominate colder and drier ones.

Autumn colour is not evenly distributed around the world. The most spectacular displays occur in eastern North America, parts of East Asia and to a lesser degree northern Europe.

The explanation is largely geological. During past ice ages, mountain ranges running east to west across Europe blocked tree species from retreating southwards, and many were lost. In North America and East Asia the mountains run north to south, so species could migrate and survive. The result is a far greater diversity of deciduous trees in those regions, and diversity is what produces the range of colour.

Not every deciduous tree drops its leaves on schedule. Young beech trees, beech hedges and some oaks often keep dry brown leaves on their branches right through winter, letting them go only when new buds push through in spring. This habit is called marcescence. Botanists have proposed several explanations, such as protecting buds from browsing deer, which find dry leaves unappetising, or releasing the leaves later so their nutrients reach the soil in spring, when growing roots can use them. None of these ideas has been firmly established.

You can see the hidden pigments for yourself with simple chromatography. Tear up a few green leaves, crush them in a small amount of rubbing alcohol until the liquid turns green, then stand a strip of coffee filter paper in it with its tip just touching the liquid. Over the next hour or so, the solvent creeps up the paper and the pigments separate into bands. Alongside the green of chlorophyll you will often see a yellow band from carotenoids, showing that the autumn yellow really was present in summer.

The actual separation of a leaf is controlled by plant hormones. While a leaf is healthy, it sends a steady supply of a hormone called auxin down its stalk, which keeps the cells of the abscission zone at the base insensitive. As the leaf ages and days shorten, auxin from the leaf declines while the gas ethylene increases, and the zone becomes responsive. Cells there then release enzymes that break down their walls and the pectin gluing neighbouring cells together, until the leaf is held on by little more than its water-conducting strands and snaps away in the wind.

Before the leaf falls, the tree also lays down a protective layer of corky cells on the branch side of the break, sealing the wound against water loss and infection. The result is a leaf scar, and it is worth looking at closely in winter. Each species leaves a characteristic scar shape, often with tiny dots inside marking where bundles of veins once passed through. Together with the shape and colour of the buds above them, these scars let you identify many bare deciduous trees, such as ash, horse chestnut and sycamore, even when there is not a single leaf in sight.

The line between deciduous and evergreen trees does not follow the line between broadleaves and conifers. Larches are conifers, yet each autumn their soft needles turn golden yellow and fall, leaving the branches bare until spring. The dawn redwood, known from fossils long before living trees were found in China in the 1940s, turns russet before shedding, and so does the swamp cypress of the south-eastern United States. These trees follow the deciduous strategy, reclaiming nutrients and dropping their leaves, but with needles instead of broad leaves, which shows that the choice depends more on climate and way of life than on leaf shape.

Evergreen does not mean that leaves last for ever. Evergreens replace their leaves gradually, so the tree is never bare. A typical pine needle lives for a few years before turning brown and dropping, usually from the older, inner parts of the branches, which is why the ground under pines is carpeted with needles. Holly and many other broadleaved evergreens shed older leaves mostly in late spring and early summer, once the new growth has taken over. At the extreme, the Great Basin bristlecone pine of the American West can keep individual needles for several decades, useful in a harsh climate where new needles are costly to build.

Leaves change colour because trees dismantle chlorophyll to reclaim nutrients before winter, revealing yellow and orange pigments that were always present, while reds are freshly produced and appear to protect the leaf during the shutdown.