Why Do Leaves Change Colour in Autumn?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Leaves change colour in autumn mostly because green chlorophyll breaks down and reveals pigments that were present all along. The oranges and yellows were hiding under the green for the whole summer. The reds are a different story, and they are actually manufactured on the way out.
Why change colour rather than just fall?
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 and phosphorus locked up in chlorophyll. The colour change is a visible side effect of that recycling process.
The pigments involved
Three 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
Why reds are different
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.
What triggers it
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.
Why evergreens do not do this
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.
Where the best displays happen
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.
The takeaway
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.