Autumn in a single leaf
Summer's green hides the yellow. The leaf manufactures the red in autumn, and short days give the starting signal, not the first frost. Mix the pigments yourself and see where each colour comes from.
Mix the pigmentsThree colours in one leaf
The green comes from chlorophyll, the paint that catches sunlight and makes sugar from air and water. The yellow and orange sit in the leaf all year, covered by green. The red appears only in autumn, manufactured by the leaf at its own expense.
Mix the pigments
Drag the sliders and watch the leaf. The buttons set a season's values.
The on-screen mix is indicative. A leaf's real colour comes from absorption spectra, not from blended pixels.
In July, abundant chlorophyll covers everything: the leaf is green. In October, chlorophyll falls, the yellow comes out, and the red the leaf manufactures is added on top.
All summer the leaf is a kitchen. Chlorophyll, in abundance, gives the green colour and puts light to work. Beside it sit the yellow and orange pigments, the carotenes and xanthophylls, the same ones that colour the carrot. They stay invisible because the green covers them.
In autumn the leaf closes the kitchen and dismantles the chlorophyll. The green dies away and the yellow comes into view. At the same time, in some species, further reactions begin that manufacture red pigments, the anthocyanins. The mix of leftover green with yellow and red gives the russet, the purple and the bright orange.
Some trees show only yellow. Many oaks turn mostly brown. The brown comes from the mix and, after the cells die, from dark products of the oxidation of compounds called phenolics.
Conifers sit this game out: their needles stay green all year and fall only after two to four years or more.
The leaf's clock
As days shorten and cool, the leaf stops making food. A layer of cells at the leaf stalk gradually cuts the connection to the branch, and the chlorophyll is dismantled. Colouring begins this way, weeks before the first frost.
Does the first frost colour the leaves?
No. Short days give the starting signal.
Cool autumn nights help manufacture anthocyanins and redden the leaf. An early frost, by contrast, weakens the bright red. Rain and overcast skies deepen the colours, and the sun plays its part too: with little sun, leaves come out orange or yellow rather than red.
1 September: 13 hours 16 minutes
On 1 September Bucharest has 13 hours 16 minutes of light. On 1 November it has 10 hours 14 minutes left: about 3 hours less.
The hard question
The yellow costs nothing: it is old paint, unveiled. The red costs: the leaf manufactures it in its last weeks of life. Scientists have argued about the reason for over twenty years. Pick a hypothesis and see what supports it and what troubles it.
What the evidence says
What supports it. Anthocyanins shield the dismantling photosynthetic machinery from light and reactive oxygen species, and the tree recovers more nitrogen into branches and trunk.
What troubles it. In a study of 55 tree species, anthocyanins did not go together with better nitrogen recovery, and the yellow species recovered better.
What supports it. Bright colours advertise strong defence, on the honest-signal principle, and insects avoid red leaves.
What troubles it. Insects avoiding red does not prove the red evolved for them. The debate with the sunscreen's defenders continues.
What supports it. In the few species with fleshy fruit ripe in autumn, the contrast helps birds find the fruit.
What troubles it. Fruitless males, fruitless species and wind-seeded species stay unexplained. The idea can hold only at the margins.
What supports it. Trees that make their own nitrogen with bacteria do not manufacture autumn anthocyanins: zero of 81 species studied, against 42 per cent of the rest. The alder sheds its leaves green.
What troubles it. The result ties the red to nitrogen hunger without choosing between sunscreen and signal.
The conclusion of the newest review is plain: there will be no single simple explanation for all yellow and red leaves. The functions mix, and their weights shift from leaf to leaf and autumn to autumn.
Two small observations help. Shaded leaves stay yellow even in species that can make red, because anthocyanins need light. And many orange leaves are really red in the making: they manufacture anthocyanins.
Six trees from Romania
Not every tree makes red, and some do not colour at all. Here is what six species you meet in Romania do, according to the botanists' accounts.
Fagus sylvatica
The beech of the hills turns yellow, gold and orange.
Quercus robur
The oak of the lowlands turns mostly brown.
Acer pseudoplatanus
The sycamore turns yellow to brown.
Carpinus betulus
The hornbeam turns muted yellow, sometimes orange in full sun.
Cotinus coggygria
The smoke tree, which also grows in Romania, colours brightly in autumn.
Alnus glutinosa
The alder of the riverbanks sheds its leaves green: it makes its own nitrogen.
Each species' colours come from its Trees and Shrubs Online page, with one exception: the pedunculate oak's page states no autumn colour, so for oaks the page follows the SUNY ESF leaflet, which says many oaks turn mostly brown.
Autumn runs late
Data from over thirty years of observation in Europe show spring advancing by six days, while leaf colouring has been delayed by 4.8 days. The growing season has lengthened by 10.8 days since the 1960s, on account of air temperature.
A 2020 study, on data to 2018, confirms the delay of colouring and ties it to warming winters and springs. In other words, the clock in the section above runs the same, but the trees answer ever later.
What the page does not show: records from Romania's phenology stations for leaf colouring. We found no suitable public dataset, so the section rests on European studies and says so openly.
What the sources do not tell us either: what share of the leaf's nitrogen returns to the tree. The page says part is recovered, with no percentage, because the sources read give none.
Play
Eight statements, each proven by a source at the foot of the page. Answer, then read the explanation.
All the answers, with explanations:
Sources and method
The page starts from primary sources: the Why Leaves Change Color leaflet of the SUNY ESF college, the University of Wisconsin–Madison page, Matile's 2000 abstract, Lev-Yadun's 2023 review (full text, open access) and the European phenology studies. Every weighty claim rests on an exact quotation, machine-checked against the raw files in the research folder.
The light curve is computed in the project folder with NOAA's solar algorithm, for Bucharest's coordinates. The on-screen colour mix is indicative: real absorption spectra cannot be blended on a screen.
Books and articles read in abstract only are marked as such. Pages blocked at access (US Forest Service, Woodland Trust) were replaced with SUNY ESF and UW–Madison.
Every photograph is served locally, with full attribution beneath it.
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