It Stays Green, and Then It Goes

The measurement this zine is built on is H. Mattila, D. Valev, V. Havurinne, S. Khorobrykh, O. Virtanen, M. Antinluoma, K. B. Mishra & E. Tyystjärvi, “Degradation of chlorophyll and synthesis of flavonols during autumn senescence — the story told by individual leaves,” AoB Plants 10(3), ply028 (2018), doi:10.1093/aobpla/ply028. Open access, read at full text.

The gradual-fade picture is not a straw man — it is what a whole-tree or whole-canopy measurement looks like, because different leaves turn at different times and averaging them together produces a smooth slope that no individual leaf ever follows. The finding required watching named, marked, individual leaves, which is exactly what the paper’s subtitle means.

Verbatim: “chlorophyll and flavonol contents were measured every morning and evening during the whole autumn with a non-destructive method from individual leaves of Sorbus aucuparia, Acer platanoides, Betula pendula and Prunus padus.” That is rowan, Norway maple, silver birch and bird cherry, in Turku, Finland. Leaves at 0.5–2 m were marked before measurements began.

The gadget is real and worth naming: a Dualex, which shines infrared and red light through the leaf and reads what comes out the other side. Because it does not damage the leaf, the same leaf can be measured hundreds of times — which is the entire reason this result exists. A method that destroys its sample can only ever compare different leaves to each other.

They checked the gadget against the hard way. Leaves were also measured destructively — discs cut, pigment extracted in solvent, quantified with a spectrophotometer — to validate and calibrate the non-destructive readings.

Verbatim, and it is the whole paper in two sentences: “In most of the studied trees, the chlorophyll content of each individual leaf remained constant until a phase of rapid degradation commenced. The fast phase lasted only ~1 week and ended with abscission.” Abscission is the leaf letting go.

One tree behaved differently and the paper says so, which is why the read-aloud says the leaf rather than every leaf: in rowan, “very slow gradual chlorophyll degradation occurred on top of the main pattern” — the switch still happened, with a genuine slow fade underneath it. Birch, maple and bird cherry showed the clean version.

The drawing is the shape, not the data. It is drawn from the paper’s described pattern rather than traced off a published figure, and the axis has no numbers on it because the instrument reports an index rather than a quantity a child could hold. What is being claimed is the shape: flat, then a cliff.

Verbatim: “chlorophyll degradation started at different times in different B. pendula leaves, and the leaves belonging to same branches did not behave similarly.” And in two species the order looked like nothing at all: “In the leaves of P. padus and B. pendula, chlorophyll degradation begins in seemingly random order.”

This spread is what resolves the contradiction on the spread before it. A tree really does turn gradually, over weeks; an individual leaf really does hold and then go in a week; and both are true because the fast changes are staggered. The smooth curve everyone pictures is an artefact of averaging.

Rowan was again the exception, in an interesting direction: its leaflets are grouped on a shared stalk, and “the timing of chlorophyll degradation differed between leaves but leaflets belonging to the same leaf had a more uniform timing.” So the unit that switches together is the leaf, not the branch.

The yellow pigments are carotenoids — carotenes and xanthophylls — and they are not autumn decorations. They are working parts of the photosynthetic machinery all summer, harvesting light and protecting the leaf from its own excess of it. Their presence in a green leaf is not in dispute anywhere.

The bars are proportions, not measurements, and the figure says so. Drawing them to a real scale would need one species, one instrument and one set of units, and the two papers this zine uses report pigments in ways that do not share an axis.

Chlorophyll is dismantled rather than discarded, and the chemistry is worth a sentence: it is broken down through several steps into a colourless product that is moved into the cell’s vacuole. The leaf does not switch the green off. It takes it apart. Spread ten is about where the pieces go.

J. Keskitalo, G. Bergquist, P. Gardeström & S. Jansson, “A cellular timetable of autumn senescence,” Plant Physiology 139 (2005), doi:10.1104/pp.105.066845, following one free-growing aspen (Populus tremula) through an autumn. Verbatim: “Lutein and beta-carotene were degraded in parallel with chlorophyll, whereas neoxanthin and the xanthophyll cycle pigments were retained longer.” Abstract read verbatim; the full paper is behind a subscription and we did not reach it.

Set that against the sentence everyone repeats. Mattila et al. give the standard account in their own introduction — “The spectacular autumn colours are partly caused by exposure of carotenoids due to faster degradation of chlorophyll” — and note the hedge, which the popular retelling drops. It is background in their paper, not their measurement; what they measured was chlorophyll and flavonols.

Both things can be true and the resolution is quantitative. Chlorophyll absorbs far more strongly than carotenoids do, so a leaf can lose most of both and still turn visibly yellow. What is not established by the measurement is the tidy causal story that the yellow simply sits there waiting while only the green leaves. One species, one tree, one autumn — and it is the only one of our sources that separated the individual carotenoids.

Why this spread exists at all. A read-aloud could have printed the tidy sentence and nobody would have complained. The house rule is that for kids is exactly the shelf where a verify-everything standard quietly lapses, and this is the sentence on this page most likely to have lapsed.

The red pigments are anthocyanins, and unlike the carotenoids they are synthesised de novo during senescence. Männistö, Holopainen, Häikiö & Klemola, Entomologia Experimentalis et Applicata 165 (2017), doi:10.1111/eea.12626, state it flatly: “Red autumn colouration of trees is the result of newly synthesized anthocyanin pigments in senescing autumn leaves.”

Keskitalo caught the timing directly, which is better than the assertion: “after this date, senescing leaves accumulated anthocyanins in response to conditions inducing photooxidative stress, but at the beginning of September the leaves did not.” Same tree, same stress, different answer — before the programme started and after it. The capacity arrives with senescence.

And the leaf is building other things too. Mattila et al.’s own finding pairs the two: “each leaf maintained steady chlorophyll content until rapid chlorophyll degradation, accompanied by flavonol synthesis, was triggered.” Flavonols are not the red pigments — the paper is explicit that “in contrast to anthocyanins, flavonols do not absorb visible light”, and that their roles are “poorly understood.” They are named here only to make the point the read-aloud makes: the dismantling and the manufacturing happen in the same week.

The photoprotection idea is that anthocyanins shield the senescing leaf from photooxidative stress at low temperatures, allowing better recovery of nutrients — nitrogen especially — before the leaf falls. The signalling idea is that red is a costly, honest advertisement to autumn-migrating insects that the tree is well defended or poor food; it is generally traced to Hamilton & Brown, “Autumn tree colours as a handicap signal,” Proc. R. Soc. B 268, 1489–1493 (2001) — a citation we pinned but did not read.

The 2021 test, in its authors’ words: Peña-Novas & Archetti, Journal of Evolutionary Biology 34 (2021), doi:10.1111/jeb.13903, analysed nitrogen content of mature and senescent leaves against autumn colours and found resorption efficiency correlated with chlorophyll resorption and not with anthocyanin production — which is the correlation the photoprotection story predicts. Abstract read verbatim.

The 2022 reply: Renner & Zohner, Journal of Evolutionary Biology (2022), doi:10.1111/jeb.13957, argue that eastern North American trees receive more autumn solar irradiation than European trees at the same latitude, which could partly explain why so many more of them go red, and raise a shorter foliage period and stronger nitrogen limitation as further factors. Abstract read verbatim.

The read-aloud does not pick a winner and neither do we. A field test of the signalling idea (Männistö et al. 2017, above) is a third strand. What a five-year-old is being handed here is not a fact but a shape: two ideas, a test, a reply, and no result yet.

Verbatim from Keskitalo et al.: “About 80% of the nitrogen and phosphorus was remobilized.” Nitrogen and phosphorus are the two nutrients most expensive for a tree to acquire, and most of a leaf’s nitrogen is locked up in photosynthetic proteins — which is a large part of why the chlorophyll has to come apart at all. Taking the leaf to pieces is how the tree gets its nitrogen back.

The recovery has a deadline, and it is not the leaf falling. Remobilisation “seemed to stop on September 29, probably due to the cessation of phloem transport” — the plumbing shuts before the leaf drops, and the last leaves hung on for over a week after that with some cells still metabolically active.

One species, one tree, one autumn. The 80% figure is aspen’s, and the read-aloud says about eighty for that reason. Resorption efficiency varies widely between species and sites, and it is the quantity the 2021 study on the spread before this one was measuring.

This is the whole zine in one picture, and it is the claim we are most confident of — more confident than the tidy mechanism on spread six, and more than anything on spread nine. Carotenoids are present in the summer leaf; anthocyanins are not. The routes really are different in kind.

Spread seven complicates the left panel and does not overturn it. How much of the yellow is still there by the time you see it is a live quantitative question. That it was there in June is not.