The green was never the only colour in the leaf. It was just the loudest one. Chlorophyll, the pigment that does the actual work of photosynthesis, is so dominant through spring and summer that it masks everything else sitting underneath it. Yellows and oranges, carotenoids, are there the whole time, quietly doing their own job absorbing light the chlorophyll misses. You just can't see them until the green stops being replenished.
The colours that were there all along
That's what shorter days actually trigger. As daylight drops off, a tree starts sealing off each leaf at the stem, a layer of cells called the abscission layer, cutting off the supply of water and nutrients that kept chlorophyll being produced in the first place. Without fresh chlorophyll, the green fades fast, and the yellows and oranges that were there all along finally get to show. No new pigment involved, just an old one uncovered.
Held up to the light, you can see what was hiding under the green all along.
The one the tree makes fresh
Reds are a different story entirely, and this is where the year-to-year variation actually comes from. Unlike carotenoids, the red pigment, anthocyanin, isn't sitting there waiting. A tree makes it fresh, specifically in autumn, as sugars get trapped in the leaf once that same abscission layer starts blocking the outward flow. Bright, sunny autumn days paired with cool nights, above freezing, are what push a tree to make the most anthocyanin, because that's exactly the combination that traps the most sugar behind the closing seal. Warm nights, overcast weeks, or an early hard frost that kills the leaf outright before this process finishes all mean less red, whatever else the season's doing.
"The reds aren't revealed. They're manufactured, fresh, on exactly the days that suit the tree to bother."
Why one October never matches the last
Which is the actual answer to why this year's display can look nothing like last year's. It's not mood, or luck, or a good year versus a bad one in any vague sense, it's a fairly precise weather record written into the hedgerows and woodland, one autumn's exact run of sunny days and cool nights against another's.
- Bright, sunny autumn days. More sugar produced in the leaf during daylight means more raw material for anthocyanin to be made from.
- Cool nights, but not freezing. Cold enough to slow the sugar escaping, not so cold the leaf is damaged before the colour develops.
- A dry, settled spell rather than wind and rain. Wind and heavy rain strip leaves early, cutting the display short before it peaks.
- No early hard frost. A sharp freeze too soon can kill the leaf outright, skipping the colour stage entirely and going straight to brown.
A duller, wetter run of weather genuinely does mean duller colour. There's a reason for it every single time, even when it just looks like the trees decided to try less hard this year.
Then it falls anyway
And then the leaf falls anyway, red or not, because that abscission layer was never really about colour. It's the tree cutting its losses before winter, shedding anything it can't afford to keep watered and fed once the light drops further still. The colour is just what happens to be visible on the way out.
Not a mood. A mechanism. Turns out they're both true at once.
The short version
Nothing about this October's colour is random, and nothing about it is permanent either. The yellows and oranges were in the leaf the whole time, waiting on the green to fade. The reds get made fresh, a direct record of exactly how sunny the days were and how cool the nights were while it happened. Good weather for colour and good weather for a walk tend to be the same weather, which is about as useful as a bit of plant chemistry gets. Worth getting out and looking while it lasts, it won't look quite the same again next year.
The Quieter Field Notes go out on the first of every month. Each edition covers what's worth noticing outside that month. If this was useful, the newsletter contains monthly things to see and do. Sign up at quieter.life. You'll get 10% off our range as well.