The leaf color changes of autumn result from a complex interplay of factors of temperature, sunlight and moisture, all dependent on timing.
Not all the processes are predictable, but it is known that sunny days and cool nights in late summer and early autumn make for a particularly bright display of reds.
The yellow display operates separately from the red display, though both result from a shutdown of photosynthesis.
Shorter days signal the development of a layer of cells at the base of the leaf, called the abscission layer, that cuts off the water supply and later severs the leaf.
This stops production of chlorophyll, the green chemical at work in photosynthesis.
The chlorophyll breaks down, unmasking pigments already present in a leaf that will “turn” yellow: the xanthophylls, or yellows, and carotenes, the same orangey-yellow chemicals found in vegetables like carrots.
Still another class of pigment, the anthocyanins, is manufactured more quickly as summer ends.
As the leaf is cut off from its circulation, its sugar is trapped in the leaf and turns into anthocyanins, which depending on the species can be red, purple or maroon.
Cool nights in autumn inhibit the loss of sugar from the leaves, but brilliant sunshine promotes the maximum sugar synthesis and its transformation into anthocyanins.
A mild to moderate drought also stimulates anthocyanin production.
Drought or stresses like insects early in the season have comparatively little effect on the eventual nature of the color display, but can cause premature browning and leaf loss.
