I’ve written quite a bit about the importance of leaf “litter” on the ground, so here’s a little info on how it gets there and what conditions make for the most vibrant leaves. While it’s understandable to think that it is the cooler temperatures of the fall season that bring about color change, there are several other factors. Besides temperature, sunlight and soil moisture influence the quality of autumn leaf color. But the process that instigates the show is actually more of a chemical process brought on by less daylight.
Darkness rules
Most plants are quite sensitive to each day’s length of darkness. In early fall, when nights begin to lengthen, the cells near the joint of the leaf and stem in deciduous trees and shrubs are triggered to divide quickly. This corky layer of cells (the abscission zone) begin to block transport of essentials such as carbohydrates from the leaf to the branch, as well as the flow of minerals from roots upward to leaves.
When plants are actively growing, green chlorophyll is constantly produced in the leaves. But in autumn, when the connection between the leaf and the rest of the plant gets more and more obstructed, chlorophyll replacement slows and then stops completely. This is when autumn colors are revealed: Normally masked by chlorophyll, yellow pigments called xanthophylls and orange pigments known as carotenoids become visible when chlorophyll shuts down. Red and purple pigments come from anthocyanins which are created (in some species) from sugars within the leaf and it’s speculated that they are a defense mechanism that helps some plants fight herbivores like aphids.
As fall moves forward, the cells in the abscission layer become drier and weaker and leaves eventually part company with the plant. Many trees and shrubs lose their leaves when they are still colorful (making for some gorgeous mulch!), while some retain the majority of their foliage through much of winter, though their leaves lose color fairly quickly. Like chlorophyll, the other pigments eventually break down in light or when frozen. The final pigments are tannins, which look brown. An example is the Oregon white oak (or Garry oak), which appears golden for just a few days before turning brown.
Recipe for color
Low temps (but above freezing) and ample sunlight following formation of the abscission layer cause quick destruction of chlorophyll and promote the formation of bright colors in some species. Stress from drought during the growing season can sometimes trigger early formation of the abscission layer, resulting in leaf drop before they have a chance to develop fall coloration, so a growing season with ample moisture that is followed by somewhat dry, warm, sunny, calm fall days with cool, frost-free nights provides the best recipe for bright fall colors.
Plant natives!
Besides offering the most ecological benefits, some native species grown in their native ground offer wonderful fall color that rivals that of nonnative plants. Here in the Pacific Northwest, some of the most vibrantly colored leaves occur on natives such as paper birch, black hawthorn, Oregon ash, quaking aspen (pictured, top), golden currant, vine maple, serviceberry, and red-twig dogwood. Enjoy!