Four main groups of biochemicals are responsible for the various yellows, oranges, reds and browns that we see in the fall - chlorophyll, carotenoids, anthocyanins and tannins. Each has its own color and chemistry. Varying amounts of these chemicals will give subtle variations in color from one leaf to the next or even from tree-to-tree.
The green color that we see on most plant leaves throughout the spring and summer is due to the pigment called chlorophyll. Chlorophyll is one of several pigments that gather energy from sunlight in the process of photosynthesis. Chlorophyll absorbs both the blue and the red wavelengths from sunlight, and reflects the green wavelengths. Nitrogen is one of the main components of chlorophyll. As daylengths shorten and temperatures get cooler, chlorophyll is broken down faster than it's produced. The majority of the nitrogen migrates back to the twigs, being placed in storage for next year's new growth. As chlorophyll breaks down, the carotenoids are revealed.
Many of the yellows and oranges we see in nature are the result of the various carotenoid compounds. The best-known carotenoids are the ones that give carrots their orange color. Carotenoids play a minor role in photosynthesis so they're present throughout the growing season, but are only revealed when chlorophyll breaks down. Brilliant colors are more often seen when the fall weather has warm, sunny days with cool nights between 32° and 45 °F. Indeed, shade-grown leaves often are a duller color than leaves that are in the full sun, even on the same plant.
The reds and purples found during the fall are due to the anthocyanin compounds. These are produced when sugars combine with compounds called anthocyanidins. Many things affect the exact color produced by anthocyanins, including the pH (acidity or alkalinity) of the cell sap in the leaves. With an acid pH, anthocyanins are often red; in alkaline solution they become purple-to-blue. Because anthocyanins need sugar for their creation, weather conditions that favor sugar production - i.e., photosynthesis - are essential. Bright, sunny fall days produce the best colors. Very little photosynthesis will occur on cloudy days while rain can actually leach out the anthocyanins and carotenoids from the leaves. Various combinations of anthocyanins and carotenoids can result in yellow, orange and red leaves all on the same tree. The exact color that is produced by a tree varies from year-to-year. Some people believe that a more brilliant red color can be produced by fertilizing with acid-forming fertilizers, especially aluminum sulfate. While this method sounds like it should work, there is no scientific evidence to support its use.
In many forests, oak trees don't add much to the collage of fall colors. They often turn brown because of a group of compounds called tannins. Tannins are unmasked when both chlorophyll and carotenoids are broken down in the leaves. While some oaks do produce a light red or pink color in the fall, our native bur oak does not.
Conifers, or "evergreens", may also change color and lose their leaves in the fall. Shedding the older needles - those toward the interior of the plant - naturally occurs every year in many conifers such as spruce and pine. This type of pattern of needle drop is normal. Larch trees are unique among the conifers in that they lose all of their needles every year. They are "deciduous evergreens." The needles turn a bright golden yellow and are beautiful to observe as they float lazily to the ground.
Credit: Joe Zeleznik, Extension Forester, North Dakota State University. 2007.
Fall Colors of North Dakota. Pages 15-16 in North Dakota Climate Bulletin.