- Author: Janet Hartin, UC Cooperative Extension Environmental Horticulture Advisor
Did you ever wonder why trees "turn" color in the fall? The short answer: It's primarily a function of long, cool fall nights and short, sunny days.
The longer answer? Chlorophyll is responsible for the basic green color of leaves we see in spring and summer and is a necessary component of photosynthesis, which uses sunlight to manufacture sugar (food) that is stored during the dormant period of the year. Carotenoids produce yellow, orange and even brown pigments in crops such as carrots, squash, bananas and many ornamental plants such as daffodils and poppies. Anthocyanins are red and orange in color and are most linked to lavish displays of brilliant fall foliage. They also give rise to coloring of strawberries, plums and cherries.
Here's the kicker: While chlorophyll and carotenoids are present in leaf cell chloroplasts throughout the entire growing season, most anthocyanins are only produced in fall due to bright light and excess plant sugars within leaf cells.
So, in reality foliage doesn't "turn" orange or red at all. Carotenoids and anthocyanins are always in the leaves; they are simply unmasked once the active growing season is finished and chlorophyll is no longer produced. This happens when nights lengthen in fall.
Interestingly, the actual timing of color change varies across species and appears to be genetically inherited. The same species will exhibit a similar color scheme in cool temperatures in higher elevations at nearly the same time as it does in warmer lower elevation climates. The intensity can vary quite a bit however.
Where do temperatures enter the picture? Both the amount of color and the overall intensity of fall color is very linked to weather conditions that occur prior to and during the actual time the chlorophyll in leaves winds down. The most brilliant displays occur after several warm, sunny days and cool, crisp (above freezing) nights. This is because although lots of sugars are made in leaves during sunny daytime hours, the corresponding cool nights prevent the sugars from moving out. The amount of soil moisture also helps ensure that from year to year fall colors vary even in the same trees. So, either a late spring or a prolonged drought can both delay the display of fall color by a few days or even a few weeks.
What's the recipe for the most brilliant fall display? Most likely a warm, moist spring followed by a warm summer and sunny fall with cool autumn nights.
Although fall color is not nearly as spectacular in lower elevations of Southern California compared to other colder areas of the nation, the liquidambar or American sweet gum (Liquidambar styraciflua) offers some pretty impressive fall color and an impressive 300 to 400-year life span. (Did you know that liquidambar got its name because it at one time was a sought-after chewing gum for Native Americans?)
To help guarantee vivid colors for years to come, growers carefully propagate trees by cuttings to yield identical clones that will produce just as brilliant fall foliage as their parents. You may have come across the popular cultivar named Festival or even have one in your own yard. I like it because it stays more compact and columnar than most liquidambars. The downside? It is less cold tolerant than other liquidambars but does well in most warmer areas of Southern California.
The cultivar Moraine is broader and a better choice if one has adequate space for a shade tree; it is also adapted to cooler temperatures and it rewards homeowners and people passing by alike with beautiful, red fall foliage.
By Barbara Ott, Butte County Master Gardener, January 8, 2016.
During the long days of summer sunlight, trees cast shade created by leaves that use sunlight to convert water and carbon dioxide into sugar. This is the process of photosynthesis, which feeds the tree and its leaves. Chlorophyll, responsible for the green color we see in leaves, is an important pigment that absorbs the light energy used in photosynthesis. As days grow shorter and temperatures begin to cool, trees respond by producing less chlorophyll, and photosynthesis slows and eventually comes to a stop. When that happens, pigments of yellow, orange and brown in the leaves (called carotenoids) begin to show through. Some trees produce pigments called anthocyanins in their leaves during the fall, creating red leaves.
A severe drought can delay the arrival of fall color by a few weeks. A dry fall with plenty of warm sunny days and cool nights contributes to vibrant late fall foliage.
Deciduous trees begin to release their leaves when cool temperatures trigger a hormone that sends a chemical message to the leaves to “let go.” Located where the leaf stem meets the branch is a thin bumpy line of microscopic cells called Abscission cells. These cells “cut” the leaf away from the stem when they receive that message. When the air moves them, these dangling leaves fall away. If the abscission cells do not develop, wind will not blow the leaves off. This is why some trees keep their brightly-colored fall foliage long after most other trees have dropped their leaves.
The yellow, orange and brown carotenoid colors in leaves remain fairly constant from year to year. Carotenoid is always present under the chlorophyll and does not change in response to weather. But when a series of warm autumn days and cool non-freezing nights occurs, it's a good year for red fall foliage. The warmth allows the leaves to produce sugar through photosynthesis, but the cool nights prevent the sap from flowing through the leaf into the branches and trunk.
Scientists offer several reasons for why some trees produce the anthocyanin that leads to bright reds. Primarily, it seems that anthocyanins protect leaves from excess sunlight and enable trees to recover remaining nutrients. This helps trees prepare for the next growing season. Anthocyanins give leaves their bright shades of red and purple. So, thanks to the warm days and cool nights we had this past fall, we enjoyed spectacular, vibrant leaf color all autumn long.