Submitted by Dennis Wilson, Forester at Land Between The Lakes National Recreation Area
The easiest way to explain why leaves change colors has to do with the time of year. The longer and cooler nights trigger the changing colors and falling leaves.
None of the other environmental influences – such as temperature, rainfall, food supply – are as unvarying as the steadily increasing length of night during autumn. As days grow shorter, and nights grow longer and cooler, biochemical processes in the leaf begin to paint the landscape with Nature’s autumn palette.
A color palette needs pigments, and there are three types that are involved in autumn color:
- Carotenoids produce yellow, orange, and brown colors in in leaves much like it does in corn, carrots, and daffodils, as well as rutabagas, buttercups, and bananas.
- Anthocyanin gives color to such familiar things as cranberries, red apples, concord grapes, blueberries, cherries, strawberries, and plums. Anthocyanin pigments are water soluble and appear in the watery liquid of leaf cells.
- Chlorophyll gives leaves their basic green color. Many of us studied in grade school science classes about chlorophyll and photosynthesis. Their chemical reaction enables plants to use sunlight in order to manufacture sugars for food. Trees store these sugars for their winter dormant period.
Both chlorophyll and carotenoids are present in the chloroplasts (food producers) of leaf cells throughout the growing season.
Most anthocyanin is produced in the autumn, in response to the bright sun light and excess plant sugars within leaf cells.
During the growing season when chlorophyll is continually being produced and broken down; leaves appear green. As night length increases in the autumn, chlorophyll production slows down and then stops. Eventually all the chlorophyll is destroyed. At that point, carotenoids and anthocyanin present in the leaf unmasked and show their colors.
Certain colors are characteristic of particular species:
- Oaks: red, brown, or russet
- Hickories: golden bronze
- Yellow-poplar: golden yellow
- Dogwood: purplish red
- Beech: light tan
- Sourwood and black tupelo: crimson
The color of maples leaves differ by species:
- Red maple: brilliant scarlet
- Sugar maple: orange-red
- Black maple: glowing yellow
- Striped maple: almost colorless
Some species’ leaves, such as the elms, simply shrivel up and fall, exhibiting little color other than drab brown.
The timing of the color change also varies by species. For example, sourwood in southern forests can become vividly colorful in late summer while all other species remain vigorously green. Oaks put on their colors long after other species have already shed their leaves.
These differences in timing among species seem to be genetically inherited, for a particular species at the same latitude will show the same coloration in the cool temperatures of high mountain elevations at about the same time as it does in warmer lowlands.
In early autumn leaves begin the processes leading up to their fall in response to the shortening days and declining intensity of sunlight. The veins that carry fluids into and out of the leaf gradually close off as a layer of cells forms at the base of each leaf. These clogged veins trap sugars in the leaf and promote production of anthocyanin. Once this separation layer is complete and the connecting tissues are sealed off, the leaf is ready to fall.
The amount and brilliance of the colors that develop in any particular autumn season are related to weather conditions that occur before and during the time of dwindling chlorophyll in the leaves. Temperature and moisture are the main influences.
A succession of warm, sunny days and cool, crisp — but not freezing — nights seems to bring about the most spectacular color displays. During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions – lots of sugar and light – spur production of the brilliant anthocyanin pigments, which tint reds, purples, and crimson. Because carotenoids are always present in leaves, the yellow and gold colors remain fairly constant from year to year.
The amount of moisture in the soil also affects autumn colors. Like the weather, soil moisture varies greatly from year to year.
The countless combinations of these two highly variable factors assure us that no two autumns can be exactly alike.
A late spring or a severe summer drought can delay the onset of fall color by a few weeks. A warm period during fall will also lower the intensity of autumn colors. A warm wet spring, favorable summer weather, and warm sunny fall days with cool nights unusually produce the most brilliant autumn colors.
For more information on our fall colors, you can go to our annual fall color press release at https://landbetweenthelakes.us/experience-fall-colors-at-land-between-the-lakes/.
For more information on where to check out our fall colors, go to the Self –guided section under the “See and Do” tab. For scenic driving, check out https://landbetweenthelakes.us/seendo/self-guided-activites/scenic-drives/.