Photos of early fall colors have been traversing the media over the past few weeks, and along with them, the incorrect explanation that cool summer weather is causing an early fall reaction in trees.
While the temperature can play a role in the saturation and vibrancy of the leaf colors, the true inciting agent of the transition from green to yellow and red is the length of daylight.
As days grow shorter, it is typical to see some trees changing color in August.
Through spring and summer, trees are pumped with chlorophyll, a pigment which gives leaves their green hue. Chlorophyll is a necessary component of photosynthesis, a process which manufactures the energy a tree needs to survive. The other necessary component is light. As the length of day — and thus the amount of sunlight — wanes, so does the amount of chlorophyll in the leaves. The U.S. National Arboretum writes:
Chlorophyll breaks down with exposure to light in the same way that colored paper fades in sunlight. The leaves must manufacture new chlorophyll to replace chlorophyll that is lost in this way. In autumn, when the connection between the leaf and the rest of the plant begins to be blocked off, the production of chlorophyll slows and then stops. In a relatively short time period, the chlorophyll disappears completely.
As the amount of chlorophyll in the leaves decreases, the other colorful pigments that are present, like anthocyanins and carotenoids, are able to shine. These pigments give trees their autumnal yellows, browns, and reds.
Timing of the color change can also depend on the species of tree. The sourwood, for example, which is native to the eastern U.S., fades to autumn colors in August. Oak trees, on the other hand, maintain their leafy green hue much longer into fall than their neighbors.
According to the U.S. Forest Service, we know these differences in timing are genetically inherited, because “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.”
Something temperature does play a role in is the brilliance of the fall colors when they finally begin to show. Leaf peepers hope for a certain combination of temperature variables that will lead to the best autumn show. The Forest Service writes:
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 lots of light-spur production of the brilliant anthocyanin pigments, which tint reds, purples, and crimson.
Though it pales in comparison to the length of sunlight in a day, precipitation is the weather variable that has the strongest influence on fall color timing. Too little rain or too much rain can stress trees, which could make them turn color earlier. A dying tree will also appear to be going through its autumn coloring process, when in reality the tree’s photosynthesis processes are simply shutting down for good.
When you put these two things together, temperature and precipitation, you have many different ways the weather could impact the brilliance of a leaf show. From the Forest Service:
The countless combinations of these two highly variable factors assure 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 should produce the most brilliant autumn colors.