It may technically be autumn, but fall weather has yet to come. For now, southerners are still sweltering in a heat wave that feels indistinguishable from the ones we experienced this past summer. Late season warmth can be frustrating to fans of fall. When Sunday football plays on every television and plastic pumpkins line the aisles of supermarkets, ninety degree days feel somewhat out of place. This frustration is only compounded by the stubbornness of native plants, almost all of which have yet to develop their bright and beautiful fall colors. Trees and shrubs are even more sensitive to weather changes than people are. When fall weather stays hot, plants respond accordingly. Here’s how the effect breaks down:
The Science of Leaf Changes
Leaves function as energy-receptors for trees. They capture sunlight and convert it into food energy in a process called photosynthesis. Plants have a special molecule called chlorophyll that is responsible for this conversion. Chlorophyll gives leaves their green hue in the summer months, when it is continually produced so that sunlight can be continually harvested.
When temperatures cool and the days become shorter, it is no longer viable for most plants to continue converting sunlight into energy. Plants go into hibernation and stop producing chlorophyll. When chlorophyll levels are low, other plant pigments are visible: red, yellow, orange, and purple. These dormant plant pigments are the vibrant colors of fall that many of us know and love well. Unfortunately, these colors are short-lived. Once useless as energy produces, leaves begin to go through a decomposition process that eventually destroys all pigment, rendering them browned, shriveled, and dead. The gradual death of tree leaves is what gives fall its gilded glow.
The Role of Weather
There is no definite time-line for this process. Depending on climate, leaves change colors at different rates. In some parts of The United States, they never change at all. Temperature and moisture levels have a lot to do with it. For instance, a wet spring can cause a barrier to form between tree branches and leaf stems earlier than usual. In turn, this can cause leaves to fall off their branches before their color even begins to turn. In autumn, cool weather and abundant sunshine are ideal for creating bright fall colors. Cool weather breaks down chlorophyll more rapidly; however, freezing weather can destroy all leaf pigments and cause premature leaf death. Unseasonably warm autumn temperatures can delay the colors of fall, but warm days are better than freezing ones, especially when they are accompanied by cool nights. Considering how often our hot days have been followed by pleasantly cool evenings as of late, I think southerners still have a healthy chance of seeing knock-out fall colors this year, though perhaps they will come a bit later in the season.