Illinois forests are dominated by a mixture of eastern deciduous forest trees and many of our urban areas mirror this forest composition creating a wonderful palette of fall colors each year as the growing season concludes and tree leaves senesce, giving way to winter dormancy. As a tree-lover, this time of year is most definitely my favorite, from an ornamental standpoint. It far outweighs the sometimes hard to detect spring flower structures that our mostly wind pollinated native trees produce.
So, what are the mechanisms in a tree’s physiology that cause this beautiful display? What makes fall color especially brilliant one year and not as great the next? As a forester and arborist, I often get numerous questions this time of year about fall color in leaves. In my excitement for fall, I always love to discuss the mechanisms at play in these amazingly productive plant structures (leaves) that do nothing short of a miracle each year to support some of the largest living organisms in our landscape.
To explain the process of fall coloration in leaves, it helps to understand the role of chlorophyll. From way back in grade school science class, we have all been taught that chlorophyll gives leaves their green color, buy why does chlorophyll create green color in the first place?
Chlorophyll is a leaf pigment designed to absorb energy from sunlight. It is essential in the process of photosynthesis, whereby tree leaves absorb energy from sunlight to convert carbon dioxide and water into sugars that are used by the tree for energy. The chlorophyll appears green because it reflects the green color in sunlight while much of the rest of the color spectrum is absorbed by chlorophyll and other leaf pigments.
Remarkably, only about 0.1% of the sun’s energy is absorbed by living things on earth, trees account for nearly 50% of all solar energy trapped by living organisms. That’s amazing!
As day length dwindles in fall, trees are cued to decrease chlorophyll production. As chlorophyll disappears in leaves, other pigments that were present year round, but overwhelmed by the vast amount of chlorophyll, begin to reflect their respective colors. Carotene begins to show up as orange colors in leaves. This is the same pigment responsible for the orange color in carrots. Whereas xanthophyll starts to display the yellow colors we see in leaves.
Bright fall sunlight encourages sugar production in leaves this time of year although leaves are beginning to shut down for winter. When combined with cooler night temperatures, leaf processes slow and leave some these sugars behind in leaf tissue. Pigments called anthocyanins are produced in some trees species as a last-ditch effort to pull out these late season sugars.
Anthocyanins are primarily responsible for the beautiful red and purple autumn leaf colors. If we have more sunny days in fall, more sugar production is sustained in leaves resulting in more brilliant red and purple leaf colors. During cloudier fall seasons, fewer anthocyanins are produced since sugar production is lessened.
Soil chemistry, which is fairly static thought the seasons, can also drive the coloration from anthocyanins. In a more acidic soil, more red color is produced, whereas in a more basic soils, the same tree species will produce purple coloration from anthocyanins.
In general, weather is usually the biggest factor in determining how colorful our autumn leaf display will be. Just as more fall sun produces more sugar in leaves, warmer temperatures later in fall allow leaves to function longer, since early cold snaps can sometimes cause abrupt leaf die off prior to fall coloration. As leaves near separation from the tree, fall rains can often be the finishing blow, easily knocking leaves off branches. In the same way, windy conditions can also encourage leaves to drop sooner.
As we all anxiously await the height of fall color, both humans and trees appreciate milder fall conditions. Nicer fall weather affords more days for outdoor enjoyment of the amazing autumn display that tree leaves, yet another incredible feat of plant evolution, perform each year. Let’s all hope for beautiful fall weather that will maximize the magnificence of our autumn leaves!