This time of year, all across the temperate climate zones of the Northern Hemisphere, an amazing thing happens. Before the long slumber of winter, deciduous trees treat us to an explosion of color so incredible it can be observed from space. These foliage fireworks paint the landscape with a mural of vibrancy that serves as a transition between the lush greens of summer and the desolate white and brown of winter.
Fall is one of the reasons we live in New England. The dripping humidity of summer fades into the crisp, cool air of Autumn. Warm days are perfect for being outside without sweating gallons, and the brisk nights give you the best sleep you’ve had in months.
But what creates our beautiful fall season? How do those leaves know to explode into color? We haven’t been able to fully understand the amazing seasonal transition, but we do know some of the science of fall and factors behind it.
Most people know the compound that makes leaves green. Chlorophyll also is the primary processor in photosynthesis, the process by which plants produce their own food. During spring and summer, when plants are bursting with green, the Chlorophyll is produced in overdrive to harness all of the energy being produced by the sun.
This brings us to the first factor in what creates our spectacular falls: the length of night! As night gets longer and longer, chlorophyll production slows. Trees have been around a long time, and one thing that doesn’t change much year to year is the daylight hours comparative to the seasons. Eventually, in the depths of autumn, chlorophyll production completely stops for the winter. Devoid of it’s chlorophyll, the colors of leaves go from a uniform green to an array of bright reds, yellows, and oranges depending on the other chemicals present in the leaf.
Leafs also contain carotenoids and anthocyanin, which are responsible for the reds, oranges, browns, and yellows. As the chlorophyll slowly drains and the leaves’ veins start to close, these pigments are allowed to show their brilliance. Different species of trees contain different concentrations of these substances, which leads to the different colors each tree tends to take up during the fall. For example, Oaks are usually deep reds, oranges, and browns, while Aspens produce a brilliant yellow.
The second factor is much more variable from year to year: the weather! The cycle of warm, sunny days, and cold (but not freezing) nights leads to the build-up of the richest colors, specifically with the deep reds and purples. Long days allow sugar to produce and accumulate, while the cold nights accelerate the closing of a leaf’s veins, keeping more of the color in the leaves. Moisture also affects fall; depending on how wet or dry the spring and summer are, it can affect when the onset of fall happens. These variables change annually, thus providing us with unique falls every year.
Why does fall happen? Well, if broad leaves stayed on trees when the temperatures plummeted, the sugars inside would freeze and they would lose all functionality. The tree would have to re-grow them anyway. Instead, trees drop their leaves, steel themselves for winter, and keep their sap running closer to the core, helping to protect them from permanent damage from the cold. Meanwhile, the fallen leaves provide important organic material to put nutrients back in the soil. The science of fall is an important part of the seasonal transitions here in New England, and a rich fall lays the ground work for a green spring. One thing is for sure: we are glad fall is here!