For many, including myself, autumn is a time to accept the ever-changing climate of our lives. This metaphorical billboard reminds us that in life, change is not only necessary, but inevitable.
The change begins in early autumn and can last for several weeks into October. Although correlated to the change in temperature, the process is actually triggered by the shortening length of the days as the northern hemisphere moves further from the sun. This process is referred to as photoperiodism.
The leaves of deciduous trees (trees that lose their leaves each year) appear green due to a compound called chlorophyll, which is necessary to promote healthy growth through photosynthesis, the process by which plants convert sunlight to nourishment. Chlorophyll is constantly breaking down when exposed to sunlight however, so plants are always making more. When light reflects off the leaves in summer, the chlorophyll gives us that familiar green color, but other pigments are also present but yet unseen, such as yellow and red.
Chlorophyll doesn’t stick around all year. Come autumn, when the photoperiod begins to change, the tree begins the dormancy process by releasing phytochromes. One of the first changes is the slowing of water and carbohydrates throughout the plant. As the chlorophyll continues to naturally break down, the tree can no longer replace chlorophyll at the levels it was able to during summer.
The chlorophyll breaks down in the leaf until it’s gone and other pigments take center stage, specifically anthocyanin (red), carotene (orange), and xanthophyll (yellow).
Other factors, such as soil acidity and weather, can affect the vibrancy of these new colors. Sugar maples turn the landscape into a fiery flame-tipped sea of orange carotene, while birch trees add to the sights by showing off their yellow xanthophyll. As chlorophyll breaks down in red maples, the pigment anthocyanin produces the startling reds seen across some landscapes.
As the dormancy process continues, the tree stops sending nutrients and water to the leaves and eventually the leaves fall. This is a particularly important survival tactic for deciduous trees, helping it survive the harsh winter. Leafed-out branches weighed down by snow can cause open wounds, for example, creating a home for infectious pathogens or parasites.
Come spring and another shift of the photoperiod, the buds will form to produce the return of bright green leaves and another growing season.
Photo of Ampersand summit by Connor John Schmitz.