Green summer foliage transforms into reds, oranges, and browns each autumn. This annual color change signals the shift from the growing season to winter dormancy, involving pigments within leaves and environmental cues. Understanding this natural process reveals how trees prepare for colder months.
The Dominance of Chlorophyll
Throughout spring and summer, leaves maintain their green hue due to chlorophyll. Chlorophyll is contained within chloroplasts. This green pigment plays a central role in photosynthesis, converting light energy, carbon dioxide, and water into sugars. Chlorophyll absorbs blue and red light, reflecting green, which is why leaves appear green. During the active growing season, plants continuously produce chlorophyll, ensuring a steady supply for energy production.
The Emergence of Yellow and Orange
As autumn approaches, the brilliant green of summer begins to fade, revealing underlying yellow and orange tones. These colors are attributed to pigments known as carotenoids, which are present in the leaves throughout the growing season. Carotenoids absorb light in wavelengths that chlorophyll does not efficiently capture, transferring this absorbed energy to chlorophyll for use in photosynthesis. When chlorophyll production slows and existing chlorophyll breaks down in the fall, these stable yellow and orange carotenoids become visible.
The Science Behind Red and Purple
The reds and purples seen in some autumn leaves are due to anthocyanins. Unlike carotenoids, anthocyanins are not present in leaves during summer but are newly synthesized in the fall as chlorophyll degrades. Anthocyanin production is influenced by bright sunlight, cool (but not freezing) temperatures, and sugar accumulation within the leaves. Trapped sugars contribute to the synthesis of these red and purple pigments, which may also serve protective functions like shielding the leaf from sun damage.
Why Leaves Turn Brown
The final stage of autumn coloration for many leaves is a transition to brown. This browning is primarily caused by tannins, present in the leaves throughout the growing season but obscured by chlorophyll. As leaves age and other pigments like chlorophyll and carotenoids break down, tannins become more visible. Brown coloration also signifies the complete breakdown of other pigments and the presence of waste products. This stage often precedes the leaf’s detachment from the tree.
Environmental Signals for Color Change
Autumn leaf color change is primarily triggered by specific environmental signals. The most significant is the decreasing length of daylight hours, known as the photoperiod. As days shorten in late summer and early fall, trees receive a cue to prepare for winter dormancy. Cooler temperatures also play a significant role, particularly in enhancing the brilliance of fall colors. Cool nights combined with sunny days promote the breakdown of chlorophyll and the accumulation of sugars within the leaves. This sugar buildup is especially important for the production of red and purple anthocyanin pigments. These environmental factors collectively orchestrate the complex biochemical processes that lead to the stunning array of colors observed in autumn foliage.