As the days get colder and shorter and winter approaches, the silver lining for many is the beautiful array of colors exhibited by trees as they prepare for winter dormancy. Maples, oaks, and beech trees all reliably transition from deep green to vibrant reds, yellows, and oranges; but how exactly does it happen? Often we see and enjoy the change without understanding what’s really going on in the cells of the leaves.

The answer is actually pretty simple and is mainly related to light. Trees, like other plants, utilize chlorophyll, a molecule capable of absorbing energy from the sun, to convert carbon dioxide and water into sugars through a process known as photosynthesis. Chlorophyll absorbs wavelengths of light from the sun from mainly the blue and red portions of the visible spectrum and reflects those in the green portion. This is why, when plants are photosynthetically active in the spring and summer, the leaves appear green.

In the fall, when available sunlight diminishes and temperatures drop, many deciduous trees enter a dormant period and cease photosynthesis. Since trees are not photosynthetically active during this time, they do not need to expend energy maintaining their leaves. Instead, they go through a process known as senescence, which involves the breakdown and reabsorption of components of the leaf, including chlorophyll. What remains behind are other molecules like carotenoids and anthocyanins, which absorb light at different wavelengths. Carotenoids mainly absorb light in the blue, green, and red portions of spectrum, and tend to reflect yellows and oranges, while anthocyanins tend to absorb greens, yellows, and blue, and reflect reds and purples. These molecules also eventually breakdown as the leaves enter the final phase of senescence before dropping. Fortunately for us, we get to witness this process first hand!

Photo Source:
https://www.newyorkupstate.com/news/2018/10/fall_foliage_upstate_new_york_adirondacks_catskills_niagara_falls.html

Written by TPS Fellow Katie D'Amico