Written by Jonathan Roth, Director of Education & Development
The northern hardwood forest and the other deciduous forests of the northeast put on quite a show every Autumn. Few places on Earth put on a show quite like it. But why do our trees make this magic happen? Why such an array of colors? In this article we will be exploring the concepts of seasonal transition and leaf color change in our region and hopefully answer some of the burning questions you might have.
To explore the topic of leaf change in detail, one must first understand the basic reason why leaves are green throughout the growing season to begin with. The green coloration found within most leaves is the result of a pigment called chlorophyll. Chlorophyll is found within the chloroplasts of a plant cell. The chloroplasts absorb primarily red and blue wavelengths of light from the sun, while reflecting green wavelengths, resulting in our eyes being able to see it. The energy that is absorbed through this process is used for photosynthesis, which fuels cellular respiration needed to power the plant’s functions such as growth, reproduction, etc. So now that we know why plants are green, we can explore why they change colors every fall.
As the temperatures grow colder and the days become shorter, deciduous tree species, the ones that shed their leaves every year, take note and start to change internally. Winter presents physiological water limitations, meaning trees do not have the same ability to uptake water, a required component for photosynthesis. As a result, the trees have developed adaptations to conserve water and store energy in the form of starches so that they can both survive to winter and grow again come spring. Many changes occur within a leaf during the autumn transition. First, a new layer of cells starts to develop an abscission zone at the connection point between the petiole, or leaf stem, and the twig. This growth of cells creates a membrane that slowly starts to cut off the vascular connections to the leaf. As the leaf begins to shut down, chlorophyll begins to breakdown in the cells. In some trees, as the chlorophyll dissipates, carotenoids that were already present become visible, presenting as yellow or orange to our eyes. Carotenoids play a minor role in light absorption during the growing season, and take longer to breakdown as the leaf shuts down for the season. As the chlorophyll disappears, it reveals these colors that were covered up the whole summer. In some trees, a red pigment called anthocyanin is actually manufactured in the leaf after the chlorophyll recedes. Evidence suggests that this red pigment plays a role in protecting the cellular structures from the sun’s harmful UV rays as the leaf shut down process plays out, somewhat similar to how sunscreen helps us to avoid a sunburn. Anthocyanin is most prevalent in autumn seasons that are sunny with cool nights. If you take a look to the tops of changing trees, the leaves that are most exposed to the sun’s rays are often the most brilliantly red. The final color that’s prevalent leaves throughout the fall season is brown. Trees with high levels of tannin, a compound that is most often used as a defense mechanism in plants, often present with brown leaves as the chlorophyll breaks down. As you might imagine, no matter the color the leaf initially turns, once the abscission layer has fully separated the leaf, all that’s left is a crunchy brown leaf that has fallen from the tree to the forest floor where it will decompose and return its carbon back to the soil.
The leaf change process, the cellularly programmed “death” of the leaf, is called leaf senescence. You can impress your friends with that if you’d like. Deciduous trees and shrubs undergo such a dramatic shift seasonally as a survival strategy throughout our cool climatic region. Trees are efficient. The process of leaf senescence takes place slowly in order for the tree to reabsorb much of the nutrients held in the leaf. Extra sugars in the leaf are converted and stored within the twigs, trunk, and roots as starch, so that come spring, the tree can tap into that energy to reawaken. These amazing adaptations that allow the plants to survive serve as some of the most dramatic and breathtaking displays in the natural world. How fortunate are we to live in a region where we can enjoy the magic of Nature in such a way?