Ohio Trees in the Fall
Fall in Ohio can be an amazingly beautiful time and if I’m being honest my personal favorite season of the year. The cooler weather, OSU football games, changes in food and drinks, the lack of bugs, and fall color changes just seem to make me want to throw on a pair of jeans with a hoodie and sit out by a campfire. While we may be enjoying these times you also can’t help but notice the changes happening in the natural world. Birds are migrating south, flowers and fauna of different regions will either begin to migrate, put on their winter coats, prepare for hibernation, and even dormancy. Shutting down to ensure that the colder temperatures will not harm their cells and they can bloom or forage again next Spring. Today we’re going to dive a little deeper into what happens to leaves and trees in the Fall, why trees use dormancy in the Fall and Winter as a natural process in their life cycle, and how it benefits them.
First off let’s go into what affects leaves in the Fall for an Ohio native tree species and the processes that are involved with this. Leaf color change and ultimately leaf drop are affected by three factors that include weather, length of nighttime, and pigments in the trees. Weather will have an effect on the trees because as the temperature, moisture, and sunlight change in this climate the amount of chlorophyll, carotenoids, and anthocyanin will be produced in differing levels of the leaves. But let’s not get ahead of ourselves, the reason that weather and temperature play a role in this change is the lack of sunlight that trees and leaves are receiving during the day and the elongated nights with colder temperatures. The water available to the tree from the soil leading up to this season will dictate the vibrance of the colors. Hotter temperatures or drought prior to fall or during this time period can lead to either a delayed color change or one not showing much color at all. For more vibrant colors we want a wet summer with cooler temperatures going into the nighttime, but not freezing. Not all trees will lose their leaves though! Evergreen trees have adapted with scale or needle-like leaves that have a thick waxy covering on the outside in order to prevent them from freezing. Chlorophyll is green pigments that are found in the chloroplasts of algae and plants that are responsible for absorbing sunlight and then transforming that energy into sugars for the tree. Chlorophyll is a crucial component of photosynthesis and one required by plants in order to make this transformation into a form of food, sugar, that the plants and algae can use. The leaf will sense these changes and begin to close a vein that connects it to the tree for nutrient and water transport. This closing of the vein restricts chlorophyll transport to the tree and ultimately halts altogether. Now that we have had a blockage in the vein connecting it to the branch of the tree, our anthocyanin and carotenoids, although present for the whole year, begin to emerge. As chlorophyll cannot leave the leaf and is no longer being produced by the leaf, the increase in sugars will activate the change from green to the fall reds, yellows, oranges, and browns. Carotenoids will be responsible for the yellow, brown, and orange colors that are reflected from the leaf. The anthocyanins are responsible for the reds that are emerging from the leaf. Once the vein has completely closed off the leaf is now ready to fall to the ground.
Now let’s talk about why trees drop their leaves and how this dormancy helps preserve them while going into winter. Trees use their leaves in order to perform the process called photosynthesis which is how they receive energy from the sun that is converted into sugars for food. The reason why a tree will lose its leaves in the fall is that the leaf is essentially too exposed to cold and frigid temperatures and consists of water and thin cell walls. The cells in the leaf held together by the cell walls will freeze and rupture under the direct influence of freezing temperatures. This is why trees drop their leaves instead of trying to keep them alive over winter. To combat freezing from happening within a tree itself, cells will actually push water out or increase the pressure within the cell as the tree is preparing for dormancy. This is used to help prevent the cells of the tree from rupturing after they have frozen. Another barrier that a tree will use is its bark. Although bark is an added layer of protection, in the winter a tree is able to use the higher temperatures we experience during the day as well as sunlight to increase the internal tissue temperatures and aid with water and sap movement. Sap moving within a tree will increase the freezing temperature allowing the mixture of water and sap to move more freely within the tree veins, keeping the tree alive.
After our hard Ohio winters have moved through it is now time for the tree to convert from survival mode to growth. A combination of higher temperatures, longer more sunny days, sufficient water, and nutrients will cause an increase in root growth in order to accumulate the water and nutrients. These factors cause the tree to elongate and grow with the pre-existing buds prepared to face another fun and exciting Ohio year.