September 22nd marks the start of autumn, or autumnal equinox every year. Autumn is known as the third season in the year, the transitional period from summer to winter. Autumn also marks the start of daytime and nighttime temperatures steadily decreasing, which leads to leaves changing colors and falling off of trees. Some people have an aesthetic appreciation for the fall transition, but may wonder the scientific process behind this and how leaves fall off of the trees.
First off, this process begins in the spring and summer months. As trees grow throughout these seasons, chlorophyll (or in other words, the green pigment present in all green plants responsible for the absorption of light to provide energy for photosynthesis) is constantly replaced in the leaves. This process with the leaves converting sunlight into energy is known as photosynthesis. This process makes trees lose a lot of water, to the point when winter arrives, the trees are no longer able to get enough water to replace it. As the nights start to grow shorter in the early fall, the cells near the center of the leaf and stem divide rapidly but do not expand. This process of the cells forming a layer is called the abscission layer, which blocks the transportation of materials from the leaf to the branch, and then from the roots to the leaves. As the chlorophyll is blocked from the leaves, it disappears completely from them. The lack of chlorophyll allows the yellow (xanthophylls) and orange (carotenoids) pigments to become visible. The red and purple pigments (anthocyanins) are created from the sugars that are trapped in the leaf. These pigments in leaves are responsible for the vivid color changes in the fall.
As fall continues forward, and leaves start to peak their colors, all good things must come to an end as the leaves will start to fall off the tree. However, the term “fall” is a bit misleading, as this implies that the trees are submissive this time of year when in fact, they are actively pushing the leaves off their branches. The changes in temperature and daylight trigger a hormone that releases a chemical message to each leaf that it is time to prepare for winter. Over the next few weeks, abscission cells form a bumpy line at the place where the leaf stem meets the branch. This process goes at a minuscule pace, as the leaf is pushed from the tree branch. This happens as a means for a tree’s survival.
This process is a gentle but seasonal cycle, however, with changes in global climate variability such as the warming trends that have been noticeable throughout the years will noticeably impact the natural cycles within this process. The peaking of the leaves will start later than it has throughout the past, as well as leaves not becoming as vibrant in color and brittle due to the lack of moisture present and general heating throughout the summer and spring months. Fall is one of nature’s greatest beauties that unfortunately may be impacted throughout the years to come if these trends continue.
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©2018 Weather Forecaster Michael Ames