Why do autumn leaves change color?

Every fall, we rejoice in the virtue of the autumn colors. The blend of purple, red, yellow, and orange is the outcome of chemical processes that take place in the tree as the seasons move from summer to winter.

During the summer and spring, the leaves work as machines where most of the tree’s foods are produced. This food-making reaction takes place in the leaf in various cells comprising chlorophyll, which gives the leaf its green color. This unusual chemical absorbs sunlight, the energy used in molding carbon dioxide and water into carbohydrates, such as starch and sugar.

Along with the green color are orange to yellow pigments, xanthophyll, and carotenes pigments, which furnish the orange color to a carrot. Most of the year, these colors are concealed by vast amounts of green coloring.

Chlorophyll and Autumn Season

In the autumn season, because of alternations in the length of daylight and variations in temperature, the leaves end their food-making process. The chlorophyll crumbles, the green color fades, and the orange to yellow colors become noticeable and give the leaves the part of their autumn glory.

Simultaneously, other biochemical changes may transpire, which form added colors through the addition of red anthocyanin pigments. Some mixtures give rise to the purplish and reddish fall colors of trees such as sumacs and dogwoods, while others give the sugar maple its sparkling orange.

Some trees show only yellow colors during the fall. Others, like oaks, present mostly browns. These colors are due to the mixing of different amounts of chlorophyll deposit and other colors in the leaf during the autumn season.

Other Changes

As the fall colors appear, other developments are taking place. When the stem of the leaf is connected to the tree, a particular layer of cells develops and slowly separates the membranes that support the blade. Simultaneously, the tree seals the cut, so that when the leaf is subsequently driven off by the wind or falls from its pressure, it leaves behind a leaf wound.

Most of the trees in the Northern hemisphere cast their blades in the fall. However, the lifeless brown leaves of the oaks and a few other species may stay on the tree until maturity starts again. In the Southern hemisphere, where the winters are warm, some of the broad-leaved trees are evergreen; that is, the leaves stay on the trees during wintertime and retain their green color.

Only Some Trees Lose Leaves

Most of the conifers – spruces, pines, hemlocks, firs, etc. – are evergreen in both the South and North. The needle- or scale-like leaves remain greenish year-round, and single leaves may stay on for 2 to 4 or more years.

Weather and Color

Weather is crucial for the autumn season of color. Temperature, water supply, and light impact the severity and span of fall color. Low temperatures above freezing will promote anthocyanin accumulation exhibiting vivid reds in maples. An early frost, however, will decrease the brilliant red color. Overcast and rainy days tend to boost the intensity of fall colors. The best time to savor the autumn color would be on a bright, fresh, and dry day.

Source: SUNY College of Environmental Science and Forestry

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