Have you noticed that autumn looks a bit different every year? Sometimes the leaves fall early. Other times they’re on the trees much longer to give a full display of color. A lot of this has to do with the temperature outside. But how does temperature affect the color of leaves?
The temperature of fall and its effect on leaves
As the nights get cooler in the northern hemisphere in September and October, we begin to see the trees change. If you’re lucky enough to live around a mixture of trees, you’ll begin to see bright red, orange, and yellow leaves appear.
Without as much daily sunlight, trees don’t go through as much photosynthesis. This is aprocess that produces sugars, which trees use as energy to grow and flower.
A reduction in photosynthesis leads to a reduction in chlorophyll as well, which is the pigment that makes leaves green. As they lose chlorophyll, they lose their green color and prepare to shed for the winter so trees can conserve their energy inside the branches and bark.
How does temperature affect the color of leaves?
But that still doesn’t explain the role of temperature.
The weather leading up to shorter days is actually quite important when it comes to determining how fall plays out for leaves .
We know that a reduction in chlorophyll leads to leaves being less green, but what makes some seasons produce more vibrant red leaves than others? Why does a tree turn bright orange one year and only a dull copper the next year?
Well, it turns out that the pigments that begin to show up once chlorophyll is reduced are dependent on both temperature and moisture conditions right before days start getting shorter. For example, some weather conditions make a leaf turn red early. It also helps it stay on the tree longer, so it goes through its full range of colors before falling off.
The role of weather in fall leaf displays
According to scientists at Michigan State (cited below), lots of warm days and cool nights narrow the veins in leaves. This helps trap the sugars made during photosynthesis in those leaves. When this happens, the sugars produce more vivid pigments.
“The most brilliant leaf displays follow a period of warm days filled with sunshine and cool nights. During this weather cycle, leaves produce an abundance of sugars during the sunny days. The cooler nights and gradual narrowing of leaf veins in the fall, means that a majority of the sugars produced are trapped in the leaf. An abundance of sugar and light in the leaf lead to the production of vivid anthocyanin pigments, which produce red, purple and crimson colors. Yellow and gold leaf colors are produced by carotenoid pigments, which are ever-present in the leaves and are therefore less dependent on the aforementioned conditions.”
Other factors in fall leaves
“Soil moisture also plays a role in the timing and brilliance of leaf color. The best displays are produced when the soil has been adequately moist throughout the year coupled with the aforementioned late summer weather. A late spring, or severe summer drought can delay the onset of color. A warm period during the fall can also decrease the intensity of fall colors by triggering early leaf drop before the colors have had a chance to develop.”
Finally, MSU explains that other factors can play a role in individual trees:
“Trees on the edge of low-lying areas, where cooler air collects at night, often display colors sooner than trees in an upland forested setting. Trees that are diseased or in decline may also display fall colors earlier than their healthy neighbors.“
And that’s why no two autumns will ever look the same. — WTF fun facts