Leaves show their true colors

Credit: Photo illustration courtesy of Daniel Tkacik/SciTech Editor Credit: Photo illustration courtesy of Daniel Tkacik/SciTech Editor

Every fall, a natural event occurs that gives the season its name: the annual shedding of leaves by trees before winter begins. Many look forward to this time of year for the wonderful colors on trees and in piles of leaves. The vibrant yellows, oranges, and reds are the result of an array of processes.

As most people are taught in their preliminary science classes, a plant’s leaves are where photosynthesis takes place. Different trees have different types of leaves; if these leaves are capable of freezing during the winter, the trees must shed them to protect themselves.

Shedding leaves ensures that the frozen sap in leaves, which could potentially be harmful, will not affect the trees’ health. In autumn, trees slowly begin to seal off the veins that carry sap from the leaves into the tree; when this vein is sealed off, the leaf falls off. According to the U.S. Department of Agriculture, trees further benefit because decomposing leaves provide more nutrients to the soil and keep their ecosystems healthy.

Leaves change color right before they fall due to a primary characteristic of autumn: the gradually lengthening nights. As the amount of sunlight that falls on trees decreases, leaves perform photosynthesis less often, which means they need less chlorophyll. Chlorophyll, a pigment naturally found in leaves, is part of the photosynthesis process; this pigment is also responsible for giving leaves their green color. Other pigments present in leaves are carotenoids and anthocyanins. The Wisconsin Department of Natural Resources explains that these pigments are responsible for producing orange and red colors, respectively, in not only leaves, but also in fruits like oranges and strawberries.

Apart from contributing to the yellow and orange colors of autumn leaves, caroteniods are responsible for absorbing excess energy from the sun, unneeded for photosynthesis. Chlorophyll and carotenoids are present in trees throughout the year, but the latter is only visible after the amount of chlorophyll in the leaves decreases in fall. Anthocyanins, on the other hand, are only produced by trees in autumn.

While the exact function of anthocyanins is still debated by scientists, two theories have been proposed. The first theory, according to The Journal of Experimental Botany, states that these pigments help trees recover as much sap as possible from leaves before the veins are sealed off. According to Science Daily, the second theory says that anthocyanins protect trees from insects; studies show that aphids are less likely to suck amino acids from and lay eggs in trees with red leaves than those with yellow ones. Hence, all three of these coloring pigments have important roles to play in ensuring that trees can continue to survive.

However, for those more interested in actual color change than the functions of pigments in trees, there are certain signs that one can watch for to see if autumn will be particularly colorful. Areas with droughts tend to see delayed falls, while those where frost comes earlier than normal see leaves turn brown and fall off without changing to bright yellows and oranges.

Places with warm and wet weather during autumn usually have less brightly colored leaves. The best autumns for producing bright and colorful falls are ones in which the days are warm and dry, while the night temperatures are not below freezing. The temperature during the rest of the year can also be important in producing vivid falls; the brightest leaves are produced after a warm and wet spring and a moderately warm summer.

All these variables show exactly how delicate such a commonplace phenomenon is, and how many factors are responsible for making autumn a much-anticipated season.