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The Leaves, They are a Changin'

written by: Atlee Hargis•edited by: Amanda Grove•updated: 8/15/2011

Every year, for those lucky enough to experience it, between late September and early November trees undergo a process of change that is both beautiful and scientifically interesting. So why do leaves change colors in autumn?

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    The Cornucopia of Chemistry

    Every autumn, the crisp, cool air meets the ever lengthening nights and combines to create a natural phenomenon that turns the landscape into a painting of colors. But behind that distinct appearance is a biological and chemical process that has been occurring for millions of years. The details on why the leaves change are somewhat varied amongst different species of plants, but the main reasoning behind it is quite simple.

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    The Days Fall Short

    The various colors of leaves in autumn As the long sunny days become less and less common and the first snow of the season approaches, trees begin to restrict the flow of nutrients to leaves so that they may prepare for the long winter ahead. As this process continues the chlorophyll which normally gives leaves their dark green hue stops getting produced. As the level of chlorophyll drops, the other pigments in leaves begin to make their grand entrance. Caretenoids and anthocyanins, which give leaves their brown, orange, gold, purple, and red color respectively lie below the layers of chlorophyll and, as they become more prominent, so too do the changing colors of the leaves.

    How exactly do these changes happen, why do leaves turn in autumn and not other times of the year? The other major factors contributing to the biological time clock of the seasons changing are not as solid as the changing length of the day. Because trees are reliant on more than just sunlight, the weather and the soil their roots dig in to also makes a difference in how and when the leaves change during autumn.

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    Gone With the Wind

    Throughout the late summer and into the early days of fall the weather will play a major role in how vividly the colors of leaves appear and when exactly they begin to turn. Should a period of unusually hot weather show up in the usually warm to cool days, the amount of sugar production in leaves will decrease, in turn lowering the levels of anthocyanin and decreasing the intensity of the fall colors.

    A group of trees in the process of preparing for winter However, should the weather be favorable, that is, if the days are mildly warm, moist, and the nights are cool and crisp, the sugar production in leaves will be spurred as the veins leading to the tree become restricted, allowing for a very spectacular array of colors to be seen.

    This all explains how leaves change colors, but not why. The primary reasoning is of course the cold temperatures of winter, but there are options for trees aside from simply dropping their leaves.

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    A Fork in the Stem

    There are two basic groups of trees, those that keep their leaves all year round and those that don't. The former being referred to as evergreens, because they're leaves are constantly green and on the tree, only succumbing to old age after a number of years. Most trees, however, will fall into the latter category, those which annually drop their leaves and re-grow them every spring. These two types of trees have very different leaves as well, most importantly is the size of the leaf, with broadleaved trees having much larger leaves to allow for the maximum amount of exposure to sunlight and thus the most sugar production from photosynthesis. The evergreens, on the other hand, have a much smaller, needle-like leaf that is coated in a waxy outer-layer in order to retain moisture and fight off the colder temperatures better.

    Evergreens in the dead of winter These two leaf types have their give and takes as well. While the broadleaved type trees are able to grow large quickly and build up vast stores of sugars for extended growth throughout the winter, they must rely on their larger leaves in order to produce the vast quantities of nutrients they need in order to maintain this growth pace as well as their survival through the cold months. But, because of the significant surface area to volume ratio of these leaves, they are unable to stave off the cold of winter and instead must be dropped to conserve those food stores for the rest of the tree.

    Evergreens, with their many tiny needles, retain their ability to produce nutrients and food throughout the entirety of the year, but because of how little area they have to photosynthesize, they must make up for it in numbers rather than size. With thousands of long, slender leaves, the evergreens are safe to grow all year long rather than stop to conserve their energy like the broadleaved trees. However, because of their inability to produce as much food, the evergreens will grow at a much slower rate.

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    The Natural Cycle

    With every fall, the intricate colors of each different tree gives a perspective on the world of plant biology, one which serves as a reminder of the details behind every aspect of life on this planet and how each species and example is different and unique in its own manner. For some, the leaves changing colors can mark a new beginning in life and for others, they can be seen as a simple picturesque landscape, one who's artist is referred to as Mother Nature.

References

  • Why Do Fall Leaves Change Colors - National Geographic
  • Images provided by Public Domain Pictures (Public Domain)
  • Large image: "The tree is on fire" by tracy from north brookfield,Massachusetts, usa - Flickr.com - image description page. Licensed under CC BY 2.0 via Commons - https://commons.wikimedia.org/wiki/File:The_tree_is_on_fire.jpg#/media/File:The_tree_is_on_fire.jpg
  • Why Leaves Change Color - USDA Forest Service