Autumn Leaves

Autumn officially began just a few short weeks ago, but we are already able to observe the changing colors and leaf fall in the woods.  There is much about nature that we can appreciate at this time of year, with observation and a little help from books, blogs and research.

Leaves on trees produce food for the rest of the tree.  They do this by a process called photosynthesis which combines carbon dioxide, water, pigments and energy to create sugars and oxygen.  The sugars are used to feed the rest of the tree.  The oxygen is a by-product that is released into the surrounding atmosphere.  The various pigments include chlorophylls, which allow a plant to absorb energy from light; carotenoids, which also assist in energy absorption; and xanthophylls, which protect the photosynthesis process from the toxic elements of light.  All of these pigments give leaves certain colors.  During spring and summer, chlorophylls are present in the highest concentrations, and they give leaves their green color.  In autumn and winter, as the number of hours of light per day gets shorter, the photosynthesis process slows down and finally ceases.  The amount of chlorophyll decreases and color from the other pigments starts to show through.  Carotenoids are orange and yellow, while xanthophylls are yellow and brown.  Another pigment that is only present when there is more sugar being produced than used by the plant is anthocyanin, and it colors the leaves red and purple.

Colors in autumn may be brilliant in some years, or more muted in other years.  This intensity is determined by the weather conditions during late summer and autumn.  Carotenoids and xanthophylls are always present in the leaves during food production season.  Dry periods in late summer reduce the amount of sugar being produced.  Thus, the red and purple hues from anthocyanin may be muted or not present at all, allowing more of the orange, yellow, and brown pigments to be displayed.  Colors may be more muted with less moisture in the leaves and the length of time the leaves remain on the tree may be shortened.  

Color is also affected by temperature.  When days are warm and sunny, leaves produce a lot of sugar.  At night, in cooler weather, leaf veins constrict and limit the amount of sugar flowing to the rest of the plant, thus creating an excess of sugar remaining in the leaves each day.  This is when we see lots of red and purple hues from anthocyanins displayed.

While leaves are falling, trees are preparing for spring in other ways.  During senescence, that time of year when leaves grow old, carbohydrates, nitrogen, sulfur, phosphorous and potassium are reabsorbed by the tree in great quantities from the leaves.  None of these elements are lost, but they are stored in twig, stem and root tissues.   They will be used in springtime to fuel the beginning of the next year’s growth. 

Another way for a tree to get a head start on next year comes from bud growth.  Buds are formed in late summer or early fall, and are covered with modified leaves called bud scales.  These will seal them against the cold and wet weather of winter.  Most of the buds that you can observe are leaf buds containing tightly packed, immature leaves.  In spring, when the weather gets warm enough for sap to start flowing, these buds will unfold into the first leaves of spring.  Larger size buds may be flower buds, depending on the species and age of the tree.  Flower buds do not change much in the winter months, but they will grow a bit larger as we get near bud burst in spring.  You may also observe terminal buds, found at the ends of branches.  Oak trees, as well as other species, add length to existing branches when these buds begin their spring growth. 

There are numerous good field guides about trees available, but I would like to mention four  that I use.  “Winter Tree Finder” by May Thielgaard Watts and Tom Watts instructs you in how to look at a twig and its structures.  Then the book guides you through an examination of a twig to identify the genus and species of deciduous trees in winter.  “Trees of Illinois” by Linda Kershaw is another book organized using keys with excellent pictures of leaves, buds and fruit for each species, along with ranges and characteristics of each.  “The Tree Identification Book” by George W.D. Symonds provides a wide variety of black and white photographs for every part of a tree including thorns, leaves, flowers, fruit, twigs, buds, bark and needles.   Peterson Field Guides presents “Ecology of Eastern Forests,” with chapters describing the plants and animals encountered in different types of forests, and how they all function together.  Other chapters talk about how forest patterns change with the various seasons.