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A Look at Why Leaves Change Color:

 

September is a month of transition for the Northwoods. Kids are back to school with busy schedules; the long, hot days of going to the beach come to an end; and the chill of fall becomes very apparent. Those crisp, shorter days also bring the transition of the landscape from the green palette of summer to the vivid golds, reds, oranges and purples of autumn. 

 

As the leaves change vibrant colors, have you ever wondered WHY they do so? Is it the colder weather that brings about the change? Why do some trees turn red and others yellow? The display of color we enjoy each fall is explained by understanding the physiology and anatomy of leaves, plant pigments, and the influence of climate and seasonal weather conditions.

 

Leaves perform two major functions for a tree.

• One is to make food through a process called photosynthesis. During the spring and summer the leaves have served as factories where most of the food necessary for the tree's growth are made. This food-making process takes place in the leaf in numerous cells containing chlorophyll, which gives the leaf its green color. This chemical absorbs energy from sunlight that is used in transforming carbon dioxide and water to carbohydrates, such as sugars and starch.
• The other function of a leaf is that they are important to the water balance of the tree. Water not used by the metabolism of the tree passes out through the leaves (through special openings called stomata- think of them as leaf pores) by a process called transpiration.

 

Most deciduous trees have a relatively short period of annual growth. New stems begin to grow from over-wintering buds when the days become longer and the weather turns warm enough to support new growth. For most trees in the Northwoods, growth is usually complete by late June. Next year’s leaf buds are usually set at this time and will not open until they experience the chill and short days of winter followed by the warmth and longer days of spring. Once the leaves are fully expanded and the buds are set, the work of manufacturing and storing carbohydrates to support next year’s growth goes full speed ahead. As the days get shorter in fall, cells near the juncture of the leaf and stem (petiole) begin to rapidly divide, but not expand. This layer of cells is known as the abscission layer.

 

Photo: Destination Door County

 

During the combination of cooler temps and longer nights sensed by plant receptors, plant hormones initiate something called leaf senescence or leaf death. Senescence happens when the leaf begins to break down starches and proteins and export them to storage cells in branches and roots. Storing these compounds permits the tree to shed its leaves while avoiding loss of their nutrients. Senescence causes chlorophyll production to slow and eventually stop which reveals the true color of the tree.

 

There are three main pigments involved in autumn color. 

1. Carotenoids are the pigments responsible for producing yellow, orange, and brown colors in trees such as aspens, oaks, some maples, and birch trees. Carotenoids also assist chlorophyll in the capture of sunlight for photosynthesis, but are not visible for most of the year because there are larger amounts of green chlorophyll.
2.  Tannins cause the brown hues in leaves of some oaks and other trees in autumn. Like carotenoids, these compounds are always present, but only become visible as chlorophyll production is halted. Considered waste products, tannins actually act as a defense mechanism in plants against pathogens, herbivores and hostile environmental conditions.
3. Anthocyanin pigments are responsible for the pink, red, and purple leaves of sugar and red maples and sumac. They are usually only developed in the fall when sugars combine with complex compounds called anthocyanidins. Their color is influenced by cell pH. They are usually red in acid solution, but may become purplish or blue if the pH of cell sap is less acidic.

 

So is there really ideal fall color weather? Well, yes. A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular displays. The yellows and browns appear regardless of weather, but these conditions allow for the formation of anthocyanin production that imparts the brilliant reds and purples of autumn.

 

Photo: Jenny Sadak

 

Soil moisture also affects autumn color. Like the weather, soil moisture varies greatly from year to year. The countless combinations of these two highly variable factors ensure that no two autumns can be exactly alike. A warm wet spring, favorable summer weather, and warm sunny fall days with cool nights will produce the most brilliant autumn colors.

Recognizing tree colors not only for their beauty, but also for the complex and vital roles the underlying pigments play in forest function and survival might just bring new awe and appreciation to the autumnal custom of leaf "peeping."

 

 Author: Jenny Sadak, Environmental Educator, Trees For Tomorrow