Triggered by changes in temperature, leaves change colors as a result of them being cut off from water and nutrients. Trees high in sugar content, like the maples, have leaves that are brilliantly colored. When sugar cannot be transported to the leaves normally high in sugar content, they change colors, often from red to yellow. Ralph LaPlant Photo

Most Minnesotans, when asked which their favorite season is, will say fall. The reasons are many, such as hunters who have their season then, bird watchers who watch the migration south, people who are just plain sick and tired of the heat, humidity, and bugs of summer (that be me), and those who observe in awe the magnificent colors of trees that fall brings. Autumn brings to us a time to reflect back on the activities of summer and to the winter ahead.
    
What triggers the change in leave colors is temperature change, and as a result, there can be differences from year to year as to when the colors start to change.
    
Between the leaves and stems is a layer called the abcission layer. Shorter days and cooler nights trigger cell activity in this layer. As a result, the leaves are literally cut off from the stems as a result of lack of minerals and water.
    
Chlorophyll is a pigment that makes leaves green. There are other pigments present that are overwhelmed by chlorophyll. Certain pigments that are now dominant cause certain colors. Carotenoid and xanthophylls pigments give leaves their yellow and brownish colors.
    
By-products cause other colors. A pigment in leaves high in sugar content, such as maples, called anthocyanin, causes their leaves to change colors. Once the abcission layer is formed, sugar cannot be transported to the stem. As a result, the leaves are changed to anthocyanin. Anthocyanin, when decomposing, causes changes in leaves colors, often from red to yellow.           
    After the leaves fall, certain things happen. For those having yards to rake, they get bagged and often dumped at site that will use them for compost. In the forest, the leaves and needles that fall are not wasted. Decomposing, they restore nutrients to the forest’s floor. A spongy humus is maintained that retains moisture that is vital to the forest’s existence. The forest ecosystem needs certain organisms and humus is vital food to them.