As cool weather comes upon us, leaves on many of our trees will start to change from their summer greens to brilliant reds, yellows, and oranges. It is an enjoyable time to be outside and enjoy the beautiful colors. Scientists have worked to understand the changes in trees during this time of year. They have learned about the processes in the leaves that lead to these colors.
The leaves of many of our broadleaf trees are not adapted to freezing weather like our evergreens. Their strategy is to produce leaves that only last one year. During the growing season the leaves are green due to chlorophyll that is being produced in the leaf. The leaf uses chlorophyll in photosynthesis to convert carbon dioxide, water, and sunlight into sugars, and oxygen. As the nights grow longer, chlorophyll production slows and stops in the leaf. As the chlorophyll in the leaf breaks down, other pigments that have been in the leaf all summer become visible. These other pigments are xanthophyll which is yellow, carotenoids which are orange, and anthocyanin which is red and purple. These pigments will eventually break down and the leaves will eventually turn brown. Warm sunny days and cool crisp nights will help to bring the most spectacular displays of color.
Leaf color is also dependent upon the tree species. Oaks will have red to brown colors. Aspens will have brilliant yellow colors. Black gum, sassafras, and sumac will have brilliant crimson leaves. Red maples will be scarlet and sugar maples will be orange to red. Each species will have their own timing of when they change color. So in a mixed species forest you will have wide varieties in colors. Depending on the time in the fall, some trees will still be green and other in in various shades of yellow, red, and purple. Stands of a single species will tend to change at the same time. A good example of this is in the Rocky Mountains where pure stands of aspen will turn a golden yellow at the same time.
Some coniferous trees will can have different colors in the fall as well. Bald cypress, a common ornamental tree will turn a coppery bronze before dropping its leaves in the fall. Tamarack, a coniferous tree that can be found in some northern Indiana swamps, will turn yellow before it drops its needles. Other conifers, even though they are evergreens, will shed needles in the fall. The older needles on eastern white pine will turn yellow and fall at this time of year.
Make sure to get outside this fall to enjoy some beautiful colors that can be seen in our neighborhoods, parks, and countryside.