Even though Illinois recently received a break from this summer’s heat and drought, the precipitation deficit that remains statewide has kicked off autumn with atypical natural events. Thus far, the year 2012 has been the fourth driest on Illinois record. However, it has been raining acorns and fall colors have been peeking through the greenery since late August—three weeks earlier than usual. This fast-forward to fall is a tree’s way of protecting itself when water is in short supply. The vibrant color displays of autumn, which seem so lively, are actually a sign that a tree is entering dormancy.
These flashes of fall colors are a result of changes in pigments. The dominant green pigment in leaves is chlorophyll. The leaves in a tree are like little factories, mixing together a recipe of specific ingredients (sunlight, carbon dioxide and water) to make food for the tree’s growth. Chlorophyll acts as the “chef” in this process, called photosynthesis; its presence is necessary in bringing everything together.
Typically, autumn’s cool nights and shortening days trigger photosynthesis to slow down. The scarcity of one key ingredient, water, is triggering this earlier-than-average dormancy. As the work of the leaves comes to an end for the year, chlorophyll breaks down and reveals yellow and orange pigments that have hidden behind its green cloak all summer. Leaves that contain the pigments xanthophyll and carotene—as do hickories, cottonwoods, elms and some maples—will change to vivid shades of yellow and orange as the green fades.
The bright reds and deep purples seen in dogwoods, sumacs and certain maples are a result of a different process. Rather than being revealed in the absence of the “chlorophyll cloak”, these colors are a result of the concentration of a sap pigment called anthocyanin, which becomes more concentrated as autumn progresses. It is this same pigment that gives the skin of an apple its red color. The more sunlight available when anthocyanin is being concentrated, the more vibrant the color. This color concentration is obvious when leaves growing directly in the sun display brighter colors than those within the same tree that are growing in the shade. (This is also why apples sometimes have one side that is red while the other is green; the redder side was more exposed to sunlight.)
Oak leaves often turn brown with very little yellow or orange coloration. This is because they contain large amounts of tannin (brown color) and relatively little carotene. For a more in-depth look at the chemistry behind fall colors check out this Chemical of the Week webpage from the University of Wisconsin-Madison.
There is no way to reverse these amazing processes that have already started in the trees, and most trees will not suffer long-term effects from this drought and the early onset of fall. For more information on trees in Lake County this season and find out how to help trees that are struggling in your yard, read this recent article from the Libertyville Review.
Come out and enjoy the early colors in the Lake County Forest Preserves on a Walk with a Naturalist or at our Annual Fall Festival. Or take the Hike Lake County Challenge and compare colors at different preserves. Hopefully, there will still be some fall color remaining for the fall Phenology and Photography workshop, too!