Image Courtesy: Jean-Pol Grandmont

The Science Behind the Fall Color Display

By Paul Trambley @yourmetpaul September 22, 2017 1:02 pm CDT

Now that we are into September, the days are noticeably shorter, the nights are getting cooler, and the leaves are starting to change. The average peak of fall foliage in Minnesota, Michigan, and Wisconsin runs from mid-September through mid-October in any given year. The timing and richness of peak color can vary each year due to the weather patterns that occur before and during the fall season. This year, expectations are running high for the occurrence of a vivid fall color display. However, weather patterns that occur in the weeks leading up to peak color can still have a major bearing on the quality of the display. The good news is that to this point, the stage has been set for a magnificent display this fall.

Now For Some Leaf Changing Science...

Although there are other factors that play a role in the timing and brilliance of fall leaf color each year, the driving force behind it is the effect that the diminishing amount of sunlight has on plants. During the growing season, leaves are uniformly green due to the abundance of chlorophyll in the leaves. Chlorophyll plays an important role in that it allows the tree the ability to harvest sunlight, thereby providing the energy it needs to carry out photosynthesis. During late summer and early fall, the diminishing amount of sunlight each day causes a decrease in chlorophyll production. Eventually, residual chlorophyll stores decay and become absent from the leaf altogether. When the chlorophyll decays, we start to see the leaves true colors that were masked by chlorophyll during the spring and summer months. A pigment called carotene (the same pigment that gives tomatoes their orange and yellow coloring) is found within the leaves throughout the growing season, but only becomes visible when chlorophyll levels decline.  This explains why you will start to see trees, such as birch and aspen, unveiling their token yellow and orange tones during the fall season. Sugar producing trees, such as maples, add an important nuance to the process of fall leaf color. During the fall, these types of trees produce sugars which are vital for the production of compounds in the leaves called anthocyanins to occur. These compounds are directly responsible for the vivid red and purple colors that appear in some maple trees as well as red oaks. A fall with more sunny days and cool nights will allow the tree to produce more of these sugars, which will result in a brighter fall display.

How Weather Patterns can Detract from or Benefit Fall Color...

The ideal conditions for a brilliant display of fall color are laid out when a warm, moist summer season is followed by a sunny and seasonably cool fall. A fall with adequate moisture along with an abundance of cool (but not frosty) nights, followed by mild, sunny days is ideal for getting the most out of a fall color display. Conversely, predominantly cloudy and rainy fall seasons can result in a more muted fall color display. An early hard freeze can ruin or stunt the display, while some light frosts can actually be beneficial. Early frosts can promote an earlier leaf change, while an extended period of abnormal warmth can delay the color change. Drought conditions can also have an impact on fall color. A modest drought has been known to make for a more brilliant fall color display, while a severe drought will oftentimes dull the colors and/or promote an early leaf drop. Essentially, you want the trees to be as healthy as possible when heading into the fall season. Anything that induces a major physiological stress on the tree can result in early leaf loss or make for a less brilliant display of fall color.

What is the Outlook for Fall Color this Year?

This fall, the upper Midwest and Great Lakes regions are expected to enjoy a beautiful display of fall color. Adequate moisture received throughout the growing season has helped keep the forests healthy and lush this year, leading up to fall. Further west, a major drought continues in the Dakotas, with some drought conditions being reported as far east as northwest Minnesota. Because of this, some of the forests in northwest Minnesota may be under some degree of stress, but the rest of the state is in good shape thanks to a moist growing season.

Much of the region has also started to see some welcome streaks of sunny, mild afternoons followed by seasonably cool nights. Although, if any hard freezes occur earlier than expected, there could be some negative consequences, as an early hard freeze can sometimes cause leaves to detach from the trees before going through their full color change. On the other hand, lighter frosts, that are expected at times, can be a boon to the fall color display.

Fall weather patterns do tend to become more volatile, as we are in a transitional time of year. This can lead to the formation of more intense low pressure systems, which are oftentimes accompanied by major winds and rain. When a strong system decides to strike during or near fall peak, a major leaf loss will occur, bringing a rather abrupt end to the fall color season. That being said, make sure to get out and enjoy the fall color while it's here, no matter how long or short the display lasts, it's always a sight to behold.

Minnesota and Wisconsin update their fall color reports each week, so you can keep an eye on what areas are at or near peak color throughout the season. If you are interested, here are suggestions for some scenic drives to take during the fall color season in the Midwest. 

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