Oaks: Great fall color!

Earlier in the fall I thought the fall color was going to be a bust, but there has been a marvelous transformation over the last week. The color this year is now fantastic. Not only the maples, which are usually the most prized, but the oaks, which are the dominant species in our area. Yesterday I took at least a 100 photos (bless the unlimited capacity of a digital camera!).

The Hill's oaks are generally the most colorful oaks in our area (they are related to the scarlet oak of Illinois), and this year turns out to be no exception. The two bright red trees in this photo are Hill's, although some other Hill's are reddish orange, red, or even yellow orange.

Mixed oak area at the north side of Toby's Prairie. The red trees are Hill's oaks.

The stand of oaks in the photo below is interesting, because they are all white oaks, and all about the same age. This is a stand at the east end of the Conservancy (those on the right are part of the East Basin savanna). The color range is really wide, varying from yellow, through orange, to rather red.

These trees are all white oaks (Quercus alba), mostly about the same age (around 50 years). Striking variation in color!

The physiology and biochemistry of fall color is fairly well understood.

The shorter daylength in fall triggers a physiological process that results in the decomposition of the green chlorophyll pigment of the leaves. Remaining behind are carotenoids, pigments which are responsible for the yellow, orange, or brown colors.

The red colors come from anthocyanins which the plant produces from sugars in the leaves. Leaf sugar increases when the temperature drops and anthocyanin production begins. The amount of anthocyanin that can be produced depends, among other things, upon the sugar concentration in the leaves. This process is highly variable from year to year, depending on weather, which affects the physiological state of each tree. Another factor is genetics, which determines which enzymes are present to carry out a particular biochemical pathway.

Considering all these factors, it is understandable that leaf color can vary greatly, depending on the amount of chlorophyll remaining, the amount and character of the carotenoids, and the amount and character of the anthocyanins. There is species to species variation, as well as year to year variation. A wide variety of colors are possible, and an individual tree can vary from one year to another.

Further, some species, such as hickories, birches, and aspens, lack the enzymes for making anthocyanins, so their fall colors are exclusively yellow or orange. 

Thus, it is not surprising that the stand of white oaks shown in the photo should vary so much from tree to tree.