Fall Color Prediction is Iffy Business

Every September, faculty in the departments of Horticulture and Forestry are approached by hungry news reporters for predictions about the timing and intensity of the fall foliage color. Occasionally, we even get e-mails from residents of other states wishing to schedule trips though Missouri at peak fall color. While we can generalize that peak color is likely to fall in mid- to late October, the odds of predicting color intensity accurately are not good. Since leaf coloration is the result of pigment changes that are influenced by the weather, long-term predictions are often no better than what might be found in the Farmers Almanac (written a year in advance). And, given the unusually cool and rainy summer that we have experienced in 2004, the odds of accurate prediction are even less favorable this year.

As most of us learned in 6th grade science class, leaves of temperate zone, deciduous trees go through a process in the fall that leads to the formation of an abscission layer where the petioles of the leaves attach to the twigs. The process is triggered in most trees by shortening days and cooling air temperatures. In some cases, the final stages of senescence and abscission occur after the first frost. Then, leaves drop to the ground where they decompose, releasing the minerals that they contain for uptake by tree roots near the soil surface. The color changes associated with leaf senescence are due to changes in the relative abundance of certain pigments within the leaf tissue. For example, the clear yellow color of green ash, ginkgo or paw paw leaves results when chlorophyll breaks down to expose xanthophylls and carotenoids that were present all along. Development of purple and red coloration is somewhat more complex and weather dependent. These colors result from the synthesis of anthocyanins, which requires that sugars be present in the leaf tissues. Cool, sunny days with chilly nights favor accumulation of sugars, since photosynthesis is maximized and respiration is minimized under such conditions. Some tree species, such as sweetgum and blackgum, may have multiple colors on the same tree or even on a single leaf. Such species can be depended upon to have reasonably good fall color every year but require good anthocyanin-producing weather to develop spectacular, brilliant red colors. Anthocyanans are red in cells with acidic cell sap and purple when the pH is more alkaline (such as with white ash). In trees such as sassafrass and sugar maple, a combination of carotenes, xanthophylls and anthocyanins produce the orange fall color.

As noted previously, the cool summer of 2004 has produced some interesting coloration on trees. Some red maples and ornamental pears began to develop fairly decent red color as early as mid-August. In most cases, the coloration is not uniform over the entire tree and is not as intense as the normal fall color for the cultivar. Interestingly, many of the trees that colored in August are right next to trees of the same cultivar that are still perfectly green near the end of September. My theory is during periods of June and July when the light intensity was very high and daytime air temperatures were in the 70s, there was significant accumulation of sugars. Since night temperatures were often in the 50s (or sometimes in the 40s), respiration was reduced and sugars were conserved, allowing anthocyanin synthesis. Perhaps the color intensity is less than that during the typical October peak period because senescence had not led to breakdown of chlorophyll masking the anthocyanin. The question that many of us have on our minds is whether the early coloration of some trees will affect color development in October. After observing temperatures in the 90s in early September, I was ready to predict that many trees would drop their leaves early and fall color would be disappointing this year. However, with the recent return of more normal temperatures and the apparent end of our mini-drought, I am more optimistic. I have seen a number of the early-colored trees, including red maples and ornamental pears that appear to be intensifying in color. It also appears that many trees are beginning to develop normal fall coloration. Certainly, the growing conditions this summer were favorable, so trees are likely to have more stored carbohydrates than in a more normal year. If the weather cooperates, this could lead to an excellent display. The wild card is always temperature. If we were to experience an early hard freeze, color development might come to a screeching halt.

Chris Starbuck, Woody Ornamental Horticulture, MU, (573) 882-9630