The so-called “evergreen” state can be pretty boring in the fall. Still, when I was growing up in Wisconsin, even if fall colors were pleasing on the eyes, it also meant raking a seemingly endless succession of leaves—and gave notification that it was the precursor to another cold, cold winter. As a purely scientific issue, the U.S. National Arboretum tells us that during the fall months
“Like most plants, deciduous trees and shrubs are rather sensitive to length of the dark period each day. When nights reach a threshold value and are long enough, the cells near the juncture of the leaf and the stem divide rapidly, but they do not expand. This abscission layer is a corky layer of cells that slowly begins to block transport of materials such as carbohydrates from the leaf to the branch. It also blocks the flow of minerals from the roots into the leaves. Because the starting time of the whole process is dependent on night length, fall colors appear at about the same time each year in a given location, whether temperatures are cooler or warmer than normal.”
I’m not sure what any of that means, but what happens next is fairly straightforward. Because of the reduction in daylight hours, the pigment responsible for the green coloration in leaves, chlorophyll—which is broken down by sunlight and must be replaced by nutrients created by the process of photosynthesis—is less able to be maintained through this replenishment cycle. With shorter periods of sunlight in the fall and winter months, along with the process of abscission, the reduction of chlorophyll allows other previously masked pigments to come to the fore—the yellow of xanthophylls, the orange of carotenoids, the red and purple of anthocyanins. These also must be replaced in daylight hours, and when the process breaks down completely by the combination of short hours and cold temperatures, the nutrient connection between the leaf and the stem ends entirely, and the only pigment that survives before it finally falls off is brown.
I don’t know why I’m talking about this, except that I really just wanted to mention what the latest forecast for this coming winter is, and I needed to fill space with something informative if otherwise innocuous. After above average temperatures in September and October in this corner of the country, the National Weather Service is predicting a 67 percent chance of a weak El Nino event this winter. This type of atmospheric event is caused by a variety of factors, including the slowing of easterly trade winds, which results in keeping warm water from the eastern Pacific tropics from migrating west, and the divergence of the jet stream.
If it does occur (not entirely certain), this will produce a 50 percent chance of above average temperatures, and a 33 percent chance of below-average precipitation, in the Pacific Northwest. The southern U.S. should see wetter than normal conditions, the Midwest dryer than normal. Most of California is predicted to see no relief in current drought conditions.