Northern Minnesota phenology report: September 2009
By John Latimer, KAXE radio, Grand Rapids
In late September and early October garter snakes begin to search out places to hibernate. It isn’t a matter of choice. Their food supply is going to disappear and cool temperatures will force all cold-blooded critters to slow down. To avoid freezing to death the snake must locate a place below the frost line.
Abandoned anthills, up-turned stumps, rock crevices, and even foundations of buildings are among the suitable choices. It is challenging for snakes to find appropriate places to hibernate. In some areas they will congregate in large numbers to spend the winter together. In Manitoba the garter snakes all gather at a rock outcrop known as the Narcisse snake dens. Estimates range as high as 8000 garter snakes using this location. There simply is nowhere else to go. Here in northern Minnesota there are a few more options but it is still possible to have dozens of snakes heading to the same spot to spend the winter.
Unlike bears or groundhogs, snakes do not sleep away the winter nor do they live off stored fat. They simply slow down. Their metabolism is reduced to the point where they may not even lose any weight during the winter. They usually avoid eating in the last few weeks before entering their hibernacula. Food taken late in the fall might remain undigested in the stomach and rot. They may be sluggish but they are alert and if necessary and possible, they will retreat further down to avoid frost.
Garter snakes are remarkably cold tolerant. They are among the last to seek out shelter from the cold. Often they can be seen basking on roads in the fall. This cold tolerance extends their range across the United States, with the exception of the arid southwest, and into sub-arctic Canada. They are the most northerly distributed serpents in North America.
As September trails off into October the deciduous forest puts on a show unrivaled in all of nature. Beginning with the black ash and continuing through to the various members of the willow family all will add their splash of color to the surrounding woods. Some are predictable and will be the same color year in and year out while others react to the conditions and surprise us with a new color each year. The sumacs shine red with a brilliance demanding your attention. The speckled alder seldom change at all and suddenly drop their still green leaves. Quaking aspen become a dependable yellow, while their cousins the big tooth aspens may follow along with yellow or they may show us a shade of orange not seen outside of the deer hunting season.
The real wonders come from the maples. Here we find the true measure of the fall. There are three factors that influence the leaf color, pigments that are present in the leaves, length of the nights, and weather. The most significant of the three is the number of hours of darkness. Rainfall, food supply, and temperature are too unpredictable for the tree to rely on as triggers for shutting down. Only the increasing length of the night hours is unvarying enough to begin the process that results in fall colors.
These colors are the result of three pigments, chlorophyll, carotenoids, and anthocyanins. Chlorophyll is responsible for the green color seen all summer. It is the foundation for photosynthesis that produces the sugars the tree stores to make new leaves in the spring. Carotenoids supply the yellows, oranges, and browns in plants like bananas, corn, and carrots.
The anthocyanins provide the reds and blues for apples, bunchberries, raspberries, and blueberries. The first two chemicals can be found in the leaf throughout the summer, but the anthocyanins are produced in the fall as a result of bright sunlight and excess sugars. As night length increases chlorophyll production ceases and the underlying colors begin to show through.
If the weather is warm and sunny with cool but not freezing nights the leaves continue to produce sugars. Because the leaf is already in shutdown mode the avenues to move the sugar to the roots are constricting. The combination of lots of sugars in the leaf and bright sunlight combine to produce lots of anthocyanins. In these circumstances we will see sparkling reds, deep purples, and crimsons. In the absence of these favorable factors the carotenoids that are always present in the leaves will show through and the colors will tend to be more yellow orange and brown.
Wet warm falls tend to favor the yellow colors. A perfect year for brilliant colors would have a warm wet spring, followed by favorable summer weather and warm and sunny fall days with cool , but not freezing, nights.
In the end what causes the leaves to fall? All the while we are enjoying the fabulous fall colors a layer of corky cells is forming at the point where the leaf joins the stem. This is called the abscission layer. Basically the tree is forming a scab to prevent the loss of fluids at the junction of the leaf and the stem. Once the scab is complete the leaf is ready to fall. In some trees the leaf drops as soon as the layer of cells is complete, in others like the oaks and ironwoods the leaves may hang on all winter. In the case of the young ironwoods it seems as if the emergence of the new leaves in spring provides the impetus to drive the old leaves off. It is often April before these leaves fall.
John Latimer is well known throughout northern Minnesota for his phenology work. He appears weekly on KAXE radio in Grand Rapids, and audio and twitter archives are available here. His work is a frequent feature on MyMinnesotaWoods. This article also appeared in the Duluth Senior Reporter. It is printed with the author’s permission.