It is that time of year again. The summer heat is behind us and the autumn rains have arrived. The moose are out of velvet and the hunters are in the field. Leaves are beginning to turn to their brilliant fall colors.
The Denali Highway is known throughout Alaska for its spectacular early September panorama. The dwarf birch, blueberries and kinnikinnick turn red, interspersed with the bright yellow streaks of the heavy willows in the draws.
So, why do some leaves turn red and others orange and yellow?
Most of us are aware that leaves appear green because they contain chlorophyll. Chlorophyll is necessary for photosynthesis, the chemical reaction that enables plants to manufacture sugars. These sugars are necessary to keep plants through their dormant winter period. There is so much chlorophyll that it masks other pigment colors in the leaves.
As the days shorten, the production of chlorophyll slows and it begins to degrade. Sugar production in the leaves makes a last surge. This final surge causes an increased production of another chemical called anthocyanin. Leaves containing this pigment appear red. Only about 10 percent of trees contain anthocyanin in most areas, but in much of Alaska's above-timberline areas, as many as 70 percent of the plants contain this red chemical.
Carotenoids are another class of pigment. They can be orange or yellow, but more commonly yellow. Carotenoids are always present in leaves but become visible only when the chlorophyll degrades. Leaves with little or no anthocyanins will show bright yellow, such as our willows.
There are other chemicals that affect leaf color to some extent. Tannins will turn leaves brown just before they fall. And yes, the word "fall" comes from just that, an old English word that means "the falling of the leaves."
Deciduous trees, like birches, poplars and willows, drop their leaves because the leaves are not tough enough to withstand freezing temperatures. They must lose their leaves or die.
Evergreens such as spruce and hemlock have waxy coatings on their needles and a natural antifreeze that keep their "leaves" from damage. Chlorophyll production becomes dormant, but does not degrade. The needles stay until they get old and die, then are replaced by new needles. Evergreens contain a lot of tannin -- hence the brown needles as they die.
Other factors affect leaf color. Moisture and temperature affects the rate of chemical reactions. The color will always be there to some extent, but timing and brilliance is partially controlled by current conditions and the preceding summer weather.
Late springs and very dry summers tend to delay the onset of color. Seasonal rains and overcast days restrict the production of red pigments so the color will not be quite so brilliant. Sunny days and cool (but not freezing) nights slow the movement of leaf sugars to the plants roots, spurring anthocyanin production -- and color. Looks to me that the hard frosts of late August coupled with steady rains of the past week might not favor the best late-season show.
Denali color usually peaks during the first few days of September. Things are a few days late this year. We recently had a couple of photographers from India stay over to get shots of the Upper Maclaren valley during the fall splendor. It was too early; they left without a good photo.
Just now, the reds are beginning to show. However, the heavy rains that are chasing the hunters from the field have also taken most of the leaves from the normally bright red blueberry bushes. The Denali fall spectacular may have to settle for a simple magnificence.
John Schandelmeier is a life-long Alaskan and two-time Yukon Quest champion who lives near Paxson and commercial fishes in Bristol Bay.