This fall’s foliage, now mixed with snow in some parts of Maine, has come mostly in yellow and burnt orange, with only a rare showing of the usual brilliant red. Not that this year’s fall colors lack beauty, but they’re giving us a different kind of beauty. The yellows are as brilliant as ever, but the oranges and the rare reds that can be seen are strangely muted. Why the change?

The primary reason is that average temperatures this fall have been higher than usual. Trees need cool nights and some early frosts to produce vivid colors. How this works is explained by an expert in the field, Professor Bill Livingston, chairman of the Department of Forest Ecosystem Science at the University of Maine.

Professor Livingston puts the warm fall in terms of degree days, a measure used by heating engineers. Since July 1, this year has had only 360 degree days, compared with an average of 565. And the first frosts around here didn’t come until a few days ago.

Normally, the bright reds of fall appear on the red maples and red oaks. (Sugar maples turn mostly yellow, and other oaks turn mostly a shiny bronze color.) Yellow pigments are always there, but in the summer they are masked by the green chlorophyll. The chlorophyll produces sugar, which flows out of the leaves through the stems into the trees to supply energy for the growth of the trunk and branches.

When the days grow shorter, the chlorophyll disintegrates, allowing the yellow pigment to show. An early frost slows or freezes the flow of sugar out of the leaves, and the remaining sugar produces the bright red pigments.

This fall, the sugar continued to flow out of the leaves, and not enough was left to produce much red. The result is that we see a lot of yellow, some rather restrained orange, and hardly any red.

It may be too late for these recent frosts to take effect, but Professor Livingston says we may yet see a few red leaves before they fall.

So enjoy what nature has given us, a lovely fall with somewhat muted colors. And if you must have those brilliant reds, you probably will have to wait until next year.