Fall's colorful last fling

Why leaves change color in fall.

I spent most of the day in New Hampshire being a leaf peeper. It was glorious! The trees were at peak color, blue skies were dotted with puffy cumulus clouds, and temperatures rose into the low 70s F. What made it even more enjoyable was that the eastern part of New England didn't have much of a leaf show last year, and I had really missed it. Several people have asked me recently why there's so much variation in leaf color from year to year. I think some of the answers on the Web are in language too technical to be easily understood. The Forest Service's explanation of why leaves change color is in between - scientific enough that you get a grasp of the principles and proper terms, but not so dense as to leave the non-science majors among us shaking their heads.

Basically (and much simplified), leaves are green because of chlorophyll, which is used by the tree in photosynthesis. When nights get longer in autumn, production of chlorophyll slows and then ceases.

That's when two other pigments already present in the leaves but hidden by chlorophyll come into their own. They provide leaves with yellow, orange, brown, and red colors. When chlorophyll stops being produced, these colors can be seen in the leaves.

You already know that certain species and varieties or cultivars of trees characteristically have specific colors -- yellow for birch and poplar, deep red for dogwood, and so forth.

What time of fall different species change leaf color is also an inherited characteristic -- some are always early and others -- oaks, for instance -- are always late.

But a big part of the variation from year to year is the weather. Warm sunny days followed by cool nights that don't drop below freezing provide ideal conditions for beautiful color. But a spring or summer drought can cause less of a colorful show. So can warm fall days followed by warmish nights.

It seems to me that a lot of rain in fall produces dull colors. And an early frost just causes the leaves to turn brown and fall off.  

Most states have websites to provide information on the progression of the fall show of leaves. If you can't find one for a state you're near, try the Forest Service's hot line at 1-800-354-4595.

There's nothing more glorious in nature this time of year than fall's last fling. Do take some time to enjoy it.

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