I must confess that reading David’s blog on his winter preparations at Turkey Hollow put me into a state of anxiety. All my friends who grew up in temperate climates look forward to the changes in the seasons, and some even plan their lives by them. But to me, fall means that winter is on its way, and that means snow. I hate snow.

I’m a tropical baby, and where I was born the seasons are called “hot wet,” “hot dry” and “cool dry.” It never snows, so I have an excuse for not knowing why fall leaves change color. What I do know is that that there is simply no better place on the planet to see autumn leaves than North America. Some places like New England are so famous for their blaze of fall colors that they draw leaf-peeping tourists from far and wide. I was a little surprised, however, when I asked my North American biologist friends to explain fall colors and they came up with conflicting old-wives tales about how leaf color intensity relates to rain, drought, or spring weather.

There are two basic mechanisms for fall leaf color change:

The first is for trees like elm, beech and poplar that turn from green to yellow. These leaves already have yellow pigments, but in summer when the tree is actively making green chlorophyll, the chlorophyll masks the yellow leaf color. However, when the days get shorter the tree stops making new chlorophyll and it begins sucking the nutrients out of the leaves into the woody parts of the plant. The old chlorophyll gradually gets photo-oxidized after prolonged exposure to light. As this happens, the leaf gradually looses the green color revealing the yellow pigments that were there all along.

The second kind of change is trees that develop red and purple coloration like red maple, sugar maple, scarlet oak and sumac. The red and purple pigments are a special kind of sunscreen made by the leaf. They prevent the chlorophyll-free leaves from being damaged by the sun’s rays while the tree is still extracting the last useful nutrients from the leaf. The thinking goes that there is an evolutionary trade-off here. Making the red and purple pigments to keep the leaf functioning at a minimal level as the tree salvages the remaining nutrients takes less energy than sacrificing the remaining nutrients in the leaves and trying to recover them from the soil as the leaves decay in the next growing season.

So what about those old wives tales about rain and cold? Basically, the more light there is in fall the more sunscreen the plants have to produce, and the brighter the colors will be. Clear days without much cloud cover are the key to bright fall colors. To prove it, why don’t you try this at home: get a thin firm flat sheet of clear plastic film and using a thick, black magic-marker, write a message or draw a design on it then fasten it to the upper side of the leaf using a paper clip. When the leaf changes color, remove the plastic sheet to reveal the design or message in green.

Brian Gratwicke is a regular contributor to davidmixner.com. More about Brian can be found at www.briangratwicke.com. Photographs by Steven Guy.