Plain brown leaves?

October 29, 2013

By Jane Garrison

It’s funny how I feel when summer ends and fall starts.

I think, “NO! Don’t let this happen yet.”

Then I get the feeling of autumn, and I revert to a crazed fall fever. Harvest time, autumn smells, colorful leaves and crisp air just win me over every year.

When I come to my senses I have more rational thoughts, such as why and how do the trees change so drastically from summer to winter? Why do leaves change color? The science behind that particular subject is fascinating and more complicated than you would think.

Gilman Boulevard is a great place to see varieties of red maple. Most of the red maples, Japanese maples and trees bred for bright color produce brilliant reds every year, but our native cottonwoods, some maples and alder just turn brown and drop their leaves. Disappointing for sure, but why?

Research on leaf color has indicated it’s not sunshine or the temperature that make a difference but the light level. Lack of light with the low sun angles and short days means chlorophyll is not produced. No chlorophyll means the leaves lose their green color. So, what we are seeing every fall is a loss of green in the leaves rather than the gaining of reds, yellows or browns — just the opposite of what we usually think.

All trees go through a process of transferring nitrogen from their leaves to their roots for winter storage. If a lot of sunlight is produced in the fall, trees need to protect themselves from it while they are in this process. Trees that turn red protect themselves with anthocyanin. Most of the trees that turn yellow produce carotenoid for protection, but only when threatened by too much sunlight.

The result is bright leaf color every year in red trees and usually only when the sun shines brightly in yellow trees. If we have a rainy fall, the native plants don’t need to protect themselves, so they don’t go through the colorful stage; they just turn brown and drop their leaves. Bummer.

Many of the trees being bred for color produce red because it’s the most popular and reliable for fall color. The yellow and brown depend more on environmental conditions.

It’s not necessary to know why the trees change color in the fall, just as long as they keep blowing us away every year with their spectacular show. Forget all this scientific stuff and just appreciate the glorious feeling of autumn.


Jane Garrison is a local master gardener and landscape architect who gardens in glacial till on the plateau.


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