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Thursday, Nov. 05, 2009

Weather affects how leaves change color

It’s that time of year again — time for our deciduous trees to warn us of the approaching winter by changing from green to yellow, orange, purple and red.

Personally, I’d just as soon they stayed green (and the temperatures stayed warm), but that isn’t the way the world works. The way it does work is fascinating enough and we have the side benefit of colorful leaves to ease the pain of change.

The leaves of trees are thin factories that take carbon dioxide from the atmosphere and with water from the soil they produce sugar (glucose) through the process of photosynthesis.

The main chemical compound responsible for the process is chlorophyll, which is also the green pigment that gives leaves their color from spring until fall.

The glucose is used by the tree to fuel the processes of life and is also stored as starch so the tree can survive through the winter months.

Some of the glucose eventually becomes cellulose, the stiff structural material that allows trees to become the largest living things in the landscape. It is also one of the main reasons that trees are so valuable to humans.

There is no advantage to a tree with thin leaves to keep those leaves during the winter. The tender leaves would freeze and the tree would lose the nutrients in the leaf, along with the ability to make sugars. Instead, reacting to the lessening of light because of shorter days, deciduous trees drop their leaves and rely on stored starch for energy until the leaves emerge again in the spring.

Conifers, on the other hand, have leaves that are relatively thick, are not broad, and have waxy coverings to protect them from freezing.

The process of leaves changing begins when a zone of cells in the stem of the leaf (called a petiole) begin to form a corky substance that eventually stops the flow of water into the leaf. Without water, the chlorophyll begins to break down and so the leaf loses its green pigment.

However, hidden behind the green of the chlorophyll for much of the year there are other pigments, mostly yellow or orange, called carotenoids. Practically overnight, a tree full of green leaves can become a tree full of bright yellow leaves, just by losing one pigment and exposing another.

In some trees, sugars are left behind during the process of shutting off the leaves.

In the presence of bright light, these sugars produce another type of pigment called anthocyanin. Brilliant reds, purples and burgundies are the result of anthocyanin production.

Leaf color varies by tree species and is influenced by environmental conditions. Hickories are always golden yellow (probably my favorite fall leaf color), while dogwoods, sweetgums and sourwoods are purplish red to crimson.

Red maple leaves are often bright yellow, but can also be clear red. There is also a chronological element to leaf color — sourwoods often start showing reddish-purple color in August, while some oaks, like scarlet oak, don’t change until many other trees have dropped all their leaves.

The quality of leaf color changes from year to year, depending mostly on weather conditions.

The best leaf color is produced when fall days are warm and clear and the nights are crisp, but not quite to freezing. Unfortunately, so far this autumn, we just haven’t had many clear days or cold nights. In fact, this has been the rainiest October I can remember in a long, long time. October is supposed to be our driest month — ha!)

Warm days produce lots of sugar in the leaves, which in turn produce lots of anthocyanin. Therefore, the purples and reds will most likely be poor this year, but the yellow pigments are in the leaves already and should be more consistent. Still, a gray, gloomy day is just that and even the brightest yellow foliage isn’t going to seem particularly cheery under those conditions.

Maybe we will get some nice, clear days before the first killing frosts and we’ll salvage something from the fall color season. Brightly colored, falling leaves are such a source of nostalgia for me, and I suspect for many people.

The only gold I may ever own in life might just be hickory leaves piled at my feet, but sometimes that is enough.

Hal Massie is a Master Gardener and garden writer. Contact him by e-mail at agardenquestion@aol.com or by writing to him in care of The Telegraph, P.O. Box 4167, Macon, GA 31208-4167.