ASK A SCIENTIST
Question: Why is tree bark brown?
Answer: This, believe it or not, is an important question that scientists have not thought about, as carefully as perhaps they should.
Let us begin by asking the reverse question: why are most plant parts green? You probably already know the answer: the green is chlorophyll, which absorbs light from the sun. Sunlight is the plant's 'food' (along with water, carbon dioxide, and mineral nutrients). Through a series of chemical reactions known as photosynthesis, plants convert the energy in sunlight to chemical energy in the form of sugar, protein, cellulose, and so on. The rest of us, the animals, get our energy by eating plants, or eating animals that have eaten plants, or animals that have eaten animals that have eaten plants, etc.
One answer to your question, then, is that tree trunks don't have chlorophyll because they usually don't get much sunlight. Especially in a forest, the trees shade each other, so that most sunlight hits the treetops, where most of the green leaves are. But this is only a partial answer, because even in a forest, trunks do get a bit of light. Walk around inside a forest and you will see ferns and other green plants growing on the ground, where they get no more sun than the brown tree trunks do. Such plants grow very slowly because they get so little light. Tree trunks also could photosynthesize, but their contribution would be small in comparison to that of the leaves up at the top of the tree.
To complete the answer to your question, we need to consider the purpose of wood: to hold the leaves up above those of other plants. Plants compete with each other for sunlight. Whichever plant can get on top of its neighbors will grab the sunlight for itself and prevent those it is shading from growing. Tall trees shade out smaller plants, but without wood they would just flop over and lie upon the ground. The chlorophyll tends to be concentrated in the upper parts of the tree, where sunlight is most likely to strike.
In nature, there always are trade-offs. Trees can shade out non-woody plants like grasses and dandelions ('herbs') because their wood allows them to grow taller. On the other hand, trees grow more slowly than herbs, because they have to make wood. The grasses and dandelions are 100% green, 100% photosynthetic (not counting the roots). Trees are not. This would be true even if there were some green in their bark, since they still would have to deposit a great deal of wood in the trunk to support the treetops. This means that when trees start out, when they are 'babies', faster growing herbs often shade them out. But if they get a head start - for instance, if the grassy plants are plowed up or killed by herbicides - they often can get up above the height to which non-woody plants can grow, and shade them out.
Global warming probably is changing the competition between trees and herbs. Through burning of fossil fuels (oil, coal, natural gas), humans are increasing the amount of CO2 in the air. CO2 is a plant nutrient, and a major constituent of wood. Increasing the amount of CO2 in the air probably helps trees compete with herbs. On the other hand, humans are also adding large quantities of nitrogen to the soil, through fertilizer and pollution. Nitrogen is especially useful to the herbs, which can only grow faster than trees if they have a good supply of nutrients. In places where smog is bad and the soils were naturally not all that great, such as the hills around LA, nitrogen in the smog has washed into the soil in such quantities that herbs have replaced woody plants over huge areas.
So, you see, your question actually is one scientists need to consider to study and hopefully predict the impacts that we humans are having on the environment.
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