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Tuesday, January 17, 2006

Why Trees have Different Shapes

There are many reasons why tree shapes differ. A tree growing in poor soil may be stunted due to lack of nutrients, and a tree growing right next to an apartment building may have more leaves on the side facing the sun. Different kinds of trees have their own unique form, but the form that any tree has is also affected by the environment where it grows.

Sunshine and water are both essential for a tree to survive, and both influence tree height, crown shape (for example, a round treetop or the cone shape of a pine tree), and the form of leaves.

Some tree species grow quite tall and receive much sunlight. But what about those trees left in the shadows? Many trees collect sunlight that is filtered through the leaves of taller trees. These shaded understory trees survive by gathering indirect sunlight or sun flecks that break through openings in the canopy. A rounded crown seems to work best for gathering filtered, understory sunlight, which comes from many different directions.

The shape of the tree's crown also has a lot to do with where it lives. Nearer to the equator, the noontime sun is almost directly overhead all year. Tall trees with flat treetops (or crowns) are very common in this part of the world because the flat shape helps expose more of their leaves to the direct, overhead light.

Up nearer to the Arctic circle, the sun is never directly overhead and is usually quite low in the sky. Trees in this part of the world tend to be cone-shaped (think of pine trees), with leaves from the top of the tree to the bottom, to make the most of this sunlight.

Finally, many of the trees up nearer to the Arctic circle (like spruce, pines, and fir trees) have needles, partly because needles are especially adapted to cold, dry climates. Needles retain water better than broad-leafed trees like oaks and maples.


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