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What’s Under A Tree? November 26, 2008

Posted by Chuck Musciano in Random Musings.

Why isn’t there an enormous hole under every tree?

Plant a seed and a tree grows.  Small at first, but becoming huge, until a multi-ton organism towers over your head.  Everything above you was once below you, drawn in microscopic amounts from the ground through the roots and sent upward to form new branches and leaves.

Granted, a lot of the tree is water, but a lot of it is not.  A tree is just hundreds and hundreds of pounds of minerals and base elements, attractively arranged in the shape of an elm or maple.  If all those elements were drawn from the ground under the tree, why isn’t there a hole of equal size under the tree?  Why doesn’t the tree slowly slump into the ground as it grows?  Just the volume of dead leaves each year is easily larger than a toddler; is there a new toddler-sized cavity under each tree each year?

I’ll concede that equivalent material may be drawn from around the tree to replace what was pulled up to create it, so that may be the answer in the wild.  But what about potted plants?  I have a plant in my office that has more than doubled in size in the past year.  Originally eighteen inches tall, it’s now easily over three feet, but it still lives in a little pot no more than eight inches square and three inches deep.  I’m guessing that if I took all the new growth on the plant and crushed and compressed it into a block, the block would not fit in the pot.  How can this be?

I await any rational explanation.



1. Mike Helms - November 26, 2008

The short answer is: trees are generally about 50% water (with considerable variation among the species). The other 50% does indeed come from the soil. Part of that “void” is occupied by roots. Part is filled in by soil’s natural tendency to settle in the lowest spots possible (from gravity and water). And vegetation does indeed change the “height” of the soil; this is why natural erosion processes like wind and water, and the rot of detritus is necessary to replenish the soil. As a sidenote … the soil level in your pot has likely gone down as the plant has grown – hopefully, the “plant people” keep the soil topped up periodically!

2. Heather Hollick - November 26, 2008

I minored in physics in college and am far from an expert.

But I always thought that part of the volume of a tree came from the fact that they convert sun’s energy into mass.

E=MC^2 you know 🙂

3. Chuck Musciano - November 26, 2008

Actually, sunlight provides the energy that drives photosynthesis, which converts carbon dioxide and water into oxygen, water, and carbohydrate. The energy is caught up in the bonds of the carbohydrate; you release that energy when you later burn the wood from the tree. It’s a mass-constant reaction, so I don’t think trees are doing any cold fusion on their own. There a nice summary of the process we all learned and then forgot in high school at http://www.backyardnature.net/photosyn.htm.

Next week, we’ll cover the Kreb’s Cycle!

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