How Trees Work

Trees' general strategy as plants is to grow slowly, put a lot of time into building a structure that gets leaves closer to the sun, and eventually out-compete the fast-growing soft plants and shrubs that can't get off the ground.  The part that makes a tree a tree, namely the wooden trunk, serves as both support for holding up leaves and the means transportation of materials from the leaves to the roots and back. 

This is a tree.

 Tree trunks are strong because their cells have surrounded themselves with lignin, the hard material in wood.  There are two basic tissues in tree trunks: wood (the inside), and bark (the outside).  Guess which one of these materials is alive?  You'll have to wait just a minute to find out.  The wood part of a tree trunk contains mostly lignin, so it's very strong.  The bark contains more suberin and much less lignin.  Suberin, the subject of a previous post, is a softer substance, and if you have ever squeezed a wine cork, you know the texture of suberin.  Wine corks are cut from the bark of the cork oak. 

The wood of a tree is produced by plant cells growing very long, surrounding themselves with lignin,  leaving a few tiny holes in each end to connect to the next cells, then dying and leaving behind hollow tubes of lignin.  Yes, the interior of trees is mostly dead.  The hollow tubes connect down to the roots and up to the leaves.  The tubes are so narrow that water can pull itself up through the tubes by capillary action.  Capillary action occurs because water is pulled more by the chemical attractions of water to the lignin than by the pull of gravity.  Capillary action works in trees as long as the lignin tubes are very thin and the tree isn't too tall.  You may remember from 9th grade Biology that the material which transports water in plants is called xylem.  In fact, wood is almost entirely xylem.

Bark, the living part of a tree trunk, is composed of mostly phloem and suberin.  Phloem transports sugars, the tree's food, dissolved in water.  If you look at bark under a microscope, you would see that it, like wood, is composed of microscopic tubes running up and down the trunk.  Unlike xylem, phloem tubes can run up or down, depending on the season.  In the summer, the phloem is busy transporting extra sugars and nutrients down into the roots for winter storage.  In the early spring, phloem brings that stored sugar back up to provide energy for the new spring growth.

A tree will die if the bark is cut all the way around the tree.  The tree below has been girdled.  It is a white poplar, a weedy tree, that is probably interfering with the native plant restoration going on in Lincoln Park.  The ecosystem manager probably also applied an herbicide to the bottom cut on this girdle so it would be pulled down into the roots.  White poplars are notorious root-sprouters, and if you don't kill the roots, you will have new mini-trees coming up all over the place.

A girdled tree.

This tree has also been girdled, but not by humans.

A girdled green ash.

Bark is infinitely variable in its patterns and characteristics.  Here's my favorite bark - white birch.  You can tell a lot about a tree by looking carefully at its bark.  This will be the subject of a future post!

Beautiful birch bark.