Friday 15 April 2005

Tree Roots Part 2

If the purpose of an inquiry is to dig deeper into the dirt, this article is no exception.

This inquiry uncovers the secret lives of roots.

In our last discussion we touched on the three basic functions of roots:

1) Physical anchoring the above ground portions of the tree

2) Absorbing and transporting water and mineral nutrients

3) Storing starches, sugars and other food reserves

We also introduced the concept of a rhizosphere composed of roughly equal parts of, water, air, mineral soil and a small component of organic matter.

Trees use fine absorbing roots to gain access to minerals dissolved in water that are vital to plant growth. The process by which water is absorbed into the plant is called osmosis. This system is entirely passive. If the root is dryer than the soil it will take in water, much as a dry sponge soaks up a spill. When the root is full of water it stops taking it in.

By the reverse process, when the root is wetter than the soil surrounding it must lose water, much as a wet sponge on a dry surface loses water.

What this means for your tree is that when soil conditions become too dry it will lose water to the soil rather than taking water in. Trees have evolved various methods to maintain water in drought conditions. One way to do this is to shed many fine absorbing roots during periods of drought. The thicker structural roots have a waxy coating that prevents moisture loss and uptake. When conditions permit the fine absorbing roots will be re-grown and the process will begin again. Trees also form beneficial associations with a group of fungi known as mycorizae.

This term translated literally means fungal roots. These fungi grow into the roots and exchange water and mineral nutrition for sugar provided by the tree. This web of fungi around the roots can extend the surface area of the absorbing roots by as much as ten times. Additionally the fungi are better adapted to adsorb soil water when it is in short supply. These fungi are very specific to the types of trees they join in partnership with. Fortunately most soils contain billions of these fungi.

So why do I have roots growing all over my lawn. Roots generally grow downwards guided by a process called geotropism. Gravity drives this process and ensures that roots end up in the soil rather than on top of it. There are two reasons why roots come to the surface. Number one, your lawn is so compacted that the roots are unable to get enough oxygen to allow them to grow at depth. Number two, the constant removal of leaves has interrupted the soil building process. Normally, in a forest setting, roots would be showered annually with layers of leaves and plant debris that would cover the roots and decompose into a rich organic soil.

Soil compaction is the process where air is excluded from the soil by packing the soil particles tighter and tighter. Soil structure is broken down and the soil can become as dense as concrete. Roots have an extremely difficult time penetrating this type of soil. Existing roots will be injured and may not recover from soil compaction. A recent study has shown that six passes over the roots of a tree with a construction tractor will result in the death of ninety percent of the roots in that area. Construction damage effects normally are not seen until three to seven years after the

damage is done. People look out one spring and the trees they bought the house for are dead.

There are things that can be done to prevent construction injury, most are related to preventing

soil compaction.

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