Tree Roots: the hidden support system

The roots are the hidden support system of our giant plants, the trees. They anchor their woody trunks to the ground, store food and bring in water, nutrients and oxygen. In this article I will discuss what is going on down there. In the next issue I will discuss more practical applications of this understanding.  

Far less is known about tree roots than about the trunk, branches and leaves. This is understandable since the roots are hidden from our view, and once you dig them up, they are no longer what they were. We are awed by the massive trees swaying over our houses and streets, but usually we give little thought to what is going on under the surface. However, when spring thawing is followed by powerful winds and we see an 80’ spruce toppled over with its roots in the air, which are only 9” deep; that gets us thinking.

         Now think about this: what tree scientist, Alex Shigo calls the “business end” of a root is non-woody and about the diameter of a sewing needle, and the only way it can move and get more water and nutrients is to push this fragile “finger” through our hard Colorado ground. Remember the last time you were armed with a steel shovel, pick or rock bar and how hard it was to get through our soil? So how do these little roots do it? They do have some kind of a toughened cap over the end which helps, but mainly the hard resistance of compacted soil and rocks tears off cells as the root grows forward, and it progresses only by replacing those cells continually. These roots usually live for only a few days or weeks. Try to remember this when you next dig a hole to plant a new tree; a wider hole makes life a lot easier for the tiny roots.

         The big woody roots that we usually see are what anchors those big trees to the earth when they whip and sway in our winds, and they also help to store energy. But the vast majority of roots are very fine and are for the purpose of absorbing water, nutrients and oxygen. These are mistakenly called “feeder roots”; roots don’t feed and fertilizer isn’t food. Roots can only absorb water and nutrients which are conducted up to the leaves where they are combined with carbon dioxide, through the energy of sunlight trapped by chlorophyll, to photosynthesize glucose. This glucose sugar is the trees’ food which is sent to the branches and down to the roots to provide them with energy and bargaining power.

         Yes, bargaining power, because the picture I painted of the little root hair pushing through the hard ground isn’t quite complete. You know how the brilliant idea you brought up at the meeting wasn’t exactly your idea because it came out of a discussion you had with your family at breakfast? It’s like that in the area around the root called the rhizosphere. (“rhiz” means root) Here there is frequently a mingling of roots and fungi (mycorrhizae) both of which benefit from their association. Here in this mix of living, dying, interpenetrating and recycling, mutual aid is a successful survival mechanism.

         The threadlike fungal mycellium, being more branching, fine and extensive, are far more efficient than roots in contacting the soil, expanding the surface area of a root by up to 700%! Therefore in economic terms (which a Colorado tree is desperate to consider), an investment of 10%-20% of its carbohydrates, vitamins and amino acids in the development of symbiotic fungi will give a return of over 100 times the value, of that same investment in root development. Therefore whatever supports the mycorrhizae fungi, like compost, oxygen and water, also supports the tree roots.

         There has long been a misconception about tree roots that “as above, so below”, but there is not a mirror image of the branches in the root system. Only about 3% of all trees have deep trunk-like tap roots, 14% have roots that grow at a variety of angles and 83% have shallow, lateral roots. In fact, 80% of all tree roots are in the top 8”-10” and almost all are in the top 2’-3’. The more dense and compacted the soil, the closer the roots are to the surface. The maximum concentration of the absorbing roots are around the dripline of the tree, but many roots extend two to three times the diameter of the tree; so a tree 30’ in diameter could have roots extending 60’-90’.

         If 12” of soil is dumped over the root zone of a tree, the tree will usually die. Likewise if trees are flooded for too long a time they will die. In both cases, oxygen starvation has been found to be the cause of death. But isn’t it true that plants take in carbon dioxide and give off oxygen? Yes, it is. I asked world-renowned tree expert, Alex Shigo, why tree roots need oxygen. He explained that trees use carbon dioxide to manufacture food, but they need oxygen to burn this food, to turn it into energy. And how does this oxygen get into trees? It comes from the atmosphere, through the soil and into the roots. It is unclear whether oxygen may enter by any other means, but Dr. Shigo made it clear that whereas the tops of trees are dormant during winter, the root system is very active, absorbing water and oxygen, growing, and converting starches to glucose and oxidizing this food for energy. From this we may deduce that the leaves are not necessary to bring in oxygen for respiration, and also that it is a good thing the roots are working while the branches are sleeping, because even a dormant organism needs energy to keep its cells alive. So this explains one of the main reasons why trees that are perfectly cold hardy, have such a difficult time in Colorado. It is because our soils are poorly aerated. In general, most of our soils (except in the mountains) are fine-textured clay soils with little organic matter, which means there is little room for air. “Don’t be fooled”, cautioned Dr. Shigo, “the number one problem for city trees is not insects or diseases, it is low organic matter in the soils.” So what does organic matter do besides make the soil more porous for oxygen movement? It supports the mycorrhizae which because of their excellent absorption efficiency, help bring more oxygen into the plant roots, especially in the winter. Higher levels of oxygen are especially important for producing new roots. If you want your newly planted tree to grow faster, loosen the soil 6”-8” deep in a 5’ diameter area around the planting hole.

         So if oxygen is so important to tree roots and organic matter helps with the availability of oxygen, how can we get organic matter into our soils? The best opportunity is when a landscape is being constructed. Then we can break up the existing soil and till in compost or composted manure. This is preferable to amending a planting hole because roots grow better without abrupt changes in the soil character. However if this is not possible, it is still good to add some organic matter to a planting hole, especially in poor, tight soils. Now what should we put on the surface over the tree roots? Is grass all right? Under some circumstances it is OK, however for a new tree, Dr. Shigo recommends a grass-free area of at least 3’ in diameter. A mulch can be very beneficial. David E. Whiting with the Dept. of Horticulture at CSU wrote that there can be  400% more development of fine roots under mulch than under grass. Organic matter, even on the surface supports both roots and mycorrhizae. Alex Shigo’s closing remark was “God cries when he sees people rake up leaves from under trees and burn them.”

         We know that raw uncomposted organic matter mixed into the soil can rob it of nitrogen, but later return it after decomposition is complete. But in the natural forest, the layers of litter add up and are recycled without harm. My recommendation is that before putting down a raw mulch like shredded leaves or wood chips, apply a fine layer of organic nitrogen fertilizer. It would be even more ideal to apply an annual top-dressing of compost to our gardens and under our trees. This service to the top soil would support soil life in general, increasing populations of earthworms, soil bacteria and soil fungi, etc. Although it would take some time, they would naturally mix this compost into the soil, thereby aerating and enriching it, and creating a healthy environment for roots, much like what occurs in the forest.

         In the next issue I will discuss tree roots and how they relate to the practical issues of watering, fertilizing and planting, with illuminating insights from Dr. James Feucht, Al Rolinger and Gene Eyerly.