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- ARTICLE The Afghan pine problem in Austin
- ARTICLE The truth about cedars
- ARTICLE Trees and roots
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Trees and roots
Tree roots have essentially four main functions. They anchor the tree, they are responsible for uptake of minerals and water by absorption, they conduct minerals and water up into the main stem and braches, and move carbohydrates down the phloem from photosynthesis, which they then store as starches in the roots.
Tree roots are either anchoring roots, responsible for holding the tree up, or fine hairy absorbing roots responsible for absorption of water and minerals. Often, the fine absorbing roots are associated with mycorrhizae, a beneficial fungus that aids with increasing root efficiency and uptake. The anchoring roots and larger woody non-absorbing roots are also responsible for storing starch and conduction.
Roots also perform some other functions such as producing growth regulators associated with branch tip growth, for example. Some trees also have a taproot which is typically 1-2 feet long (max) and is generally associated with anchoring the tree when it is young. Most tap roots vanish when a tree matures; very few mature trees have tap roots. I have had more than a handful of people tell me that tap roots and even roots in general go down as deep as the tree is tall. This is an urban legend, and is absolutely not true!
Most tree roots extend down to about 2 feet into the soil. The absorbing roots, however, do not extend deeper than 12 inches below the surface. In fact, in poor rockyAustinsoils, the absorbing roots are much shallower even than that. The reason is that roots need both oxygen and water to thrive. They will only grow where they get both in the right combination, which is close to the surface. The deeper roots that extend below that are the woody, anchoring and conducting/storing roots only. If you have ever seen a tree that has been blown over, (see picture), you will see where the roots are located.
Roots are happier and healthier when their absorbing roots have a good topsoil or humus and a healthy layer of mulch or leaf litter on top. This increases air space in the soil, or pore space, and also keeps the roots cool and aids in water retention by reducing evaporation and causing an insulating effect. Tree roots prefer to be cool and insulated during the summer, and the absorbing roots die back and have to continually regenerate during droughts and heavy freezing when they are continually exposed. Mulch also boosts beneficial microorganisms in the soil, which also helps increase root health.
Most roots extend to the canopy, and about a third past it. Some trees under ideal circumstances, however, may have roots that extend 2-3 times past the drip line! It is important to note that even in extreme cases, the vast majority of a trees absorbing roots (often up to 90 percent) are located from the base of the tree to the drip line, with the densest roots closer in to the trunk.
With respect to watering, if mulch is put down around the tree, then you can water every week or two during extreme droughts for 15 minutes or so in the morning or the evening, and the water will stay in the soil for much longer. Watering too long can be wasteful and will go through and below the absorbing roots. More frequent weekly or bi-monthly watering is usually better depending on the species. A large, healthy tree like a maple can pull up to five or more gallons a day through its vascular system via evapo-transpiration on a really hot dry day. It’s no wonder silver maples dry up and die, or loose their upper canopies in Austin summer droughts!
Sometimes, during storms, tree roots can be saturated by rain water and the soils can become unstable. When this occurs, trees can uproot. Sometimes, prior to failure, the soils raise up on one side of the tree, creating large air spaces below the roots. This is almost always a telltale sign that the tree is about to fall over. I have seen this many times, and when this happens, the tree is an immediate hazard and needs to be removed right away.
Sometimes roots simply have forces above ground that they cannot handle. High winds associated with extreme storm conditions we are often faced with here in the Austin and surrounding areas can often snap and shear roots and literally push trees over. When winds get upwards of 60-80 miles per hour here, all bets are off.