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It Does Not Take Much to Disrupt Them
This statement deserves repeating and repeating. The delicate "threads" that hold these powerful associations together need to be recognized and respected. Trees in cities grow only so long as these "threads" remain connected.
Trees grow as large oscillating pumps, with the top trapping energy and pumping it downward. The bottom absorbs water and elements and pumps them upward. The pumps have developed over time to work on the basis of many synergistic associations that maximize benefits for all connected members and to minimize waste.
Many of life's essentials for the bottom associates come from the top of the tree. And, the top works only because the bottom works. Energy is required to move things, and elements and water are required to build things.
Tree Treatments and the Rhizosphere
When trees are over-pruned, the top will be injured first. When it is injured, it will not serve the energy requirements of the bottom. Soon root diseases start and are blamed for the decline or death of the tree. Where over-pruning is common. so are root diseases.
Compacted soil blocks air and water to the bottom and crushes all the microcavities where the microorganisms live. In nature, decomposing wood and leaves keep conditions optimal for the rhizosphere inhabitants.
Over-watering stalls the respiration processes in the roots. When respiration stops, carbonic acid is not formed. When carbonic acid is not formed, ions necessary for the absorption process do not form. When absorption is down, the tree system is in trouble. Fertilizers can be of great benefit to trees growing in soils low in or lacking elements essential for growth.
Elements or molecules made up of a few to many different atoms enter the roots as ions. An ion is a charged atom or molecule. Ions with a positive charge are cations, and those with a negative charge are anions. Each particle or granule of fertilizer is a salt made up of a lattice of anions and cations, just as ordinary table salt is made up of a grand lattice of connected sodium cations and chloride anions. When salt as sodium chloride dry granules is poured into water, the sodium and chloride ions separate. When they separate, they carry electrical charges and are called the sodium ion and the chloride ion. When a cation enters a root, another cation must exit. This is very important, as we will see. When nitrogen enters a root as nitrate anion, an anion of bicarbonate ion from carbonic acid exits. The bicarbonate ion is probably the second most important compound in nature, next to water, because it drives the absorption process. When a bicarbonate ion exits into the rhizosphere, the pH increases.
When urea is used in fertilizers as the nitrogen source, the pH in the rhizosphere could increase to 2 or more pH units. The chemistry behind this is complex, but here I present only the conclusion, because a common problem with trees in some high pH soils is chlorosis. There is no easy field method for measuring the pH of the one millimeter wide rhizosphere. The rhizosphere could be pH 8, and the bulk soil would measure pH 6. As pH increases, the availability of elements such as iron and manganese decreases. In soils, it is one thing to have an element present and another to have it in a form available to the plant as an ion. As pH increases, iron and manganese element, form molecules that precipitate in water rather than ionize. If they are not available as ions, they will not be absorbed. And, if they are not absorbed, several of the enzymes essential for chlorophyll formation and photosynthesis will not form.
When the energy flow from the top of the pump is blocked, then the bottom does not get enough energy for growth and defense. The pathogens invade, and the tree declines. This scenario does not mean that every time you use urea, trees will decline from chlorosis. But the use of urea could be a contributing factor where trees with genetic codes for growth on low pH soils are planted in high pH soils. If fertilization is a desired treatment, then a fertilizer that has nitrogen in a positive charged ion, such as an ammonium ion, would help to reduce the rhizosphere pH. When the ammonium ion enters the root, a proton of positive charge will exit. The protons in rhizosphere water will bring about more acidic conditions, so there is a way out.
In summary, fertilizers can be very beneficial for healthy survival of trees planted outside their forest homes. How beneficial will depend greatly on an understanding of many of the points mentioned here and some basic chemistry.
Primary Causes of Diseases
It is often very difficult to have people recognize the importance of small organisms in small places doing big things. Blame for the death of a tree is often placed on big things that can be seen or felt. Most pathogens are opportunistic weaklings waiting for a defense system to decrease. Many small disrupting events often lead to the decrease in a defense system. Then after the tree has been weakened, the final agent comes along and gets the full blame for the cause. A perfect example is the cankers on honey locust. Flush pruning is usually the real cause.
Pumps and Food
Trees are oscillating pumps. When the pump begins to wobble, some parts will begin to weaken. When they weaken to the point where some other agent causes a part to break, the pump will stop.
It is very difficult to determine where problems start in an oscillating pump. Symptoms may be in the bottom, but the cause may have been in the top. Or, it could be the other way around.
I go back to two points that may be part of the answer: exudates and the self-thinning rule of ecology. All living things require food and water for growth. Leaves and photosynthesis provide the energy at the top of the pump. The nonwoody roots and the rhizosphere provide the elements and water at the bottom. Photosynthesis will not work without water and elements, and the absorption processes will not work without an energy source.
Trees became trees growing in groups in forests where the self-thinning rule had strong survival value. Not only did exudates provide quick energy for the rhizosphere organisms, but the carbon in the wood of the trees that fell to the ground also provided a long-lasting energy source for a succession of organisms.
Reports from some countries indicate an abundance of soluble nitrogen compounds in runoff water and even in ground water. This is a strong indication that the carbon-nitrogen ratio has been disrupted in the soil. It is well established from studies of the physiology of fungal parasitism that the degree of parasitism is often determined by the carbon-nitrogen ratio. It is probably similar for other organisms.
The organisms in the rhizosphere and surrounding soils have many different ways to weather rocks and to get nitrogen and other elements essential for their growth. What they cannot get in the soil is a sufficient energy source. Yes, some small animals die and provide carbon, and some microorganisms can get energy by chemosynthesis, but the requirements for carbon are much greater than what could be supplied by those sources alone. Carbon must come from the top of the pump. When the energy source from the top begins to decrease, the rhizosphere organisms will begin to starve.
The oscillating pump model soon takes on the form of a circle, because now it could be said that the top did not work efficiently because the bottom had a problem first, and this could be so. My point is that the energy problem does play a key role in declines. If a single tree is already very low in energy reserves, it cannot contribute much to the rhizosphere even if the genetic codes rule that exudates should increase as a tree begins to decline. Soon we will be faced with the chicken or egg problem.
I believe there is a way to decrease the potential starvation problem. In forests, more wood should be left on the ground, and in cities, more composted wood and leaves should be added in correct quantities to the soil about the base of trees. Incorrect treatments of pruning, watering, planting and fertilizing should be corrected, because they often start the pumps to wobble. If these simple adjustments can be made, rhizosphere starvation will decrease and our trees will lead healthier and longer lives.
Much of the information presented here has come from several books that I found very helpful in preparing for this article. I recommend these few books to people who want more information.
1. Foster R.C , A. D. Rovira, and T.W. Cock. 1983. "Ultrastructure of the Root-Soil Interface." The American Phytopathological Society, St. Paul, MN.
2. Kilham Ken. 1994. "Soil Ecology." Cambridge University Press. Cambridge, Great Britain.
3 Wild, Alan. 1994. "Soils and the Environment: An introduction." Cambridge University Press. Cambridge, Great Britain.
Dr. Alex Shigo is a noted authority in the field of modern arboriculture. An author, lecturer and consultant, he is the owner of Shigo, Trees & Associates in Durham, New Hampshire.