Overstory #70 - Troubles in the Rhizosphere
This is the third in a three part series by special guest author Dr. Alex L. Shigo, retired chief scientist of the U.S. Forest Service and author of numerous books including Modern Arboriculture. Dr. Shigo describes the importance of the root-soil interface (the rhizosphere), and how trees depend on healthy relationships with numerous soil organisms.
Troubles in the Rhizosphere
The rhizosphere is the absorbing root-soil interface. It is the zone, about one millimeter in width, surrounding living root hairs and the boundary cells of mycorrhizae as well as hyphae growing out from some mycorrhizae.
The rhizoplane is the boundary where soil elements in water are absorbed into the tree. Under an electron microscope, the rhizoplane appears as a jelly where microorganisms and tree cells mix, making it impossible to tell which side is tree and which is soil.
A constantly changing mix of organisms inhabit the rhizosphere and surrounding soil. Bacteria, actinomycetes, fungi, protozoa, slime molds, algae, nematodes, enchytraeid worms, earthworms, millipedes, centipedes, insects, mites, snails, small animals and soil viruses compete constantly for water, food, and space.
The rhizosphere is a battleground and the wars are continuous. Amoebae are eating bacteria. Some bacteria are poisoning other bacteria. Fungi are killing other fungi. Nematodes are spearing roots. Fungi are trapping nematodes. Earthworms are eating anything they can find. Sometimes the victors benefit the tree and sometimes they do not.
Every tree treatment affects the rhizosphere in some way. The more you know about the rhizosphere, the better the chances are that your treatments will lead to benefits rather than harm.
Tree Declines and the Starving Rhizosphere
Go anywhere in the world and you will learn that some local trees have a "new" decline problem. Declines usually mean the trees are sick because there is a problem in the rhizosphere.
Trees die, as all organisms do, in three basic ways: depletion, dysfunction and disruption. Disruption means wounding, severe mechanical impacts and fracturing. Dysfunction means some parts and processes of the living system have developed problems that retard or prevent their functioning and growth. Depletion means that the basic substances for life begin to decrease to the point where injury and death are certain. One of the ways depletion injures organisms is by starvation.
Soils and wood share a common problem: They are thought of as dead substances. This has come about because wood-products research gained an early lead over research on wood in living trees. With soils, many texts still define soils as "loose material of weathered rock and other minerals, and also partly decayed organic matter that covers large parts of the land surface on Earth."
Sapwood in living trees has many more living cells than dead cells. In upper layers where most absorbing roots of plants grow, soils have more soil organisms than grains of weathered rock. In great disrespect, most people still refer to soil as dirt! When researchers first discovered the great value of soil microorganisms for human antibiotics and profit, the living nature of the soil began to emerge.
A more correct definition of soil should be that it is a substance made up of sands, silts, clays, decaying organic matter, air, water and an enormous number of living organisms. Survival of all living systems depends greatly on synergy and efficiency to optimize the functioning of all processes and to keep waste as low as possible. When synergy and efficiency begin to wane, declines follow.
Trees are dependent on the light energy from the sun for their energy, water and 14 elements from the soil for their building blocks of life. Some trees decline when incorrect treatments or abiotic injuries lead to starvation of organisms in the rhizosphere. When there are troubles in the rhizosphere, there will be troubles with the tree.
Energy & Root Exudates
Microorganisms compete in the rhizosphere, an area rich in exudates from the tree. The exudates contain carbohydrates, organic acids, vitamins and many other substances essential for life. From 5 percent to 40 percent of the total dry matter production of organic carbon from photosynthesis may be released as exudates! When trees begin to decline, the amount of organic carbon released as exudates increases. Mineral deficiencies, low amounts of soil air and severe wounding are major causes for the increase. An increase in exudates can be caused by over-pruning, construction injury, planting too deeply, over-watering, compaction and planting trees in soils that have a pH too high or too low for their optimal growth.
You would think that a tree in decline would decrease, not increase, exudates. A possible explanation might come from the self-thinning rule of ecology, which states that when energy input into a site equals output, there will be no further growth unless some trees die. As many suppressed trees die, a much fewer number continue to grow bigger. Simple. Or, on the basis of the mass-energy ratio law, as some trees on a site get bigger, many smaller suppressed trees will die. As the suppressed trees decline, they contribute a higher percentage of their soluble carbohydrates to the rhizosphere.
The increase in exudates from a declining tree with a defense system weakened by low energy reserves would give root pathogens an advantage over other soil organisms. When the tree dies, its dead wood adds a great amount of carbon to the soil, thus benefiting all soil organisms. If this scenario is correct, then the codes for the increase of exudates as trees decline would have been set in the genes of the forest trees. Then, even after trees are taken out of their groups in forests and planted as individuals, the genetic codes for increasing exudates as the tree declines for reasons other than crowding would still be in effect.
A tree does not "know" why it is dying. In a crowded, young, growing forest, the self-thinning rule of ecology does benefit tree survivors and all soil organisms. But, when one or two trees in a yard, city or park start to decline, their early death may benefit only the root pathogens. And even worse, since the tree will be cut and removed from the site, there would be no benefits from added carbon to the soil.
A Closer Look at Roots
Woody tree roots are organs that support the tree mechanically, store energy reserves, transport water and the substances dissolved in it and synthesis substances such as growth regulators, amino acids and vitamins that are essential for growth.
Trees have different types of root systems. For example, mangroves along coastlines have stilt roots. Many trees growing in tropical areas have aerial roots that become prop roots when they grow into the soil. Other trees have strangling roots that eventually kill the host tree that first supported their growth. Trees in sandy soils can have roots that grow downward over 90 feet. Palms have roots that are adventitious and grow from meristematic regions in their base. Many tree species have deep roots when they are young and more shallow roots later. It would be nearly impossible for the strongest person to pull out young saplings of beech, oak or hickory from forest soil.
It is important to remember that woody roots store energy reserves, and root defense is dependent on energy reserves. When reserves are low, defense is low. When defense is low, weak or opportunistic pathogens attack. It is nature's way.
Non-woody tree roots are organs that absorb water and elements dissolved in it. The two basic types of non-woody roots are:
- Root hairs on non-woody roots are extensions of single epidermal cells. Common on seedlings, root hairs grow to maturity in a few days. They function for a few weeks and then begin to die.
On mature trees, they are usually not abundant. When they do form, they do so when soil conditions are optimum for absorption of water and elements. I have found root hairs growing in non-frozen soils beneath frozen soils in winter.
- Mycorrhizae are the other type of non-woody roots. Mycorrhizae are organs made up of tree and fungus tissues that facilitate the absorption of phosphorus-containing ions and others essential for growth.
The fungi that infected developing non-woody roots to form mycorrhizae were very "biologically smart." Rather than competing with other microorganisms in the rhizosphere for exudates from the tree, the mycorrhizal-forming fungi went right to the source inside the tree. And, even more to their advantage, many of the mycorrhizal fungi grew thread-like strands of hyphae-long, vegetative tubes of fungi-out from the mycorrhizae. This inside and outside presence gave the fungi a distinct advantage over other microorganisms in the rhizosphere.
The tree gains efficiency with mycorrhizae in several ways.
- With their extended hyphae, mycorrhizae not only greatly extend the absorbing potential into the soil, but the hyphae may connect with other hyphae on other trees. In this way, the mycorrhizae serve to connect trees of the same or a different species. This leads to the conjecture that the natural connections that developed over long periods in the natural forest may have some survival value. That is why forest types are often named for the groups of species commonly found growing together. For example, we speak of the birch-beech-maple forest, or the pine-oak forest. From a practical standpoint, when trees are planted in cities and parks, there may be great survival advantages by planting groups of trees made up of the species that are normally found together in natural stands.
- The mycorrhizae have been shown to provide some resistance against root pathogens. It may be that the pathogens would have difficulties in building their populations in the rhizosphere dominated by the mycorrhizal fungi.
Perhaps the most important feature of the mycorrhizal fungi is that their boundary material is mostly chitin. Chitin is slightly different from cellulose by the replacement of some cellulose atoms by a chain of atoms that contain a nitrogen atom. This slight change in some way makes chitin a material better suited for absorption of elements. Remember that the fungus hyphae gain all their essentials for life by absorption through their boundary substance.
There are other advantages, to the chitin and the tube-like hyphae that ramify the soil in the rhizosphere and beyond. When the hyphae die, they add a nitrogen source for other organisms. Also, when the hyphae are digested, they leave tunnels in the soil that are about eight to 10 microns in diameter. For the bacteria, these small tunnels may mean the difference between life and death. The bacteria quickly colonize the tunnels. The survival advantage here is that the major threats to their survival are protozoa that are usually much larger than 10 microns. So the hungry amoebae are not able to get at the bacteria inside the eight-micron tunnels.
A common treatment for compaction is to fracture the soil and add water. The fracturing allows air to penetrate the soil, but does not provide any eight-micron tunnels for the bacteria. The only way to bring back the tunnels is to bring back the fungi in well-composted wood and leaf mulch, as nature does, or by inoculating the mulch with mycorrhizal fungi.
Who Was First?
I do not know if the fungi were the first to grow into the root to get first chance at exudates or whether it was the bacteria. Regardless, bacteria and their close relatives, the actinomycetes, also infect non-woody roots to form organs that serve for the fixation of atmospheric nitrogen. Fixation means that the nitrogen that makes up almost 80 percent of our air is converted to a soluble ionic form by the action of the bacteria and actinomycetes within the nodules on the roots. (Some free-living soil bacteria can also fix nitrogen.) An enzyme called nitrogenase is the catalyst for the reaction that will take place only under very exacting conditions. There must be soluble molybdenum and iron and no free oxygen available. These conditions are present within the nodules. Here again, the microorganisms benefit the tree by providing a source of soluble nitrogen, and, in turn, the bacteria and actinomycetes get first chance at exudates. Even more importantly, the nodules protect them from foraging protozoa.
Infections that result in benefits to both parties are called mutualistic. When the benefits are greater than the sum of the parts, the association is called synergistic.
Species of legumes commonly have bacterial nitrogen-fixing nodules and mycorrhizae. The mycorrhizae facilitate absorption of elements, and the nodules provide a nitrogen source. Many species of trees have actinorhizae, which are the nodules formed by the root infections by actinomycetes. Species of Alnus have very large nodules. The actinorhizae are common on tropical and subtropical trees, and especially on trees that have adapted to soils low in available elements essential for life.
On some subtropical and tropical trees, such as the macadamia, multi-branched clusters of non-woody roots called proteoid roots form. The proteoid roots alter the rhizosphere by acidification processes that facilitate the absorption of phosphorus-containing ions. When I examined the roots of dying macadamia nut trees in an orchard in Hawaii, I could not find proteoid roots, yet only a few days earlier I had found them on macadamia nut trees growing in the wild. I learned later that the orchard where trees were dying was heavily fertilized on a regular basis with phosphorus.
Another type of nodule forms on species of cycads. These nodules harbor blue green algae, or cyanobacteria, that have the ability to fix atmospheric nitrogen.
My point is that many different synergistic associations have developed in, on and about non-woody roots that provide elements, not an energy source. These associations are of extreme benefit to all connected members. At the same time, the conditions that provide for the associations are very delicate and exacting. It does not take much to disrupt them.
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 very beneficial for healthy survival of trees planted outside their forest homes. How beneficial will depend greatly on an understanding of some basic chemistry. For example, the use of urea could be a contributing factor to decline 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.
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.
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 leaves and photosynthates from the top of the tree. When the energy source from the top begins to decrease, the rhizosphere organisms will begin to starve.
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 imbalance that leads to decline. 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.
Foster R.C., A. D. Rovira, and T.W. Cock. 1983. "Ultrastructure of the Root-Soil Interface." The American Phytopathological Society, St. Paul, MN.
Kilham Ken. 1994. "Soil Ecology." Cambridge University Press. Cambridge, Great Britain.
Wild, Alan. 1994. "Soils and the Environment: An introduction." Cambridge University Press. Cambridge, Great Britain.
About the Author
Dr. Alex L. Shigo is considered by many to be one of the foremost authorities on trees in the world today. Retired from the U.S. Forest Service, he is an internationally recognized researcher credited with the development of expanded interpretations of decay based on compartmentalization and microbial succession. His research includes over 15,000 longitudinal tree dissections with a chain saw. He has published over 15 textbooks used in many universities worldwide. For more information about seminars and to order Dr. Shigo's publications, contact: SHIGO AND TREES, ASSOCIATES, P.O. Box 769, Durham, NH 03824, USA. Phone: 603-868-7459; Fax: 603-868-1045.
This excerpt of "Troubles in the Rhizosphere" was reprinted with the kind permission of the author. More information and book purchase can be found at Shigo and Trees, Associates LLC or Shigo and Trees, Associates LLC, P.O. Box 769, Durham, NH 03824, USA. Phone: 603-868-7459; Fax: 603-868-1045.
Related Editions to The Overstory
The Overstory #61--Effects of Trees on Soils
The Overstory #33--Mushrooms in Agroforestry
The Overstory #8--Mycorrhizae