By Norman Helie
The annual death rate of newly planted trees in the city is high. Many trees die during the first season. The first season of life at the new site is very important. Regardless of the tree planting technique, bare-root, container, or balled and burlap, the tree roots system is disturbed. This disturbance breaks the continuity of water from the soil to the root. In nature, we generally call this continuum of soil water into the root “establishment.”
The details of this establishment include fine root development to an existing root system with secondary growth. Fine root development is destroyed during the planting process. Fine roots, including fungal roots, have great surface area and account for more than half of the plant’s ability to absorb water. To make matters worse, the soil planting process disturbs all soil particle arrangement and structure surrounding the tree’s roots. The primary goal of successful tree planting should be quick “establishment,” not the one-year warranty. The one-year warranty method keeps the tree watered and the soil saturated with water. This gives no opportunity for the regeneration of fine roots. Rapid establishment requires understanding the root system and the movement of water in the disturbed soil profile and the root zone.
The Importance of Fine Roots
The efficiency of the established tree’s ability to provide moisture to the canopy is not understood. Fine roots lack secondary growth and are living cells that need more oxygen for respiration. Established trees in undisturbed soil maintain turgidity via water as a liquid and a gas. In soil and trees, these systems can be considered open and or closed containers during varying seasons and conditions. Therefore, all the laws of temperature on water as a gas (vapor) and a liquid apply.
Water in the vapor form in equilibrium with its counterpart liquid, provides the best environment for fine roots. High humidity is always present in the soil, even during the driest times. Water as a gas permits space for oxygen which is essential for cellular respiration. Both plant and fungal fine roots need ample oxygen. This gas and liquid equilibrium in the soil requires soils to dry out several days after watering. Soil wetting and drying cycles are vital for root and soil health. Tree transplants suffer moisture stress the most during this time because they lack fine roots in disturbed soil. Fine root re-establishment is dramatically hindered when liquid water is added to compensate for this problem because of the lack of oxygen. This process repeatedly continues as the caretakers continue to water the tree. But remember, the first year of fine root development is vital for the tree’s establishment. The disturbed soil will also correct itself, but this intricate adjustment takes at least two years. Soil wetting and drying cycles promote this soil restorative process too. Constant wet soil hinders soil health.
Watering transplanted trees is not the same as watering crops or a perennial garden. Small annual and perennial plant root systems are larger and robust, requiring less oxygen than the delicate fine root systems of the tree. Fine roots are defined as diameters less than one millimeter. Most of these fine root diameters are half a millimeter. Fine roots are very powerful and can penetrate a rock to mine for elements the tree needs. On the other hand, fine roots are also very delicate, unlike the larger roots we commonly call fibrous roots. These roots are usually unnoticed and, in many cases, unappreciated by most people. Fine roots are very thin, delicate, and considered insignificant. However, these roots are vital because they enhance the soil/root surface interface, allowing roots to absorb vital micro-nutrition and water from the variable soil surfaces. Fine roots act as filters against elements that may be excessive and/or toxic. Without fine roots, trees are gluttons for unnecessary organic and inorganic toxic elements. Fine roots are continually cycling through phases of life and death. In these cycles, the root death contributes to the organic matter content in the soil. Like leaves on a tree, fine roots senesce frequently. To put fine root significance into perspective: removing all fine roots from small root branches is like removing all leaves from small branches of your young tree. Of course, removing all leaves may kill the tree but, removing all fine roots during planting does not harm the tree but, it can be a problem if their re-growth is inhibited. Fine roots can grow back. Thankfully, larger and smaller fibrous roots can also absorb water. However, their capacity to absorb minerals is reduced significantly without fine roots. It is, therefore, imperative to consider fine root regeneration for the rapid establishment period.
Water Movement Within a Tree?
It can be said that the most difficult trees to transplant are also the trees that rely heavily on fine root mass for important resources like water and nutrition. Oak trees are an example of a tree that relies heavily on fine roots for their water and selective nutrient needs. These species need site-specific watering instructions and treatments to avoid the downfalls of overwatering a disturbed root ball surrounded by disturbed soil.
In the upper 12 inches of soil, fine absorbing root development is important, but in the subsoil, larger roots develop. These roots function for anchorage, storage, and water conduction. It is common to instruct people caring for trees to: “just keep it watered.” When it’s hot, they add “Water is all the tree needs.”. Berms around trees are created and the instructions are to fill these berms up with water. Other watering suggestions are to fill bags of water and the bag’s emitter will slowly water the entire root ball. Regardless of the method, applying water on a schedule may add more than enough water to the tree. Large anchoring roots can absorb water. These roots can also transfer more water. This is one of their functional designs. This design is exploited during the transplanting process. Roots with secondary growth have larger vessel elements. In many species, the vessels in the roots are larger than the vessels in the shoots (trunks). This allows water to flow freely from disturbed soil into these spaces. In nature, this process happens each and every spring when soil conditions are saturated and the tree is in a post-dormancy phase. This is called root pressure. This phase enables the tree to overcome sapwood embolism in the lower trunk and branches as warmer daytime temperatures seasonally arrive. Sugar maple flow is very dependent on this phenomenon. At this time sugar maple trees have very low numbers and activity of fine roots. The anatomy of the large root serves the purpose of both absorbing and translocating large volumes of water. During the summer, fine roots maintain the trees’ turgidity. This phenological tangent stresses the importance of planting trees early. When trees are in the ground well before bud break they have time to regenerate a fine root system that they need to survive. Late-season plantings are thrust into the summer’s high heat without any fine roots. This makes watering the tree for rapid establishment more difficult. Furthermore, it can lead to overwatering the restricted lower root zone in the subsoil.
The bottom of the root ball is smaller than the surface area on the top of the root ball. The larger area of topsoil is created to promote the re-generation of a fine root mass faster. But even a bare root or container-grown plant is placed into a disturbed lower subsoil. When the topsoil is watered frequently on schedule this water quickly filters down through the soil by gravity (Brady, 1984). If too much water is applied to the topsoil it begins to accumulate in the lower subsoil. The disturbed and often compacted soil surrounding the lower root ball does not permit water to flow freely or any further (Jury et al., 1991). It accumulates and creates an anaerobic, saturated condition. If large roots from the transplant are in this sub-soil they begin to die. This microbial decomposition creates toxic gasses that move up towards the surface of the root ball. This reduces the volume of soil for healthy roots. It is not noticed because it happens below ground. But as the tree continues to be watered, the gasses become trapped between the surface and the saturated subsoil. More water is added, and now the trapped gas is blended into the saturated sub-soil water. The tree is now wilting from high ethylene production from its own root system. This production of ethylene sends signals to the foliage first. This hormonal stress-induced wilting is from overwatering. The caretaker of the tree sees dry topsoil and adds more water because the tree is wilting and has dry topsoil. Ethylene-induced wilting is not easily discerned. Is the tree wilting from not enough water or too much water? It looks the same as a small perennial plant in need of water. The caretaker doesn’t want to nullify the warranty and waters again without any instantaneous leaf-turgidity improvements. At this time, if the saturated subsoil levels of oxygen do not improve, the tree will begin to die.
Watering for Tree Success
When trees are watered properly, they may slightly wilt, but turgidity should recover in less than six hours. If the watered tree is wilting the next day, the lower root zone has a saturated subsoil. Diagnosing this condition can be done by digging down alongside the root ball. Soil moisture, color, and odor are useful characteristics to look for in detecting lower subsoil saturation problems. When soils are wet, gray, and have an unpleasant smell, the soils have been saturated too long. This may also be a condition of the existing subsoil that has poor drainage. If the site is known to have slow draining subsoils, watering the tree with less water is advised. Species selection may also be a problem with slow draining subsoils. Checking the root system of the transplant for color and health is also recommended. Roots should be light in color, not dark black or blue. If roots are dark, the tree’s roots are likely to become infected with disease.
Advanced diagnostic and research tips are to install moisture sensors into the lower root zone at the time of planting. Careful installation will permit monitoring of the water content of the lower profile. The meter’s sensitivity is enough to flag water-saturated soil. Sometimes these conditions happen naturally during heavy rains. Recording this information helps people know how much water is needed to assist the soil’s root zone in the upper 12 inches. At this time, it is advised to use wetting agents and humectants to keep the soil and larger roots moist in the upper soil profile only. The application must be uniformly applied to the upper 12 inches of the soil root ball. The addition of bio-stimulants can be added to this treatment.
Understanding the complexities of watering trees is the first step in reducing tree planting losses. Transplanting trees destroys all natural soil arrangement and aggregation. This soil disturbance takes several years to self-correct. The re-establishment of fine roots production is scarcely mentioned by professionals when they communicate how to water trees to their clients. Present-day recommendations are to keep the tree alive by saturating the soil and the larger root systems without any consideration for fine root re-establishment. Keeping the topsoil wet at all times does not permit natural soil wetting and drying cycles. This directly hinders the fine root establishment and soil aggregation. Overwatering trees is the leading cause of tree planting failure when trees are watered. Rapid establishment watering techniques can be accomplished in one year.
Brady N. C., 1984. The Nature and Properties of Soils. 9th ed. pp 74-108.
Jury W. A., Gardner W. R., Gardner W. H., 1991. Soil Physics 5th ed. pp 73-110
Leuschner C., Hertel, D., Schmid, L. et al, 2004. Stand fine root biomass and fine root morphology in old-growth beech forest as a function of precipitation and soil fertility. Plant and Soil 258, pp 43-56.
Pregitzer K S., 2002. Fine roots of trees – a new perspective. New Phytologist 154, pp 267-273.
About the Author
As a young boy, Norm built numerous tree forts. He would also tunnel underneath the trees, where he made many observations about how trees grow.
These simple investigations lead him into a career in arboriculture. Using scientific literature gathered from various fields, he enjoys reading about plant anatomy, nutrition, physiology and soil sciences. Taking long walks in the woods with his dog Finn helps him process this information and develop solutions to many challenges of growing trees.
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