Tips for the Municipal Arborist: Root Physiology

From City Trees, The Journal of The Society of Municipal Arborists
Vol 35, Number 4
July/August 1999

How do roots grow and develop into an extensive network capable of holding a tree 100 feet in the air, against 50 mile per hour winds? This article will describe how roots grow and interact with the rest of the tree and the soils in which the tree is growing. This article will be followed by an article in the next issue of City Trees about solving root problems in our urban environments.

The roots of all trees are required for anchorage, absorption of nutrients and water, and the storage of starch. The development of a root system is dependent upon the plant genetics, the soils, and the environment. The first part of this article will describe how roots grow and function.

A tree's root system must balance its shoot system, but not in weight or dimensions. The root system must be able to supply the shoot system with sufficient moisture and nutrients so that the shoots can manufacture enough food to support the root system. This balance will be disturbed by pruning branches, damaging roots, or attacks by insects or disease.

Roots develop internally rather than from buds as occurs on stems and the parts of the tree above the ground. To do this, the root has several parts. They are described as follows from the youngest part of a root to the oldest part:

Root Cap - This part of the root is at its very tip. The cap protects the tip of the root as it is forced through the soil by the elongating tissue behind it. Old cells on the cap will slough off the surface and lubricate the movement of the root through the soil. These cells are constantly being replaced.

Apical Maristem - This part of the root provides the cells for the root cap in front and for the region of elongation behind this area.

Region of Elongation - Active cell division and elongation in this part of the root forces the cap through the soil against the mass of the tree.

Region of Root Hairs or Differentiation - It is in this area that cells develop a more mature form and are differentiated into the epidermis and cortex. Root hairs are formed in the epidermis or surface layer of cells. The root hairs serve to absorb water for the plant and live for only a short period of time. They do not become lateral roots. Firs, redwoods and Scots Pine do not have root hairs. Instead, they absorb water and nutrients through the thin walled epidermis. In contrast to this, other trees such as the Redbud and Honeylocust have root hairs that last for several years. Within the cortex are the endodermis, pericycle, phloem, and xylem.

Lateral Roots - In the area behind the region of root hairs, lateral roots are formed. Lateral roots begin by sending out a root cap, apical meristem tissue, etc. into a new area of soil.

Root Forms

Most root systems of trees grow in the top three feet of the soil. This is where oxygen and organic matter are most prevalent. The small feeding roots are in the top six inches especially in a forest or mulched area. In ideal conditions, a tree's roots will spread out two to three times the crown diameter or a radius distance equal to the height if the soil is right for the tree.

Roots need a minimum of 3 percent oxygen in the soil to stay alive. Twelve percent is needed for new roots to form. Fortunately, most normal soil is about 20 percent oxygen. However, if surface changes have occurred, and the soil became compacted and contained only 5 percent oxygen, the existing roots would survive but new roots could not grow, therefore the tree would become stressed.

Soil types can also influence root development. While clay soils might have insufficient oxygen levels at three feet deep, sandy soils could have 15 percent oxygen at 5 feet deep.

Root Microorganizms

Other microorganisms consist of algae, bacteria, and fungi:

Algae assist in dissolving minerals and creating soil.

Bacteria decompose organic matter. Some bacteria are very useful in compost piles, while other bacteria will thrive in anaerobic composts. Certain bacteria are also useful in legumes for fixing nitrogen to the roots while others denitrify nitrates. There are also some harmful bacteria that cause diseases in trees.

Fungi are the most important microorganism in the soil. Although many fungi are responsible for causing diseases such as rots and wilts, they are important because they form a symbiotic relationship with a plant's roots. This association is called mycorrhizae. The word mycorrhizae comes from myco meaning fungus and rhiza meaning root. A tree can not survive without mycorrhizae.

Mycorrhizae

Mycorrhizae roots not only increase a tree's drought tolerance, they increase tolerance to soil compaction, high temperatures, toxic materials, salt, extremes of Ph, and root diseases. Three types of the most common mycorrhizae are:

• Ectomycorrhizae which are fungi that form a mantle or sheath around roots with no fungal penetration of root cortical cells.
• Endomycorrhizae which are fungi that penetrate cortical cells of fleshy young roots but form no mantle or sheath.
• Ectendomycorrhizae are those fungi that both penetrate root cortical cells and form root-surrounding mantles.

Runner hyphae follow roots as they grow in the soil. Sometimes runner hyphae grow out into the soil probing for more roots.

Penetrating hyphae branch off runner hyphae and infect fleshy young roots usually within a short distance of the root growing tip. Once inside the root, these hyphae pierce root cortical cell walls and form arbuscules between the root cell wall and cell membrane, called the plasmalemma. Arbuscule are organs for exchanging nutrients with root cells. In addition to arbuscules, penetrating hyphae also produce storage organs called vesicles in spaces between root cells.

Finally, absorbing hyphae branch off of runner hyphae in a fanlike pattern. They grow into soil surrounding the infected root and absorb water and nutrients. The absorbed water and nutrients subsequently move to the penetrating hyphae and then into the infected root.

Phosphorus is mainly insoluble and immobile in soil. VAM fungal hyphae explore the soil beyond the root zone and acquire phosphorus for roots. In the past, researchers thought that all VAM fungi, regardless of species or geographic region, were functionally equivalent. This meant that an increased uptake of phosphorous was the sole cause of VAM promotion of plant growth. Based on that assumption, researchers thought a plant's need to be mycorrhizal was related directly to the amount of phosphorus in the soil. If soil phosphorus was low, then mycorrhizal fungi were necessary to facilitate phosphorus extraction. However, if soil phosphorus was adequate or high, then mycorrhizal fungi became little more than root parasites levying a carbon tax on the plant.

The concept of mycorrhizae does not mean that a root/fungal association is always beneficial. Situations occur in which a fungus benefits the plant under one set of conditions but harms it under another, or the fungus benefits one plant but harms another. Researchers are learning from this increasingly complex picture of VAM fungi that it is inadequate to measure the efficiency of a mycorrhizae solely in terms of host plant phosphorus nutrition or an enhancement of plant growth. Other adaptational responses may be equally important. For example, under drought conditions, VAM promotion of growth should take a back seat to improved plant water relations. Or, plants recently transplanted into a landscape could benefit more from VAM fungi that promote root exploration of soil rather than promote shoot growth.

During the 1980's, researchers noticed that some mycorrhizae caused the roots of herbaceous plants to become thicker, less branched and more elongated. Recently, plant scientists at Arizona State University discovered that the VAM fungus, suppressed shoot growth and caused roots of a landscape tree to become thicker and less branched under drought conditions, compared with drought stressed trees not infected with VAM. Mycorrhizal suppression of growth and thickening of roots may be a drought survival mechanism promoting storage of carbon reserves in roots until drought conditions improve.

Compared with a highly branched or dichotomous root system, a sparsely-branched or herringbone root system would be more beneficial under conditions of soil stress or intense root competition because of its ability to explore a larger soil volume. In addition, mycorrhizal colonization of sparsely branched roots increases the total root absorbing surface because fungal hyphae act as root conduit extenders. Such changes in root branching caused by VAM fungi could have significant ramifications for successful transplant establishment of landscape trees and shrubs.

Roots attract mycorrhizal hyphae by excluding organic acids that act as chemical signals. As root tips continue to grow out into the soil, the infected root segment begins to harden. This hardening causes an end to the mycorrhizae and ensures that fungal colonization of roots occurs only on fleshy portions of the root system that are most active in absorbing water and nutrients.

Root Interaction with Soils

Planting sites should be large enough to accommodate the tree's roots at maturity. This means 4 square feet of surface area for every inch of diameter the tree is expected to attain, or 2 cubic feet of soil for every square foot of the future crown. The soil should also be between 20 and 30 inches deep for maximum vigor. The more surface area the better. Sites with pavement and barriers to healthy root development should be avoided. Proper planting techniques are essential for tree root survival.

If the soil is typical of an urban site - concrete, metal, building materials, clay, etc. or the soil can not be improved by tilling, it must be improved if trees are to grow. The improvement should consist of removing some of the existing material and incorporating organic matter such as compost, peat moss, peat, etc., to improve the soils's permeability. If the soil is heavy clay, sand should be added by as much as 50% in volume. If the soil is pure sand, organic loam should be added by as much as 30% in volume. Gypsum should be added if the soil test indicates high saline condition also known as a high salt index. Sulfur or aluminum sulfate should be used to bring down a high pH, although the planting of alkaline tolerant trees is a better option. Permeability of the soil is essential if water, air, and roots are to move through the soil.

Tree roots also need warm soil so they can continue to grow all year round. Even when the soil is frozen, the roots will continue growing until the soil reaches a temperature of 240F (-5°C). Mulch or forest soil will protect the roots and usually prevent the temperature from going below 240. Tree tops on the other hand can survive in temperatures as low as -20°F (-29°C).

Soil Preparation

When transplanting trees into urban soils, arborists should select the sites containing as much high quality soil as possible. High quality soils have adequate aeration, moisture, a stable pH, and a good amount of organic matter. If good soils are not available, amendments such as organic fertilizer and soil conditioners should be added. Biostimulants, surfactants and water absorbing gels can be added to the soil. Inocculants of mycorrhizae and beneficial bacterias as found in products such as Mycor Tree, Tree Saver Transplant, or equal, are also desirable at planting time. Mature trees will also benefit from mycorrhizal injections into the soil before, during, or after a major construction project over their roots.

These tips will encourage tree survival through consideration of root microorganisms:

• Amend disturbed urban soils with native topsoil when landscaping. Native topsoil will have mycorrhizal fungi.
• Maintain a well aerated soil environment because this stimulates mycorrhizal activity.
• Avoid using vermiculite and perlite as landscape planting amendments because they inhibit the formation of mycorrhizae essential for tree root development.
• Avoid overusing inorganic fertilizers, especially phosphorus, because they inhibit the formation of mycorrhizae.
• Avoid unnecessary use of fungicides unless their is a major outbreak of harmful fungi.

How do you grow trees in downtown soils? How do you compact soil for sidewalks, curbs, and streets and not compact the soil for street tree planting? Cornell University has developed a technique called Cornell's Structural Soil mix. It is basically crushed rock varying in size between 1/2" and 1-1/2" diameter. The rock is easily compressed so any sidewalk or street construction has the necessary compact soil. At the same time, the large pores between the rock particles can contain soil, roots, moisture, and air. The soil is stuck to the rock by using hydrogel as a sticking agent. The root can grow between the rock particles seeking moisture and air.

The rock is prepared by being spread out near the construction site. The hydrogel and soil are added on top of and mixed into the rock. The ratio is 30 grams of hydrogel to 100 kilograms of soil to 500 kilograms of rock.

The entire mix is then moved to the construction site so the sidewalk or street can be built. The trees can be added later. The mix is spread at least three feet deep and under as much paved surface as feasible. The mix allows the pavement to be built properly and the trees to grow with their roots penetrating the voids between the rock particles.

Another growing technique for dealing with poor urban soils is the Amsterdam Tree Soil which consists of 91% coarse sand, 5% organic matter and 4% clay. This soil is especially suited for growing trees surrounded by sidewalks. The sand provides the stable base for the walk construction while tree roots can utilize the minerals and organic matter.

If the existing soil is so bad that it can not be amended to meet the ideal conditions, the soil should be completely removed and replaced with the Cornell Structural Soil mix, the Amsterdam Tree soil, or other soil improvement techniques. The soil should be amended as necessary before trees are planted. Attempts to grow healthy trees in poor soil will not succeed. Where growing conditions are ideal, roots can grow 10 to 15 feet per year.

Sources:

Society of Municipal Arborists
Norma Bonham, Executive Director
P.O. Box 11521
St. Louis, MO 63105
Phone: 314-862-3325
Website: http://www.urban-forestry.com/

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