Basic Needs Plant growth is a very complicated process, but it is not necessary to understand all the intricate details in order to successfully manage the urban forest. However, understanding the basic requirements of tree growth will help. The primary environmental factors are:
—	Soil is a reservoir for nutrients and moisture, and provides mechanical support.
—	Sunshine furnishes heat and light.
—	Air supplies carbon dioxide and oxygen.

Plants in the wild obtain these basic needs from a complex, but finely balanced biological system. Cultivation often disturbs the natural balance. Any factor in the plant's environment that becomes less than optimum will limit its growth. Naturally, quantity is important, but quality also affects the vigor of trees. Air pollution, construction activity, and soil compaction place stresses on trees. Healthy trees can better with stand these stresses year after year. This section describes principles of tree growth and their relationship with the environment. Knowing these basics will help the people responsible for tree management make wise decisions. Relationship of Air and Water in the Soil Soil provides a foundation for tree growth-structurally and biologically. Soil supports a tree's physical weight and resists the forces of wind. Soil al supplies water, air, and nutrients. The value of the soil as structural and biological support is related to soil structure. Soil structure influences plant growth, because it affects moisture, aeration, heat transfer, and the mechanical resistance to root growth. Most people think of soil as being a mixture of solid elements, such as minerals and organic matter. But, open spaces in soil, called pores, are equally important. The size and distribution of soil pores affect the movement and availability of moisture and air through the soil. The air-water relationship is a critical one for optimum plant growth. The number of pore spaces must be adequate and the variety of sizes appropriate. Pore size determines the relative amounts of air and water that soil will hold. Gravity quickly drains water from larger pores, making them good sources of air. Smaller pores tend to hold water against the pull of gravity, making them good for water storage. Because plants need both air and water in the soil, a good balance of large and small pore spaces is required for optimum growth. Sand, with its open structure of many large pores, is called a "coarse" soil. Sandy soils generally drain well. They contain ample amounts of air, but little water. "Fine" clay soils drain poorly and tend to hold water in their many small pore spaces. But, pores in clay soils can be so small and hold water so tightly that tiny feeder roots cannot extract the moisture.

Organic matter in the soil affects both the physical and chemical properties of the soil. A fertile soil rich in organic matter is literally alive. Although insects and earthworms are the most obvious inhabitants, microorganisms, such as bacteria and fungi, constitute the largest population by weight in the soil community. Organic matter in the soil comes from decomposed plant and animal tissue as well as the micro-organisms themselves. Organic matter enhances the aeration and water-holding capacity of the soil. It also affects the soil's chemical properties by supplying necessary plant nutrients. The phrase "effective rooting depth" describes the portion of the soil where conditions are favorable for root growth-most often in the top three feet of the soil. Effective rooting depth may be limited by circumstances that restrict soil porosity or hinder plant growth. Four relatively common problems are surface crusting, high water tables, poor sub-surface drainage and claypans or hardpans.

In extremely compacted soil, or soil without adequate aeration, roots are found growing on the surface of the soil where conditions are more favorable for growth.

Soil Compaction Soil compaction is the major cause of death or decline of mature trees where efforts have been made to save them. It poses a very serious threat to good soil structure. Delicate soil pores are easily crushed, decreasing their capacity for water and air movement and hindering root growth. Wet soil is particularly vulnerable, because water lubricates soil particles and loosens binding agents. Small particles slip between the larger particles, filling the pore spaces. Loose soils will compact more than tight soils, and soils that have a broad range of particle sizes can be more severely compacted than more uniform soils. Few soils can withstand traffic without becoming severely compacted. Compaction depends not only on the amount of pressure exerted, but also on the duration and frequency of exertion. For example, the heel of a shoe exerts force per unit of area equal to that of heavy equipment (although the turning, starting and stopping of heavy equipment increases the force). Pressure spreads with depth, so the compacting effects of pedestrians and vehicles usually occur to the top four inches of the soil. Compaction is easier to prevent than to remedy. Avoid cultivating wet soils. Keep foot and vehicle traffic over the roots to a minimum. Where traffic is necessary, confine it to a few paths that stay well away from the driplines of trees. Light-weight vehicles with large, smooth, low pressure tires compact the soil less than heavy vehicles with smaller, heavily treaded, high pressure tires. A thick, coarse mulch spread on the soil surface where travel must occur helps to disperse the load.

The Root System A healthy root system serves multiple functions for the tree. Roots collect water and minerals from the soil, store nutrients, and anchor the tree. Fibrous roots are primarily responsible for the absorption of water and minerals from the soil. They most often grow in the top six inches of the soil. Woody roots provide a framework for anchoring the tree in the ground. They support the weight of the trunk and branches, including leaves, snow and ice. And, wind pressure adds to the load. Woody roots radiate out horizontally and vertically, decreasing in size as they get farther from the trunk. Woody roots seldom penetrate the soil deeper than three feet. Because they are relatively shallow, roots are vulnerable to damage from activities on the surface. It's important to be aware of roots, because they are not easily seen, and structural deficiencies often remain unnoticed until the top of the tree dies back or the entire tree falls over.

(Fig. 2)

(Fig. 2.) Zones where root defects are commonly found in container grown trees.

Root defects commonly occur in two forms (Fig. 2). When roots grow in a circle around the root mass it's called girdling. Roots may also be kinked into a characteristic "j-hook" growth pattern. Improper treatment in the nursery often causes these types of defects. To avoid problems, inspect the roots in the nursery before purchasing trees. Girdling roots may also occur after the tree is planted into the landscape. This happens when roots grow around the edge of the planting pit instead of out into the field soil. A lack of structural integrity in the root system may cause the planting to fail early in the tree's life or much later, but failure is sure to occur.

(Fig. 2.) Zones where root defects are commonly found in container grown trees.

Figure 2. Types of root defects and the zones within the root mass where they may be found.

Structural Strength of Tree Crowns A tree's crown must be strong enough to withstand wind forces, and carry the weight of snow, ice, and branches covered with thousands of leaves. To assess the structural strength of a crown, look at these characteristics: the spacing of lateral branches along the main trunk, the presence of a clearly dominant leader or main trunk, trunk taper near the ground, and angle of branch attachment. A tree with branches spread out along the trunk generally has better structure than one with all its branches growing close together on the trunk. A clearly dominant leader or main trunk provides a strong framework-called a scaffold-that supports lateral branches. Several leaders of similar size offer a weaker scaffold. Even round, oval, and vase shaped trees that commonly have more than one leader should show a clear hierarchy of leader sizes. This distinction of leader size is a sign of a good scaffold. Trunk taper at the base of the trunk helps support for the entire crown. Young trees with no trunk taper usually have to be staked, so they don't topple over in high winds or under heavy loads of ice and snow. Finally, branches that have a wide angle of attachment or have connective tissue in the crotch are usually more structurally sound than those with sharp angles of attachment or imbedded bark in the crotch.

Plant Stress Trees, regardless of species or location, have the same basic needs that must be met for the tree to remain healthy and grow. Basic requirements for plant growth include water, carbon dioxide, oxygen, nutrients, sunlight, appropriate temperature, and sufficient space. A balance must be maintained between the amount of water lost through the leaves and that taken up through the roots. Soil conditions must allow enough root growth to supply the leaves and branches with nutrients, gaseous exchange, and water. Photosynthesis (the manufacture of sugar by the green parts in the presence of sunlight) must be able to supply the energy needs of the tree. And this energy must be conveyed from the leaves to the stem and roots.

The trees planted in the median strip outside this row of fraternal houses near the University of Washington campus are suffering primarily from soil compaction associated with the recreational use of this space.

Last but not least, the strength with which the tree grows , known as vigor, must remain high enough to prevent attack by disease-causing agents, such as bacteria, fungi, and insects. The urban environment often places tremendous stress on trees that natural defenses may not adequately protect against. The urban environment changes rapidly, at least in comparison to the life span of most trees. The amount of stress experienced by a tree is directly related to the rate of unfavorable change in its environment, the quality of the planting location, and the level of maintenance it receives. Noninfectious diseases, which are not transmittable from a diseased plant to a healthy plant, will be especially threatening to trees whose defenses are weakened by urban pressures. Most urban stresses can be divided into the broad categories of environmental stress, animal injury, and people pressure. Most urban stresses can be divided into the broad categories of environmental stress, animal injury, and people pressure.

Environmental stress has the most severe effects on newly planted trees that have not yet adjusted to their new conditions and on established trees whose surroundings have changed, usually from the activities of people. Temperature stress results from extreme heat and cold as well as rapid changes. High light levels are usually associated with high temperatures. Thinning or clearing operations can expose remaining trees to excessive sunlight threatening them with light and temperature stress. Minimize the negative impacts of high temperature and light by planting shrubs and ground covers to create an "understory" beneath and around the trees. These new plants will help keep the area cooler. In areas surrounded by concrete or asphalt group the trees together or plant an understory to buffer high temperatures.

Soil compaction, altered surface water drainage patterns, and mechanical damage to this tree's roots are three stress factors represented at this freeway construction site

Low temperature can also injure trees. "Hardiness" refers to the adaptations of woody plants to cold conditions through patterns of growth and dormancy. While plants can often tolerate extreme cold during the dormant season, they could be injured by the same temperature during the growing season. Select plants with the extreme rather than the average temperature in mind. Plants in containers are more susceptible to extreme and rapidly changing temperatures, because their roots are not protected by the "thermal mass" of the earth. Moisture stress is one of the most common problems. Too much or too little water both inhibit root function, and therefore, the growth of the entire plant. Physical damage from snow and ice, poor water quality, and erosion could also be classified as moisture stress. One of the most important factors contributing to moisture deficiency is the amount of space available for the roots. If roots are restricted, they cannot spread through enough soil to gather the moisture needed to support the crown.

Another type of moisture stress occurs mostly in broadleaved and needled evergreens. During the winter animals --both pets and wildlife and spring, the air may be windy and warm, but the soil frozen or dry. Known as "winter drying" conditions, more water is lost through leaves than is taken up by the roots. Excess water from flooding, saturated soils or over watering can also place stress on the tree. Like other parts of the tree, roots need to ex change gases, such as oxygen and carbon dioxide, in order to survive. This exchange of gases occurs between the fibrous roots and the air in the soil's large pores. If the pores are filled with water, the roots suffocate. To avoid problems with excess moisture, determine the soil moisture and drainage patterns of the planting site, select a tree that thrives under, or at least tolerates, those conditions, or modify the soil situation.

Road construction is one of the most damaging forms of tree stress in urban environments.

Soil stress can be classified as either chemical, biological, or physical. Chemical stress involves unfavorable pH, an imbalance in nutrients, or even the presence of toxic materials discharged into the soil by other plants or human recklessness. In cities, soil stress is usually related to physical changes affecting soil aeration and moisture. Physical stress involves something that restricts root growth, including streets, sidewalks, buildings, construction activities, construction debris in the soil, large boulders, trenching, grade changes, soil compaction, improperly used herbicides, and the roots of other trees.

Cultivation of wet soil leads to soil compaction and should be avoided.

Animals injure trees in several ways. Animal waste, especially urine, adds toxic compounds to the soil. Animal traffic compacts the soil. A host of animals --both pets and wildlife inflict direct injuries to the bark: birds peck it, cats shred it, and rodents chew it. Dogs are particularly destructive when chained to a tree. People apply a variety of pressures that adversely affect the health of trees. Trees are easily disturbed by construction of buildings and roads, soil compaction, chemicals, air pollution, and improper tree maintenance also fall into this category. In addition to the stresses that come from the direct or indirect actions of people, urban trees also suffer from human inaction. Well meaning people sometimes impede appropriate urban forest management. Education is one way to increase the knowledge of urban forestry and to get people to work together to support it.

Restricted root space is capable of causing roots to grow in a girdling pattern, creating a structural deficiency in the anchorage of the tree.