By their very nature, trees and green space provide benefits and add value to developments. The ability of trees to improve and maintain the quality of water, soil, and air and to remove pollutants from the air is well known. Trees also provide shade and help lower temperatures during hot weather. Trees enrich people’s lives and beautify landscapes.
Preserving/planting trees in a development has positive effects on the image and attractiveness of the developments and enhances a developers’ reputation and profit, including increasing a project’s attractiveness, monetary value, and marketability by providing aesthetic and functional values. Land where trees are planted/preserved can be sold more quickly and at higher prices.
Research has shown that mature trees increase the worth of a property up to 12 percent. Developers who understand these values realize that it is in their best interest to encourage the preservation and planting of trees and green spaces.
Developers can take advantage of different opportunities when considering the preservation/planting of trees. Individual historic, landmark, and ornamental trees are all good choices for preservation, planting indigenous trees within their local bio-diversity is always preferred.
Opportunities differ from one development to another, but many of the recommendations for preserving trees remain the same.
An experienced arborist, after a site visit, can identify “ideal” trees for preservation and provide a suitable tree list for planting, considering the geographical location/annual rainfall, the overall development plan, and taking into account the architectural look and feel of the development.
Having an arborist on site is critical in monitoring tree health before/during and after construction. If necessary, this specialist can then recommend correct watering schedules and/or fertilizer programs for the trees, and/or provide treatment for a pest or disease problem.
Although care should be taken when working around all trees, some trees are more sensitive to construction than others. For example, older and larger trees may need specialized attention, such as fertilization and irrigation, if they are disturbed by root pruning, soil compaction, or other construction activity.
Soil for healthy trees
A good soil for plant growth is a mixture of clay, sand, and silt called a “loam.” The ideal soil for tree growth is a loam that is well drained and aerated, contains 1 to 5 percent organic matter, has a covering of leaves and other organic material, and has an established population of living organisms such as fungi, bacteria, and earthworms.
Healthy trees need healthy roots and healthy roots need good soil structure. Care must be taken during construction to protect the structure of soil. Compacting soils harms trees by decreasing the ability of their roots to take up water, oxygen, and nutrients. Compacting soil with heavy equipment or stored materials destroys good structure of the soil by crushing and closing the pore spaces. Even foot traffic beneath trees in parks and around buildings can compact soils enough to be detrimental to tree health. Soil compaction slows or stops rainwater from infiltrating and increases runoff, which reduces the amount of water available for plant growth. Compaction also interferes with the aeration of the soil, which lowers oxygen levels and raises carbon dioxide levels around tree roots.
Preconstruction activities
The goal of preserving trees in development projects is to protect adequate space for trees with the best health, structure, and appearance, while removing hazardous trees, lower quality trees, and others that are in the way of construction. Above all else, preserved trees need adequate space for root and canopy function and growth.
It is important to preserve only those trees that are healthy and structurally safe. Dead and dying trees and trees that are hazardous should be removed. Tree health is evaluated by observing crown density, foliage colour and size, insect and disease problems, injuries, and amount of deadwood. Tree structure is a system of many interactive factors.
It is commonly thought that trees can withstand the removal of 30 to 50 per-cent of their root system, but structural stability may be compromised after the removal of more than one-third. Because it is difficult to estimate the full extent of a tree’s root system, it is difficult to know when a certain percentage of roots has been affected. Providing an adequate tree protection zone (TPZ) helps preserve needed tree roots. A minimum TPZ for a tree can be determined by using the following rule: Measure the diameter (in inches) of a tree trunk at a point 1.5 meters above ground, known as the diameter at breast height (DBH). Define a circle around the tree with a diameter in cm or inches (every2.5cm/1 inch) equal to the number of meters/feet (0.3m/1foot) of the trunk’s DBH.
For example, a Celtis Africana with a diameter of 25 cm /10 inches would have a TPZ of 3 meters/10 feet in diameter. This rule defines the minimum distance to keep construction activity and storage of equipment and materials away from a tree. The more undisturbed space that can be provided around a tree, the better the tree’s chances of survival and subsequent growth.
Considering alternative construction techniques
A variety of construction elements and techniques can be used to help preserve trees while providing space for development. For example, crib and retaining walls can be used to limit the length of manufactured slopes and keep soil cuts and fills out of a tree protection zone. Engineered pier or grade beam footings, which reduce pruning damage to tree roots, can be used when working close to tree trunks. Self-supporting concrete or asphalt sections can be engineered to reduce soil compaction and root damage when sidewalks, parking lots, streets, and driveways are placed under or close to trees.
Consider energy needs of buildings
Strategic tree and building placement can reduce annual energy use for heating and cooling by as much as 10 percent.
Consider replacing or moving trees
Smaller trees sometimes can be replaced for less than it would cost to preserve them. Nursery plants or trees can be placed in just the right locations and are often of equal or greater value than existing smaller trees. Usually, it is more beneficial and effective to save larger trees, because larger trees can provide greater functional and aesthetic benefits and have surprisingly high monetary values in landscapes.
Existing trees could be transplanted to other locations on the site. Trees that are 10 cm/4 inches DBH or less could possibly be moved with backhoes or other equipment already owned by most contractors. However, such an operation requires a knowledgeable and experienced person to be successful. Tree spades or other equipment for moving trees up to 25cm/10 inches DBH are available. Larger trees can be moved with cranes and other special equipment, but the relocation costs may exceed the costs associated with saving them at their existing locations.
The transplanting process requires cutting many of a tree’s roots, which puts the tree under severe stress. Even with special care, such as staking, irrigation, and fertilization, it often takes several years for a larger tree that has been transplanted to regenerate enough roots to continue growing at a normal rate, for this reason it is always preferred to bring in container grown trees and not ex-open ground trees. All these factors and costs must be considered when deciding whether it is better to preserve a tree at its existing location, replace it with nursery stock, or relocate it on a construction site.
Dirk Stoffberg
Owner of Watercombe Farm Indigenous Tree Nursery
@watercombe_farm_tree_nursery
www.watercombefarm.co.za
