There are plenty of good reasons to protect trees when building on previously undeveloped land.
Healthy trees can shield a house from noise, block cold winter winds and hot summer sunlight, and make backyard recreational space more attractive. Strong, healthy trees also boost the value of a home or building lot, so preserving trees saves money that might otherwise be spent for landscaping. Given these benefits, it pays to put some effort into protecting what’s already there.
During excavation and grading, trees on the job site receive the worst treatment they will ever get. Specifically, it is the root system of trees that is most frequently injured by excavation equipment. The depth and spread of a tree’s root system depends on the underlying soil. Trees growing on sites with heavy clay soil or where ledge is close to the surface have shallow root systems that are especially susceptible to damage during construction. But even trees with deep roots in sandy soil or on sites with deep topsoil can be injured or destroyed when the soil grade is altered, when roots are severed, when tree trunks are skinned, or when soil is compacted. By simply using some forethought during design and construction, getting advice from the right experts, and carefully overseeing site work, you can prevent much of this damage.
Raising The Grade
Roots absorb oxygen, water, and nutrients — which the tree needs for growth and survival — from the surrounding soil. Raising the grade around a tree — even temporarily — can impede this absorption, effectively suffocating the roots (see "Suffocating Roots," below). The degree of damage depends on the type and depth of fill around the tree. A porous gravel or sand fill is less damaging than a more impervious clay fill. Even if the grade is not changed but an impervious layer is added, such as asphalt or concrete, the effect will be the same. (Planting a young tree in a predominantly paved area, however, such as in a city, is very different from paving around an existing tree, since the new tree’s growing roots will adapt to the conditions as necessary.)
Many people build small wells around trees to prevent injury when raising the grade, but this seldom succeeds. Most of the root system is still prone to suffocation by the elevated grade because the roots extend well beyond the trunk and usually even beyond the “drip line” — the outermost edge of the tree’s branches, or “crown.” To be successful, especially in clay soil, a well needs to extend at least out to the drip line (see "Sizing Tree Wells," below). If the grade must be changed and important trees need to be protected, a skilled arborist might be able to design a system of gravel fill, landscape fabric, and drainage tile to aerate the root system. But because such measures are expensive and cannot guarantee success, they are usually considered only as a last resort.
Injury From Surface Grading
When bulldozers or other heavy equipment nick or skin the trunk of a tree, the wound is obvious and highly visible. This kind of damage opens the tree to attack from insects, fungi, or disease. If skinning does take place, it can be treated by coating the wounded area with a compound available at a garden supply store. But skinning may not be as serious as the more hidden damage that grading does to roots.
Scraping topsoil. Woody plants, like trees and shrubs, have most of their roots in the top 18 inches of soil, with a large number of feeder roots located within the top 6 inches. Roots adapt to the ground they are growing in. On hilly ground, for example, a tree’s roots will grow along just beneath the surface, rising and falling with the topography, with feeder roots extending almost to the surface. Consequently, even shallow scraping of topsoil around trees can cause considerable damage. Severing large roots close to the trunk is more damaging than severing smaller roots near the drip line, but both injure the tree. If you want to stockpile topsoil from the site for use in landscaping around the finished house, limit the scraping to the immediate excavation site.
Lawns. Most finish grading is done in preparation for planting lawns, but lawns can compound the injury to trees. Grasses are more successful competitors for nutrients than trees, especially in poor soils. In fact, some grass roots may extend even deeper than the feeder roots of trees. When you scrape off the topsoil around a tree in preparation for a lawn, you also remove many of the tree’s feeder roots, increasing the odds that the grass you plant there will win in the competition for nutrients and water. Obviously, this can further damage the tree and threaten its survival.
Trenching And Compaction
Digging a trench near a tree is often the most damaging site work of all, since it severs all the roots that intersect the trench (see "Trenching and Tunneling Near Trees," below). How much injury trenching causes depends on the depth of the trench and its distance from the tree. Obviously, a trench right next to the trunk will do a lot more damage than a trench out near the drip line.
There are two ways to avoid damage from trenching: Relocate the trench, or tunnel under tree roots. Keep in mind that the extra cost of a longer trench or tunneling will usually be recovered through the direct economic value of the trees you save, either in reduced landscaping costs or resale value of the house. If your usual approach for a buried electric or gas line, for example, is to keep the run as short as possible to save money in materials and trenching, consider digging the trench around valuable trees. Tunneling under tree roots is more difficult and time consuming,but you can use a power soil auger to speed things up.
When roots are accidentally severed during trenching or tunneling, cut the ends cleanly with a saw or clippers. Then refill the trench as soon as possible to keep the surrounding soil from drying out and further damaging the remaining roots.
Soil compaction. Both vehicle and foot traffic can compact soil, reducing its ability to absorb surface water and to provide oxygen to tree roots. When growing tree roots encounter compacted soil, they often turn away just as they will from barriers such as boulders or a foundation wall. Compaction is likely to be most severe in heavy clay soils because they are poorly drained and poorly aerated.
First Aid
Determining the exact cause of tree damage is often extremely difficult. The symptoms from different types of injury are the same, so you can usually determine the cause only by figuring out what type of site work was done. But if you can discover the cause, you can take corrective measures to improve the tree’s chance of survival.
When roots are injured or severed, estimate the extent of the damage. If 20% of the root system is damaged, for example, thin the crown to compensate. Otherwise, the crown will require more water and nutrients than the roots can provide, a condition known as rootshoot imbalance, which can result in dieback.
You can also apply phosphate fertilizers, which aid root development, although this should be done by a professional arborist. Avoid fertilizers high in nitrogen until a few years after root damage has occurred to prevent unbalanced shoot development.
Finally, mulch the area under the drip line to reduce competition from grasses for nutrients and water, and to improve oxygen absorption. If the soil has been compacted, aerate it to a depth of about 8 inches with a crowbar or needle-type soil aerator.
All injured trees should be watered regularly during dry periods to compensate for reduced water uptake by the roots.
