Everyone knows that trees add value to a landscape. A mature specimen tree, twenty years old or more, can have an appraised value of between $1,000 and $10,000, according to the American Nursery & Landscape Association. In addition, well-placed trees can save substantial dollars on energy costs ? as much as $200 a year under ?best-case? scenarios.
There are a tremendous number of variables in tree appraisal. However, like antiques, condition is critical. A hazardous tree can be an outright liability. So, it pays to take care of mature trees.
During construction pro-jects, spending a few dollars and taking a little extra time can result in actual cash value. However, effective tree protection practices are often the exception rather than the rule.
?Tree protection is one of the biggest boondoggles ever,? says Dennis Swartzell, marketing manager at Mountain States Wholesale Nursery in Las Vegas, Nevada. For more than twenty years, Swartzell was arboretum director at the University of Nevada, Las Vegas, and watched the 337-acre facility grow from open park-like land to a bustling campus with facilities for 24,000 students. In other words, the grounds were under constant construction, which often conflicted with the existing trees.
?No one thinks about tree protection, especially the contractor,? says Swartzell. ?At the completion of the projects everyone is shrugging their shoulders and pointing fingers. The trees end up being the losers.? In most urban areas, preserving mature landscape trees during renovations and upgrades is the primary concern. However, new construction is also a large concern in suburban and even rural areas of the nation. As cities sprawl over the countryside, countless trees are demolished to make way for housing tracts. In many cases, with a little forethought, the homes in these developments could have an ?instant? landscape feature that would add value and provide energy savings from day one.
Whether you are looking at specimen trees in an existing landscape or in a new construction setting, most of the same rules apply. Protecting trees is a process. If you follow a few simple steps, you should successfully save your most prized specimens. If you are not trained to work with trees, look into hiring a certified arborist or a consulting arborist who specializes in tree protection. Although the up-front cost may seem high, it will pay off handsomely in the long run.
First of all, you need to determine if a tree is worth saving in the first place. Although this may seem obvious, trees with structural problems or are at the end of their life cycle may look fine to the untrained eye, but could easily become hazardous when subjected to the stresses of construction.
As a general rule, the younger the tree, the better chance that it will adapt to its new environment. Some tree species also tolerate construction stresses better than others do. If you are working with Ulmus spp. (elm), Populus (poplars and cottonwoods, but not aspens), Salix spp. (most species of willow), Platanus (sycamores) and Gleditsia and Robinia spp. (locust), chances are good that the tree will survive and thrive. However, Fagus spp. (beech), Liriodendron tulipifera (yellow poplar, which is really not a poplar at all, but actually in the Magnolia family), Carya spp. (hickory) and Betula spp. (birch) present a greater challenge. Oak trees can range from fairly hardy (white oak) to susceptible to any damage (scarlet oak). Most maples would prefer to remain undisturbed; the sugar maple is probably one of the most sensitive, while the silver maple can be the toughest (although many arborists would argue for any silver maples? removal!). In general, conifers should be well protected.
If you are working on a large site with many trees, the first thing you need to do is take an inventory. Look for trees that are damaged or decayed, leaning, or show excessive insect or disease damage, and slate them for removal. Trees that are removed should be cut down flush to the soil, and the stumps should be ground out with a stump cutter. Using a backhoe or large equipment to rip trees out of the soil is not recommended, because the resulting soil compaction can impact the remaining trees.
Speaking of soil compaction, that is probably the biggest problem during construction activities. Traffic from equipment or people can literally squeeze the life out of a soil. The pore space, where life-giving air and water resides, vanishes when the soil is compacted. Since this is where plant roots forage for nutrients, decline and death are sure to follow.
So, once you have determined which trees you want to keep, it?s essential that the soil beneath them be kept free of machinery, equipment and even foot traffic, if possible. This also goes for solitary trees in existing landscapes that may be victims of building, remodeling or landscape retrofits, such as patios or pools.
Traditionally, the ?drip line,? the area from the edge of the canopy to the trunk, has been suggested as the root protection area. But more is better. ?Protect as much of the area beyond the tree?s drip line as possible,? advises Gary Johnson, University of Minnesota Cooperative Extension. ?Some healthy trees survive after losing half of their roots. However, other species are extremely sensitive to root damage even outside the drip line.?
Johnson advises calculating the ?critical root radius,? especially for trees that are growing in groups or that have a narrow canopy. Measure the tree?s diameter at dbh. (This stands for ?diameter at breast height? but is generally taken about 4.5 feet above the ground. You can divide the circumference by three to get a working number.). Measure in inches. For each inch, allow for 1 to 1.5 feet of critical root radius. If a tree?s dbh is 10 inches, its critical root radius is 10 to 15 feet.
Since these trees are about to face the greatest stresses of their lifetime, do everything you can to ?ease the pain.? Be sure they are in the best health possible. Prune out any dead wood, but do not prune heavily. Be sure that soil moisture is adequate; irrigate if necessary. You might even want to lightly fertilize if any nutrient deficiencies are noted. If the tree has any minor insect or disease problems, they should be treated.
Although orange snow fencing is often used to delineate the root protection zone, most consulting arborists suggest more secure methods. ?While fencing of this type can be very useful to delineate protected areas if properly placed, they are easy to breech,? says Dennis Brown, consulting arborist and owner of Urban Forestry Resources in Austin, Texas. ?When they are run over or moved to accommodate activities on the site, the result is loss of control of the site and poor tree protection.? Sturdy wood fences or chain link fencing are generally better choices, especially on busy construction sites.
In spite of your best efforts, chances are there will be some traffic in the root protection zone. To further protect the tree, a thick layer (about six inches deep, if possible) of organic mulch is highly recommended. Wood chips work well and are inexpensive or free in most places. You can also use gravel to a depth of about four inches if it is more readily available.
By fencing off the critical root radius and mulching, you should be able to minimize both soil compaction and physical damage to the tree, two of the biggest problems during construction. Grade changes can also have a serious impact and should be avoided. The ?root flare? of a tree is actually a misleading term. This area between the trunk and the soil line is actually more closely related to trunk tissue than root tissue. In other words, it cannot live beneath the soil?s surface. Piling soil up at the root flare, or literally burying this trunk tissue, will lead to slow decline and eventual death as the tissue dies and decays.
Chemical (paint, solvent, etc.) and concrete spills and debris disposal are also common problems on construction sites. Any spills should be cleaned up as quickly as possible. Although this may seem like common sense, most of the time spills are ignored by the construction crews. These can lead to long-term damage long after the project is completed. If the chemical is toxic, soil removal and replacement may even be warranted.
Paving over tree roots should be avoided, but sometimes a project might demand it. If at all possible, use porous pavers laid over a sand/gravel base. Using this technique at the existing grade causes the least amount of damage. If concrete or asphalt must be used, simply smoothing and tamping the existing grade in preparation is obviously the best approach.
Realistically, though, sometimes a tree?s roots will need to be disturbed or removed. Some trees can survive and even thrive with as much as one-third root loss; again this depends on the species and condition/age of the tree. If root removal is necessary, use the same techniques as you would in fine pruning. In other words, make clean cuts. Ragged cuts are an open invitation for decay-causing microorganisms.
Sometimes, trenching will be called for. If possible, you should remove soil by hand to minimize damage. Install irrigation lines or wiring below large tree roots. Mechanical trenchers should be avoided. The teeth cause ragged cuts that can lead to decay and decline. There are soil removal systems that use water or compressed air that can be used for trenching, and cause minimal root damage.
Now that the project is finished, are you ?out of the woods? yet? Not necessarily. The tree should be monitored carefully. If any limbs have been damaged, they should be pruned carefully. Again, avoid any unnecessary pruning. Be sure to carefully monitor soil moisture.
Jeff Iles, professor at Iowa State University, suggests that trees receive at least one inch of water per week over the entire root zone during the growing season after a construction project. Leaving that layer of organic mulch, and even renewing it, will keep the soil friable and moist.
Iles warns that large amounts of nitrogen or phosphorus can increase salt levels in the soil and discourage root growth. Fertilize only if nutrient deficiencies are detected. Nitrogen can cause flushes of leaf growth at the expense of the roots, further stressing the tree.
Since more and more communities are recognizing the contributions trees make to the environment, tree protection ordinances are becoming the norm. Although there is no magic formula that can guarantee a tree?s survival, doing a little research and carefully monitoring the construction site will give you the edge when protecting these valuable resources.