Reaping the Benefits of Existing Tree Cover
It’s a paradox in residential construction: Developers clear a wooded lot to create housing, and one of the first things the new owners do is buy small trees to plant on the property.
There are reasons for this, of course. It’s much easier to build when there’s a blank slate to begin with and crews don’t have to dodge large trees here and there. Perhaps developers tried to preserve large trees in the past only to watch those trees succumb to the compacted soil and trunk injury that’s par for the course on a construction site.
But the fact remains that people love large trees. They hold romantic notions of lounging on the lawn in the shade of a giant oak, of building tire swings and playing hide-and-seek with their children. They marvel at the tree-lined streets in historic neighborhoods. So for those builders who can manage to keep a handful of specimen trees around, the rewards are there to reap.
Mature Tree Values
The curb appeal of properly maintained landscapes, especially those with mature trees, has long been recognized and documented. Over 50% of prospective home buyers admit that proximity to natural areas and parks will influence their buying decisions, according to a survey conducted by the National Association of Home Builders (Quint, 2013).
A landmark publication released by the University of Georgia Cooperative Extension in 1996 lists the many values that trees can contribute to homeowners, neighborhoods and society in general. This exhaustive list cites psychological and monetary, as well as aesthetic, benefits. Trees increase property values, reduce noise pollution, and lower summer cooling costs by as much as 27%, and increase positive interactions with neighbors (Coder, 1996).
As for saving energy, the U.S. Department of Agriculture (USDA) compares the net cooling produced by one well-situated tree to that produced by ten small air conditioner units running for 20 hours a day during the cooling season (USDA Forest Service, 2004).
It is difficult to place an economic value on individual trees, because there are many variables to be considered. But it’s fairly easy to see that an 8-foot, $100 tree planted on a site is worth far less than an existing mature tree. The latter may be appraised at anywhere between $1,000 and $10,000, according to the Council of Tree and Landscape Appraisers (CTLA, 2000).
And while many resources look at what a single tree can add to a property’s value, a whole forest—even a small one—is a valuable asset to communities as well. The USDA, for instance, says that a 1-acre forest captures 6 tons of CO2 annually and in turn generates enough oxygen to keep 18 people alive (USDA, 2013).
So once both clients and builders recognize the value of leaving some mature trees and small woodlots behind, the next question is, how?
Unfortunately, as many builders and remodelers already know, it isn’t as simple as piling a few straw bales around the trunks to keep equipment away. Trees are terribly sensitive to soil compaction wrought by heavy equipment, stacks of material, and piles of excavated material—spoils—and in many cases, a grade change can kill a tree. Each tree preserved will need special attention to ensure that it remains healthy throughout the construction process, and for that reason, only the best and healthiest trees should be selected for preservation.
When it comes to choosing which trees to preserve, it’s important to involve all stakeholders. What does the client want? What’s practical for the construction crews? Which trees will contribute to the long-term value of the development? Are there trees that simply aren’t worth preserving?
These questions must be addressed well before excavation begins, and before any heavy equipment is moved on the site.
An arborist or consulting forester should be involved in identifying the trees that should be preserved, because not all trees respond to construction pressures in the same way. Colorado State University Extension and University of Minnesota Extension have both published tables that rank trees by how well they adapt to environmental changes. White and green ash, American elm, hackberry, and willows all adapt well to these pressures, for instance. Northern red oak, Colorado blue spruce, and black locust don’t adapt well at all. Falling in between are white oak, black walnut, and pine species (Dennis & Jacobi, 1999) (Miller, Rathke & Johnson, 1999).
It is also important to consider epicormic branching. This is a shoot growing from a bud that lies beneath the bark of a trunk, stem, or branch of a tree. It is a natural response that can detract from the attractiveness of such species as tulip poplar trees and white oaks if too much sunlight touches their trunks after adjacent trees are cleared.
It is imperative for a professional to factor in all these variables—along with the needs and wants of the ultimate property owners—during the tree selection process.
Generally speaking, a mature tree doesn’t handle stress as well as an actively growing one. In some cases, therefore, it might be better to preserve trees that are 8 inches in diameter or less, rather than their full-grown counterparts. But not in every case.
Finally, it is important to choose healthy trees, as I said above. Here again, a forester or arborist can detect signs of canopy decline or borer evidence in trees early on. Construction injury to a tree that is already unhealthy could prove to be the proverbial straw that breaks the camel’s back.
Protecting Soil and Roots
When protecting mature trees, it is important to note that
approximately 90% of a tree’s roots lie within the top 6inches of topsoil; and
the roots extend well beyond the tree’s dripline.
Identified Benefits of Community Trees and Forests. University of Georgia Cooperative Extension, 1996.
Colorado State University Extension and University of Minnesota Extension have both published tables that rank trees by how well they adapt to environmental changes.
Coder, K. D. Identified benefits of community trees and forests. Athens, GA: University of Georgia School of Forest Resources, 1996.
Council of Tree & Landscape Appraisers. Guide for Plant Appraisal (9th ed.). Savoy, IL: International Society of Arboriculture, 2000.
Dennis, C., & Jacobi, W. R. Protecting trees during construction (PDF). Fort Collins: Colorado State University, 1999. .
Miller, N. L., Rathke, D. M., & Johnson, G. R. Protecting trees from construction damage: A homeowner's guide(PDF). Minneapolis, MN: University of Minnesota, 1999.
Quint, R. What home buyers really want. Washington, DC: National Association of Home Builders, 2013.
USDA Forest Service. The value of trees. Washington, DC: USDA Forest Service, 2004.
USDA. Longleaf pine initiative. Washington, DC: USDA Natural Resources Conservation Service, 2013.
This being so, it is obvious that anything parked or piled up to several feet away from a tree’s trunk will inflict damage to feeder roots. In order to maximize a mature tree’s chances for survival, a visible barricade fence should be erected around the tree’s critical root radius, and signs in both English and Spanish should be attached every 20 feet along the fence. To calculate the critical root radius, measure the tree’s diameter in inches at 4.5 feet above the ground and multiply the result by 1.5. For example, suppose that an ash 16 inches in diameter is identified as a tree to be preserved. A fence with a radius of 24 feet (or 16 X 1.5) should be established around this tree.
This illustrates why it’s important to select the trees to preserve early in the construction process. It may not be feasible to protect all the trees on the site and still have reasonable access to the site, so developers and owners should choose their battles carefully.
Before the fences are erected, it’s a good idea to mulch the critical root radius with 4–6 inches of wood chips. This mulch layer will absorb some of the possible impact to the root zone and will also preserve soil moisture around the tree.
It is sometimes also a good idea to subcontract with a certified arborist who can deal with the tree preservation aspect of the project. Many professional tree care companies specialize in this field, and have crews who do tree preservation exclusively.
It should go without saying that in addition to protecting the root zone, construction crews must avoid damaging trunks and limbs with cranes, excavators, or other machinery. Just as with human skin, the slightest wound in a tree’s bark can open it up to a host of potential pathogens and insect problems.
Property owners today are savvy about their homes’ outdoor environments. Large, healthy trees aren’t just about charm anymore. Rather, today’s homeowners are likely aware of at least some of the ways in which trees can lower costs and enhance quality of life. Developers would do well to recognize this and build upon it.
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