Leaf Litter

Working in the Shade: Achieving Sustainable Canopy Cover During Construction

Kevin Heatley of Biohabitats ISM advises – and muses – on the protection of our leafy kin during construction.

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The older I get the more I identify with the sedentary lifestyle practiced by our most graciously sheltering of biotic associates – the trees.  I also find myself empathizing with their notorious intolerance of construction and site disturbance.  I freely admit to being perplexed by the bizarre compulsion of our species to continuously modify and “improve” the landscape. Better to subjugate the land and its denizens in an attempt to pay homage to John Locke’s.

Unfortunately, in the pursuit of this quest we inadvertently lose our woody brethren and the very real ecological and economic benefits they provide. Given that trees are keystone species in many ecosystems and can take decades to achieve maturity and maximize their function in the landscape, the loss of existing tree cover at a project site is, in effect, the destructive liquidation of an accrued natural capitol asset. As an example, Urban Ecosystem Analyses performed by American Forests in several major metropolitan areas have documented an average 30% decline in tree cover over the last several decades.

“How can we stop this madness?” scream the collective voices of Leaf Litter readers. Well, apart from a mass chaining of the readership to threatened plants (a project initiative that is unlikely to receive federal funding under the current administration), there are simple strategies that can be employed to help assure the retention of a mature tree canopy during any construction project; be it a site restoration, residential development, campus expansion, utility placement, or commercial installation. The limiting factor is not the arboricultural tools available for protection but rather the recognition of the value of the tree cover and an understanding of tree biology.

The critical first step lies in recognizing what the focus of our efforts should be.  What are we trying to protect when we approach a stand of mature trees? Individual relic specimens or a functioning ecosystem? Our ultimate goal will impact our retention methodology. Frequently, an easily measured parameter is used as a default for intelligent tree and forest conservation. For instance, it is common for land managers to focus on preserving DBH (diameter at breast height) by targeting “historic” and “significant” trees. However, unless we anticipate munching on wood fiber or harvesting firewood, DBH may not directly correlate to the ecological and economic benefits we seek to preserve. This type of focus upon individual tree retention can fragment the overall canopy, destroy structural and age class diversity, incur excessive project costs, reduce tree survival, and ultimately, destroy the possibility of sustainable regeneration of the forest system. We are left with a skewed population of older plants, a population that is inherently less tolerant of the site modifications infrastructure installation entails. The forest is lost and in its place is a homogenized, energy-intensive landscape of turf and trees. Retention actions need to take this structural dynamic into account.

Biohabitats supports the use of a “sustainable canopy” model during tree retention on our projects. This model recognizes that the protection of a healthy level of tree canopy on a site will, by default, protect a greater proportion of the ecosystem functions and values we associate with an intact forest system. It is based upon restricting the percentage of canopy loss to a predetermined design threshold and preserving forest blocks as opposed to individual trees. Successful implementation of this model requires a systematic, broad-based approach involving:

  • Pre-Disturbance Resource Assessment
  • Design Constraints & Construction Modification
  • Education & Monitoring of Contractors
  • Enforcement


Each one of these elements is important to achieving our goal of tree and forest conservation. First off we need to know what is on the site. As a famous man once said, “All trees are not created equal”. (OK, so he is not quite famous yet but somewhat notorious). For instance, if our site is dominated by Norway Maple (Acer platanoidies) for the good of North America, we may want to prune each tree at the base.

Following our site assessment we will need to integrate the biological constraints imposed by tree physiology into the planning and design process. While the visual, above-ground portion of a trees system (stem and roots) tends to be of high concern during a site modification project, it is the subterranean portion (the roots) that is of primary importance. Damage to the root system, be it in the form of mechanical severing, chemical destruction, or smothering from compaction or excessive grade change, will result in a reduction in tree vigor. Ultimately, this damage can result in tree loss, either directly or from secondary stress factors such as drought or pathogen activity. Unfortunately, the process of tree loss may take a period of several years. This temporal separation can mask causality and, in the case of contractor inflicted tree injury, complicate the collection of damages.

Biohabitats recommends strict adherence to the concept of the CRITICAL ROOT ZONE (CRZ) during any site disturbance. The CRZ can be defined as the area where the vast majority of a tree’s fine root system is located. These roots tend to be within the top eight to twelve inches of soil, an area rich in oxygen and nutrients. The critical root zone is also frequently referred to as the Tree Protection Zone; however, it is our opinion that referencing the roots in labeling this zone helps to draw attention to the object of primary concern. Although there are several methods of computing this rooting area, the Trunk Diameter Method has been shown to be extremely effective at encompassing the vital root system. It is ordinarily estimated as 1 to 1.5 feet of radii from the trunk for each inch of DBH.

Modifications to standard construction activities such as trenching, grading, material storage, and the use of power equipment within the CRZ are required in order to protect this sensitive area. Any activity that results in compaction and/or disturbs the oxygen level of the soil is prohibited. Alternative strategies such as directional tunneling (as opposed to trenching) and the use of an approved temporary construction mat (to disperse vehicle weight) allow for access to these areas while minimizing the adverse impact upon desirable vegetation.

Of course, while there may be a general consensus at the design table as to the need to preserve trees, a different story frequently unfolds when the backhoe scoop hits the dirt. I have personally witnessed, and still bear the emotional scars, of several instances where thousands of dollars of design work and tens of thousands of dollars worth of tree canopy were wasted in the space of one afternoon. If there is inadequate communication to the general contractor or sub-contractor as to the importance of respecting the CRZ you might as well slash and burn the site ahead of time. The importance of contractor communication and continual on-site monitoring of performance can not be overemphasized. Relying on blaze orange plastic mesh fence to do the job is an exercise in futility.

A system of incentives and penalties needs to be agreed upon and contractually assigned at the start of a project, should, despite your best efforts as a dedicated champion of phyto-rights, negligent damage occur to the vegetation from the actions of the contractor. As the arboricultural tools available to remediate construction damage are limited and, for the most part, not scientifically validated as to their efficacy, following the classical mantra of “an ounce of prevention is worth a pound of cure” is a prudent strategy.

To borrow some imagery from The Lord of the Rings ...until the day comes when the mighty ENTS (giant trees) march out of the forest and storm the gates of Isengard it will be left to those of us on the front lines of sprawl to apply our botanical and regenerative design skills to protecting our leafy kin. The weapons are simple but the battle is long.

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