• By Stephanie Free
• July 20, 2020

Updating NCPC's Tree Replacement Policy

Balancing the Growth of People, Places, & Trees
Trees are a valuable resource for a variety of reasons. Their benefits range from environmental to visual, and at the end of the day no one ever says, “I wish we had less trees.” So why does it seem that trees are so easily removed when a new development is built? In many cases, especially on federal land in the National Capital Region, development seeks multiple alternatives to plan and design around existing trees so that as few trees as possible are disturbed. Yet in some cases, the only way to produce a viable project is to eliminate the trees that have staked their claim on the parcel in question. What happens then? Does the 10-inch diameter oak tree get replaced with another 10-inch diameter oak tree in another location? Can this oak tree be moved to another site?

Replacing Trees for Long-Term Success
These types of questions are regularly considered by planners, landscape architects, engineers, and arborists when trees are impacted by development. It is through the planning and design process that these questions are answered but sometimes the results are not successfully understood. The reality is that tree replacement is tricky. There is not a one-size-fits-all approach because trees are unique and so are the communities that they live in. For example, the trees in an urban triangle-shaped park downtown have different soil, sun, nutrient, and water requirements than a one-acre forest, or trees located in a wetland buffer. While the removal of trees in any environment should always be carefully considered, the methods to replace them will vary greatly.

Let’s look at the urban park scenario. There may be two or three healthy, 10-inch diameter trees in the park that provide a good amount of shade. If left undisturbed, an arborist’s evaluation may say that these trees would live for another 15 or 20 years. However, this park may be a prime location for a new memorial and the location of the existing trees conflicts with the memorial’s main features. After considering other design alternatives that would prevent removal of these trees, it is determined that the best way to achieve the memorial program and also meet other external requirements (e.g.; code, accessibility, etc.) is to remove the trees and plant new ones. In this case, and in many other situations, the trees planted in replacement are much smaller than the trees that were removed. Is replanting one or several small trees the right approach when removing a mature tree? Do several small trees provide the same benefits as one mature tree?

It is true that smaller trees are easier to transplant. However, ease of transport is not the main reason to plant smaller trees; they also will acclimate to the new site sooner than a larger tree would and will generally be healthier over their lifetime. In addition, the nursery industry typically grows trees to be sold in certain size intervals. For trees more than seven-feet in height at the time of planting (such as shade trees), the most common sizes found in nurseries are 1 to 2-inch caliper, 3 to 4-inch caliper, and 4 to 6-inch caliper. Once the trees reach these sizes, their roots are balled and burlapped^[1] so the tree is ready to be transported to the development site for planting. A tree over a 6-inch caliper may have more difficulty reestablishing itself at a new site, resulting in additional maintenance to help the tree succeed. Sometimes even with the additional maintenance larger trees may not fully recover from the initial “shock” from being transplanted.

Therefore, while it may seem that planting several smaller shade trees in replacement of one healthy, mature shade tree is not a fair trade-off, there are some notable short- and long-term benefits. At the outset, planting several smaller trees on the same site where the mature tree was removed can cumulatively replace the canopy area that the mature tree was providing. The long-term result will be a net increase in the canopy area on that site as those smaller trees mature. As noted above, since these smaller trees will acclimate more easily than a large tree would, the odds of achieving this long-term benefit are increased.

Transplanting Large Trees
This is not to say that transplanting trees larger than 6 inches is impossible. Whether the large tree is transplanted from a nursery specializing in growing large trees, another location on-site, or a location off-site, there are many factors to consider. The current and proposed location of the tree, as well as the soil conditions, available light, water, and the amount of the critical root zone that can be retained should all be considered. The critical root zone (or CRZ) is essentially a circle on the ground that corresponds to the dripline of the tree. It is usually 18 to 24-inches deep and contains the tree’s most important roots for absorbing water and nutrients. Retention of the CRZ is a significant factor in the tree’s establishment after it is transplanted. Another essential consideration for transplanting trees is aftercare maintenance. As discussed above, large trees have a harder time acclimating to a new site than smaller trees. Activities such as frequent watering, misting, mulching, and follow-up visits should be outlined in a maintenance manual prepared by an arborist or qualified tree care professional, and carefully followed to facilitate the tree’s ability to get reestablished in a new location.

New Comprehensive Plan Guidance and More Information
NCPC proposes updates to its tree replacement policies for federal development sites which are found in Section G of the Federal Environment Element in the Comprehensive Plan. The updates aim to bring the existing policies into alignment with current best practices. The Commission reviewed a draft update to the policies at its July 9, 2020 Commission meeting and voted to release a draft of the updates for public comment. The draft is available for public review and comment through September 14, 2020. You can read the text and proposed policies, provide comments, and learn more on NCPC’s Tree Replacement Policy initiative webpage.

Initiative Page