Keeping Room for Urban Trees

by Peg Staeheli, MIG|SvR

Urban tree canopy is a hot topic these days given its documented benefits to human health and ability to help combat climate change (USDA; ADF) and many cities have a long history of tree planting campaigns. However, in order to reap the benefits for health and habitat, we need to continue to help urban trees thrive long after they’re planted. How can we give back to our trees? Let’s consider how we might improve conditions by keeping them from harm and providing nourishment.

Improving Conditions – Do No Harm

“First, do no harm” (primum non nocere) – we have all heard this common phrase from the Hippocratic oath. In brief, it is a principle that says whatever the intervention or procedure, keep the wellbeing of the person as the primary consideration. Let’s apply this to evaluating preservation and protection of urban trees as we “intervene” to build housing, replace utilities, improve our store fronts, and repair our various paths of travel. We are often asked about the best tools and techniques to protect trees during construction and improve trees’ ability to thrive. We believe that there are three tiers of tree practice: caring, implementing and continuing.

Tier 1 is caring. The project team, including the owner, designers, and the review agency, all need to understand and believe in the value of saving trees. This results in caring not only about the trees but also the process required to save trees during design and construction. Depending on location, site development rules may need to allow for adjustments from standards to create room for viable root zones and branching structures, while also providing means to achieve allowable floor area. Many cities and regional jurisdictions have incentives or grant stormwater credit for tree retention (WA State DOE or Washington DC for example). Once an approach is approved, it is critical that the caring attitude be extended through to the construction team.

Tier 2 is implementation. Generally, the best strategy is to fence off the critical root zone to protect the tree volume from the roots through the canopy. In tight urban sites however, allowing the full protection is often not an option. If this is the case then have the specific tree condition and the planned construction details reviewed by an ISA-certified arborist with construction experience.

Together with the design and construction teams, appropriate practices can be determined. Practices could include root pruning, air spading, vertical mulching, soil enhancements, adjacent soils cells, and branch structure or pruning modifications. It is best if the protection and enhancement methods are employed at the start of construction to allow the tree to adjust to the changed conditions and then monitored to determine if additional interventions such as biological or chemical amendments would be useful.

Finally, as part of implementation, signage should be placed on the tree fence indicating protection status, value of tree, fine for damage, and a number to call if disturbance is observed. Many cities have specific language required for signs during construction.

Tier 3 is continuing. Continuing maintenance of the tree protection fencing, nourishing the tree, and observing the tree during construction is critical. Everyone that has practiced tree protection has had the experience of working diligently to preserve a tree only to have one mishap stemming from equipment, storage leak, human error, or drought take a tree out. In our experience, the key areas of continuing tree protection are water, fence repair, and materials storage.

Nourishing the Tree

All too often it is believed that mature trees don’t need supplemental water, however, under the added duress of construction, the root zones of these trees do need surface compost and water. Maintaining the tree protection fencing is a daily task as humans often nudge the fence over just a tad and forget to put it back. Storing materials and equipment right up to the fence edge is also common yet harmful. Heavy materials compact the adjacent soils, reducing opportunities for growth. Other materials may spill out, creating soil plugging or contamination. Equipment storage can result in fuel leaks, air pollution, and inadvertent backing into the tree zones. All these mistakes may result in tree loss.

Evolving Practice

The good news is that the design and construction industries are learning how to save trees during design and how to protect them through construction. Most permitting agencies are catching on that inspecting sites for tree protection results in both improved outcomes for the trees and a positive response from the community. Improving conditions for existing trees along our streets and plazas is still an evolving practice.

Stay Tuned

Tree pit design — the space and conditions for planting trees in hardscape zones — is improving. However, there are many trees barely surviving in constrained tree pits and within grated surfaces. Retrofitting tree pits to improve conditions for these trees and allowing them to grow past their typical 15-year urban life requires additional analysis, tools, and techniques that will be discussed in our next blog post.



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