Tree-Friendly Development: Building a Culture of Creativity
By Joe Thomas, Donor and Public Relations Coordinator
When it comes to the effects of development on the health of a tree, the most appropriate comparison may be an iceberg – what happens below the surface is sometimes more important that what happens above. This isn’t extremely well-known – it certainly didn’t occur to me until after I had worked at Trees Atlanta for a while.
What I’ve learned in that time, though, is that while tree removal is certainly the most obvious harm done to our tree canopy, failure to properly protect the underground area around the tree during construction can have an equally dramatic long-term effect. To me, that presents Atlantans with a challenge, but also an opportunity – with a little bit of education and willpower, we can continue to build density while ensuring that our urban canopy is healthy and thriving.
What is the critical root zone?
A quick vocab lesson here. To understand what is happening underground, we need to know a little about underground tree biology. The critical root zone (CRZ) is the land around a tree that houses that tree’s root system. It’s where the tree is stabilized, and where it draws water and nutrients from the soil.
Disturbing the land in the CRZ can have a devastating effect on the tree’s health, but CRZ protections are often overlooked since the damage isn’t as noticeable as damage to the trunk or branches. This damage can come in the form of actual physical harm to the roots, soil compaction from vehicle traffic and material storage, and either under- or over-supply of water as terrain and irrigation shifts.
Usually this happens when designers and builders don’t account for the impact of their work around trees. Even when the actual structure isn’t being built over the critical root zone, simply storing building materials or repeatedly driving over roots can compact soil and damage the tree. The damage may not be immediately evident, either, often showing up years later when it’s someone else’s problem.
What are some tree-friendly development techniques?
Trees Atlanta isn’t against development – we love that people want to live and grow in our city. With our blossoming community of architects and planners, we can meet the challenge of building density while preserving and even expanding our tree canopy, through creative and principled design.
The cool folks over at SmartCities Dive published some thoughts on tree-friendly design earlier this year. Here are a few of the methods they suggested for preserving canopy while developing land:
- Encourage structures that use post and pier construction or discontinuous footings to preserve structural roots that hold the tree up.
Single-slab foundations don’t allow for root systems to grow. By using post and pier or discontinuous footers, the building footprint is more flexible, allowing for more creative design.
- Create tree pits, tree islands, or root bridges to avoid drastic changes in grade around trees.
“If extensive grade changes are called for, trees will either be buried too deep or roots will be exposed or damaged. Root bridges can be used to carry pathways and sidewalks over tree roots. Trees can be kept at their existing grade with tree pits or raised islands (ed. note: these are landscaping tools that make sure the proper amount of soil remains around the CRZ despite changing topography) the size of the CRZ or as advised by a Consulting Arborist.”
- Specify low-impact landscaping around trees.
Plant small plants or seed within the CRZ to minimize root disturbance. Understory plants should be selected based on their ability to keep moisture and nutrients in the ground to feed the tree, rather than stealing those resources.
- Consider installation effects, even with “green” products.
“Pervious pavement and artificial lawns may seem like eco-friendly design elements, but their installation may still involve excavation and grading. Consult an arborist before including these elements within the CRZ.”
Why does this matter to Atlanta?
Now is a critical time in the growth of our city. As we continue to build infrastructure to support a rapidly-expanding population, we have to think intentionally about what is important to us – what values do we prioritize when decisions must be made about balancing the built environment with preservation of our existing, natural infrastructure?
Without prioritizing tree canopy, we risk losing our civic identity as the City in the Forest. The reality is that the overwhelming majority of our city’s trees are on private property, so it is up to owners and developers to preserve that identity. In order to do that, the design-build community needs the tools to make informed decisions not just about tree removal, but tree protection during construction as well.
Let’s build a civic culture where we reward forward-thinking, creative design, praising property-owners and builders who are conscious of trees in their work. We’ll preserve our canopy, and in the process build interesting spaces that can inspire other growing cities.