Finally, LEED Directly Encourages Trees
Does LEED encourage planting trees? It’s a strange question to be asking 23 years into the rating system. Sure, we know trees are good; they provide shade and wildlife habitats, stabilize soils, and create oxygen, but there is no LEED credit that simply encourages design teams to plant more trees—until now. The publishers of Pilot Credit 158, Assess and Increase Onsite Carbon Sequestration Through Plantings, seem to feel the point needs to be hammered home: PLANT MORE TREES (and shrubs)!
The mechanics of the credit are simple enough: establish a baseline measurement of carbon sequestering potential on the site, and then plant enough trees (and shrubs) to increase the carbon sequestration by 10%. There is also the somewhat ubiquitous language for those often using the Whole Building Life-Cycle Assessment option in LEED v4 and v4.1 that the baseline and proposed site “must be of comparable size, function, orientation, and climate zone.” Presumably the baseline and proposed cases are the same site, so this seems redundant; however, the subjective nature of the definition of the term comparable “function” will likely encourage project teams to use the “existing condition” option vs. the “initial proposed design” option.
At first glance, this seems like an easy credit for many projects. With the baseline defined as the “existing condition of the site pre-design,” for many projects starting with a cleared site or an urban infill site, or demolishing an existing structure, the number of trees and shrubs needed to hit the 10% threshold would be minimal. What could be interesting is how an urban site with zero lot lines might achieve this credit. It could encourage the design of more intensive green roofs with deeper planting beds, a strategy that will help meet stormwater management requirements to retain water onsite in many cities.
There are two tools referenced as acceptable for this pilot credit. The first one, i-Tree Planting from USDA Forest Service with collaboration partners, is the simpler of the two. With some basic location and tree information, you can build a group and get the total CO2 sequestered. The output is in pounds or kilograms, which you then have to divide by the area of the site to get the metric in the credit metric, pounds per square foot or kilograms per square meter. When comparing trees in the tool, not all are created equal. A small dogwood sequesters 5,717.2 of CO2 vs. the Ginko bilboa, known for its pollution tolerance, at 17,214.4 CO2 vs. a tulip tree clocking in at 21,998.1 CO2 sequestered. It seems unlikely landscape designers will select trees based purely on this performance factor, but maybe it will tip the scales if multiple species are viable options.
The second tool is Pathfinder by Climate Positive Design. This one is a bit more complicated; it is basically a life-cycle assessment tool for the landscape only. You have to build a baseline model, then a proposed case model to compare. The database includes paving, site furniture, and plantings. It’s a more complete picture but cumbersome to build the models initially.
Overall, this is an interesting pilot credit addition to the LEED library. The low threshold for achievement sends the signal that this credit is about more than just planting trees: it’s about modifying the mindset of design teams that the landscape is a resource.
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Sarah Buffaloe is an architect on WSP’s Built Ecology team. She has been working in the sustainable building industry for 14 years, including five years as USGBC staff during LEED v4 development. Her expertise is in LEED Materials and Resources as well as embodied carbon and whole-building life-cycle assessment.