Below the Surface: Evaluating Urban Soil Performance Over Time

HONOR AWARD

RESEARCH

Boston, MA | Reed Hilderbrand LLC / Halvorson Design Partnership

PROJECT STATEMENT

Landscape architects are dedicated to planting trees in the city, but the contemporary city is not hospitable to healthy mature trees. In response, we rely on often-competing planting strategies and details. Rarely, however, do we analyze the relative outcomes of our choices of soils systems, pavement details, and management practices.

In response, two landscape architects, an arborist, and a soils scientist joined forces to undertake a comparative study — through fieldwork and laboratory testing — of plantings that have endured the stresses of an urban environment in Boston for between 5 and 45 years. We wanted to understand how the trees have grown and how the soils have performed. Our goal was not to crown one planting strategy superior to others, but to understand the dynamics of these complex systems, including what we can’t normally see below grade and to evaluate whether the results are worth the investment.

PROJECT NARRATIVE

Replace Conjecture with Data

We know that natural soils are dynamic and living systems. But in urban conditions, where soils are constructed and often installed below pavements, we don’t have an understanding of how the often novel and technically-advanced planting systems we deploy change and mature over time. This project aims to replace conjecture ⎯ often influenced by embedded biases of those who represent alternative planting systems ⎯ with data. We posed a question: Are designed soils meeting the performance standards we set in our specifications, and are they holding up over time?

Looking Broadly and Deeply

Approach: We selected seven Boston-area projects between 5 to 45 years in age that represent three different soils systems. In each case we gathered drawings and/or specifications to understand design intentions and had access to the site for testing.

Field Work: At each site we selected three trees representing different growing conditions and extracted up to one meter of the soil profile with a soil augur. We took weight measurements and samples at multiple depth horizons, producing a total of 79 soil samples. Samples were characterized in the lab according to texture, structure, density, and microbial biomass and respiration. To evaluate tree response we extracted tree cores and documented tree health using an accepted qualitative assessment methodology.

Findings: A reading of the data shows that urban soils, even under pavement, are dynamic over time. They change and mature, developing horizons as a native soil would. The research also supports or dispels a few common beliefs about soils under pavement: organic matter decreases over time because of lack of leaf litter (false), loams in CU soils migrate down the profile over time (true), tree growth rates tend to be slower when under pavement (true). Equally important, the data shows that post-construction maintenance of a landscape is a greater indicator on tree health than any specific soil condition.

Collaboration, Data and Ideas

This project was undertaken and is submitted jointly by landscape architects from two local firms and two practicing scientists — the kind of collaboration required to bring rigorous metrics to our work. The resulting data set was limited in scale but returned statistically significant distinctions that have enriched the context in which we make decisions about alternative soil systems as a way to promote longevity and robust health in urban trees and how we provide counsel to clients maintaining these systems. Our team also developed a set of new drawing types to illustrate baseline data and findings to a broad audience of scientists and landscape architects alike. We continue to engage the project and are looking to implement strategies to enlarge our data set through crowd-source data collection from a broader array of sites around the country. This is very much just the beginning

"This has been an ongoing discussion and heated debate because there’s still no 100 percent right way to work with the soil."

- 2015 Awards Jury

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