For the past year, the Tree and Soil Research Subcommittee of ASLA, comprised of Jim Urban, FASLA, Chair, José Almiñana, FASLA, Eric Kramer, ASLA, and Peg Staeheli, FASLA, has been working on ways to engage ASLA members in tree and soil research. This includes making the existing research more easily accessible to members, helping them to incorporate research into their own practice, and encouraging members to participate in new research efforts. To that end, the subcommittee, with the support of ASLA staff, is launching the Tree and Soil Research Blog to serve as a platform for discussions on tree and soil research. The site is intended to allow members to ask questions, post their own experiences related to research, develop community discussions and research-based best practices, and give researchers the opportunity to post their work for review and discussion.
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In 1998 Leslie Sauer Jones wrote The Once and Future Forest: A Guide to Forest Restoration Strategies. Embedded within the book’s forward, landscape architect Ian McHarg implored: “We must participate, with action and all the experience we can bring” in order to attempt to reverse environmental degradation and we can no longer expect our actions to be reversed with inaction. He further suggested that we embrace, “important havens, such as the interstices of cities” as critical canvasses for habitat enhancement and expansion for our native plants and animals.
Within our cities, large, contiguous tracts of vegetation, such as urban forests and riverfront corridors, offer critical ecological value potential. However, in more densely developed fragments of the city, where landscape design increasingly occurs, researchers are discovering that purposely selected woody plants can similarly provide animal species with viable urban habitat. Conceptualizing the ecological value of these urban interstices may be a function of perspective, or scale.
Soil structure (how soil particles are held together to form larger structures within the soil) is recognized as an important property of a healthy soil. Grading, tilling, soil compaction and screening soils during the soil processing and mixing process damages structure. Structure makes significant contributions to improving root, air and water movement thru the soil. Soil screening is extremely damaging to structure but is included in most soil specifications.
Why do we screen soils and what happens if we do not? Prior to the mid 1970’s soils were rarely screened and landscape plants performed quite well. Installed soil was moved with clumps or peds throughout the stockpile. In the last 15-20 years farmers who have stopped tilling their soil have found significant improvements in soil performance. Several new research projects suggest that elimination of the screening and tilling processes in favor of mixing techniques or soil fracturing that preserve clumps of residual soil structure may improve landscape soils.
Data, data, data.
We know that all good science is based on adequate data. And if you’re reading this page, you probably also already know there is a lack of adequate data when it comes to the real-world performance of urban tree planting soils. This post is your chance to change that and add your own information to a shared database of soil performance data.
A bit of background: In 2014 we (Eric Kramer, ASLA, and Stephanie Hsia of ReedHilderbrand; Robert Uhlig, ASLA, of Halvorson Design; Bryant Scharenbroch, PhD of University of Wisconsin – Stevens Point; and Kelby Fite, PhD of Bartlett Research Labs) undertook a micro-study of seven sites in Boston — constructed urban landscape projects anywhere from 5 to 45 years old. Each project took a pro-active approach to designed soil systems, using suspended pavements, Cornell University structural soils, or sand-based soils. We took soil cores, recorded soils horizons, took lab samples and compared findings to what we knew about what had been installed. We also assessed the performance of the trees over time.
Stem girdling roots, kinked roots, J roots, T roots, and root collars buried deeply in the root package are one of the principle reasons whey trees and large shrubs fail to recover from transplanting or decline and even die at a young age after planting. These problems are typically created in the nursery by practices that do not produce plants with radial root architecture and place the root collar close to the surface of the soil. As a plant moves thru the production process from propagation to delivery at the site, there are many opportunities for root problems to develop in the plant.
Most plants are started in small containers and then gradually moved into larger containers. If the plant is sold in a container there may be three or four different container sizes. Each of these containers may result in a series of roots circling around the edges of the pot forming circling roots. Any of the circling roots above the root color can eventually choke the tree. Other roots may be deflected from the bottom of the container and grow upward to the surface forming a sharp kink in a root that may eventually become an important structural root. If these misshapen roots are not pruned at each shift in pot size they form an imprint of constricting roots in the next container. As trees are repotted they are also often placed too deeply in the next pot. Trees lined in the field may also be buried in the soil. This places the roots too deep in the soil where oxygen is less available at a critical point in the trees development.
When high-intensity rainfall events roll through cities, particularly those with combined sewer systems, peak flows increasingly overwhelm grey infrastructure, compromise water quality, and induce sedimentation and erosion. New research suggests that engineered soil and purposely selected plants within green infrastructure may help offset these flows by offering more benefit than most stormwater engineering models and municipalities acknowledge.
A handful of progressive entities – like the Chesapeake Stormwater Network and the Commonwealth of Virginia – now award extra stormwater credit for management approaches that deploy high-performance engineered soils, dense and varied planting palettes, or an inter-connected series of green infrastructure elements. More research is needed, however, to mobilize engineers, designers, and policy makers to rely more heavily on the “green” in green infrastructure.