238 investigated using static pull tests and photogrammetry- derived computer models (Miesbauer et el.). Roots at mul- tiple depths in a 90° wedge on the side opposite of the pull direction were the most significant factor affecting tree stability. Detter et al. explored if, and to what extent, trees can regain anchoring strength after their root systems have been overloaded. London plane trees were subjected to destruc- tive winching tests until primary anchorage failure occurred. Retesting these trees over time revealed that par- tially uprooted trees may reestablish stability, but some will not and can fail. Silver maple (Acer saccharinum) and red maple (Acer rubrum) roots from interior and edge positions of a planta- tion were examined to observe cross-section radial growth patterning in response to wind (Grabosky). The loss of observed upward radial growth bias very closely coincided with the ending of the zone of rapid taper. Observations suggested a morphological response to wind exposure. Planting Site Design Urban landscapes frequently require trees to be planted in confined spaces. Work aimed at improving the quality of those spaces continues. Structural soils and suspended pavement systems are designed to support pavement while providing for root growth; these systems continue to evolve. Root densities of Accolade™ elms planted in , overlaid with porous or nonporous CU-Structural Soil© asphalt over a twelve-year period, were measured with Ground Penetrating Radar (Bassuk et al.). Root develop- ment was better under the porous asphalt. Shoot growth was reduced sooner in trees that grew under the nonporous asphalt. Smiley et al. directly compared supported pavement and structural growing media systems in two studies. Trees grow- ing in the supported pavement treatments with low-density soil media resulted in significantly greater tree growth and a healthier appearance. Tree Roots and the Built Environment Urban sites and tree roots are often in conflict. Activity around trees can damage trees if it encroaches too far into the root zone. Trees can damage infrastructure by heaving pave- ments, contributing to foundations subsiding, and invading pipes. Moore et al. investigated root penetration into PVC pipes in two experiments. Properly installed pipes were impene- trable, but the width and number of openings in a pipe influenced the capacity for penetration. Species had differ- ent capacities for penetrating pipes, and appropriate species selection could reduce incidences of pipe damage. Watson: Introduction to a Special Issue Tree Production, Planting, and Establishment Container systems remain a mainstay of the nursery indus- try, though natural, lateral root spread is very limited, and root defects are common. New bare root systems are being used to increase fine root density and improve planting success throughout the season. The Missouri Gravel Bed system (MGB) has been used to enhance fine root growth of bare root stock prior to planting. Fite et al. compared gravel beds with mulch beds and showed very few differences, suggesting that the less expensive, more readily available substrate of wood chip mulch should be considered in these aboveground growing systems. Managing Urban Soils Urban soils may be the least studied and understood of all soils. Compaction and lack of structure and organic matter are common problems. Though not represented in this spe- cial issue, the proceedings will include information on compost and biochar amendments to improve the soil, rebuilding the soil profile, and understanding how natural soils provide a root environment that is closer to nature where the trees evolved. Looking Forward The 2018 Landscape Below Ground IV conference punc- tuated 25 years of tree root and soil management research. The articles included in this special issue contribute to the growing body of knowledge on these topics and will undoubtedly be an important resource for practitioners, managers, and policymakers involved in tree care in cities. Despite considerable advances, the field of study has many knowledge gaps. Future research can build upon the last quarter century of research by addressing these and other gaps. Acknowledgements The 2018 Landscape Below Ground Conference is gener- ously supported by Platinum Sponsors the International Society of Arboriculture, Bartlett Tree Experts, The John and Eleanor Kuser Faculty Endowment for Urban and Community Forestry of Rutgers University, and the Arbo- ricultural Research and Education Academy, and Gold Sponsors J. Frank Schmidt Family Charitable Foundation and Davey Tree Expert Company. Gary Watson (corresponding author) The Morton Arboretum 4100 Illinois Route 53 Lisle, Illinois, U.S.A.
[email protected] ©2019 International Society of Arboriculture
November 2019
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