GETTING TO THE ROOT OF TREE STRESS ALONG HIGHWAYS^©

Soil compaction has been identified as a major contributor to urban tree failure. In order to develop criteria to increase rates of survival of outplanted trees in roadside environments this study investigated the influence of bulk density as an indicator of soil compaction on tree morphology and physiology. In 2012, four # 10 container-grown tree species were planted into a total of 37 quadrats at two highway interchanges in the Niagara Region, Ontario, Canada. Four data collection cycles were conducted and measurements included: tree height, caliper as well as chlorophyll content of leaves, soil moisture tension and stomatal conductance. The soil texture was mainly comprised of fine particles (clay and fine silt). Average soil bulk density for Site 1 was 1.45 g·cm^-3 and was 1.55 g·cm^-3 for the 0-10 and 20-30 cm depth respectively. For Site 2, the average soil bulk density was 1.49 g·cm^-3 and 1.67 g·cm^-3 for the 0-10 and 20-30 cm depth respectively. The results suggest soil bulk density was consistently above root limiting levels at both sites for samples collected at the 20-30 cm depth. These findings illustrate the importance of developing root systems with shallow structural roots that are radially oriented around the trunk in the nursery for trees that will be outplanted into urban soils.