Greening the highways is a series of mass out-plantings, studying long-term survival and growth of transplanted deciduous trees in urban highway right-of-way (ROW) environments in relation to species, site and production techniques. This alternative landscape often appears underutilized and stressful; however, may contain valuable space for building the urban forest and desired green infrastructure, which is often limited within built urban contexts. The research conducted is the first of its kind in North America, and is located in Ontario, Canada, and Ohio, United States.
Studies conducted in Ohio looked at survival rates of Acer rubrum (Acer), Betula jacquemontii (Betula), Celtis occidentalis (Celtis), and Syringa reticulata (Syringa) and caliper and height growth of Celtis and Syringa. Ohio studies focused on understanding selected biological, chemical and physical soil property differences occurring between two sites and survival and growth in relation to two production techniques and physical soil properties. Production techniques included the addition of a hydrophilic polymer, Geohumus® at 0, 0.5, 1, and 2% by container volume, and three growing environments; outside on a gravel pad, and in a flat and a peak retractable roof greenhouse (RRG). In Ontario at four ROW sites (Sites 1, 2, 5, and 6) species and site differences were evaluated for survival of nine species; Acer xfreemanii `Autumn Blaze’ (AFA), Acer pseudoplatanus (AP), Betula lenta (BL), Betula papyrifera (BP), Celtis occidentalis (CO), Gingko biloba (GB), Gleditsia triacanthos (GT), Quercus coccinea (QC), and Quercus robur (QRO), and caliper and height growth for four species; CO, GT, QC, and QRO.
In Ohio, site 2 had 32.9% greater mean predicted probability of survival (PPS) than site 1, and Celtis and Syringa having higher PPS compared with Acer and Betula. Production environment did affect height growth prior to installation into the ROW; however, not after. Syringa caliper was positively effected by 0.5% Geohumus® following installation; however, not before installation. At site 1 a significant decrease in PPS occurred after installation with a combination of 1% Geohumus® and production in a Peak RRG, and no other significant Geohumus® effect occurred. Soil properties at the sites in Ohio varied by site and depth. Greater correlations were found between physical soil properties and Celtis and Syringa survival rates compared with Acer and Betula. Survival at site 1 increased with increased bulk densities, whereas no correlations for survival were seen at site 2 and is thought to be related to soil texture and water holding capacity. All species had significant survival correlations with sand silt and clay, likewise caliper and height growth of Celtis and Syringa caliper were effected by soil particle size and gravel content. In Ontario sites 5 and 6 had greater survival compared to sites 1 and 2; however, sites 2 and 5 had better growth than 1 and 6. GT, CO, and QRO had greater than 50%, and BL had less than 50% PPS in all four sites other species varied by site. Above and below ground micro-site conditions and installation size may offer reasoning for differences between species and sites.