By Emily J. Jack-Scott, Yale School of Forestry and Environmental Studies
New Haven, CT (February 2012) – Urban street trees face adverse growing conditions: compacted soils, extreme heat, lack of nutrients, drought, car damage and vandalism. Limited funding, however, is cited by urban tree-planting organizations as their major obstacle. To maximize budgets, many organizations along the eastern United States have planted bare root trees as a less expensive alternative to balled-and-burlapped (B&B) trees. Existing research indicates equivalent survival rates between bare root and B&B trees, but no research has examined this in community group-planted urban street trees.
Bare root trees are additionally advantageous in community-based plantings because they are much lighter and easier for volunteers to handle. This study evaluated the influence of stock and other site factors on street tree survival and growth measures (diameter at breast height, percent canopy cover, and percent live crown), while controlling for species and age.
Site factors included street traffic intensity, site type (curbside, park, yard, or commercial corridor), wound presence, and sidewalk pit cut dimensions. 1159 trees (representing ten species) planted by Philadelphia community groups under the guidance of the Pennsylvania Horticultural Society from 2006-2009 were sampled.
Overall, trees showed a high survival rate of 95%, with no significant difference between B&B and bare root trees. Species with the highest survival rates were Prunus virginiana (chokecherry), Platanus x acerifolia (London plane tree), and Acer tataricum ssp. ginnala (Amur maple). Heavily trafficked streets exhibited lower survival, percent canopy cover and percent live crown.
Larger growth measures were expected and found in B&B trees, as they have historically been planted larger than their bare root counterparts. Findings support planting larger trees (such as B&B and/or larger bare root trees) along commercial corridors. Species in the Rosaceae family (Amelanchier spp., Malus spp, and Prunus virginiana) exhibited lower percents canopy cover. Wound presence and pit cut size were not major factors affecting the 1-5 year old street trees sampled in this study.
The major management implication of these findings is that bare root trees are a viable alternative to B&B trees in community-based urban forestry initiatives. Tree-planting campaigns with similar climactic conditions to Philadelphia can use this study to inform selection of stock and species.