User:Cdelmer/Urban forestry

Street trees provide valuable ecosystem services including stormwater mitigation, air pollutant removal, and shade to mitigate the urban heat island effect. Since street trees are often planted in areas with a high percentage of impervious surfaces, they are an important fraction of an area’s overall urban tree cover. When planting street trees, there are many factors to consider and difficulties to overcome. Depending on climate, soil moisture, nutrient dynamic, and location much planning goes into planting street trees. If done incorrectly these trees can cost a municipality time and money to maintain and remove. Urban site conditions, Species selection, and tree management are three key aspects of cultivating street trees.

Urban sites present many challenges to street trees because of their adverse conditions. Limited soil volume, high soil compaction, and intense microclimates are common where street trees are planted. Because of these adverse conditions, street trees typically have lower growth rates and lower survival rates than trees planted in nurseries or more natural settings. There are also conflicts between tree parts and urban infrastructure because of dense urban environments. Tree roots are known to inflict costly damage by fracturing pavement, which is a common cause for tree removal. In order to receive the full benefit of ecosystem services of street trees, urban foresters aim to minimize these conflicts and provide young trees with the highest opportunity to reach maturity.

A guiding principle of urban forestry is to plant the right tree in the right place. Certain species are more tolerant of adverse urban conditions than others, and urban foresters strive to select species that will maximize benefits and minimize costs for a specific site. For example, yellow-poplar (Liriodendron tulipifera) is known to be intolerant of poor urban soils, and therefore is rarely used as a street tree. There is no tree species perfectly suited for every site so characteristics of each species are scrutinized to determine their suitability for planting as a street tree. Some important characteristics of street tree species include tolerance of alkaline soils, compacted soils, low soil volume, de-icing salts, drought, and having good structure. Blackgum (Nyssa sylvatica) and swamp white oak (Quercus bicolor) are species renowned for their adaptability to urban environments, but even they have drawbacks such as Blackgum being difficult to transplant.

Planning is an important step in the establishment of street trees. Policies and guidelines are beneficial in the street tree planning process by lowering costs and improving the health and safety of a municipality. Studies have shown that municipalities that do not abide by policies and guidelines are shown to have higher costs in economic and environmental aspects. Models and formulas may also be used to warrant adequate species diversity for more resiliency to disturbances and stressors. An example of a formula that municipalities abide by in planning is Santamour’s 10-20-30 rule. This formula allows for no more than 10% of the same tree species, no more than 20% of the same genus, and no more than 30% of the same family. The Species Selection Model and the Analytic Hierarchy Process are other models that can be used in street tree planning. The Species Selection Model focuses on procedures that create a suitable street tree by surveying common species used in urban areas. The Analytic Hierarchy Process is a three layer structure that includes an objective, criteria, and factors. Some factors that may be included in street tree establishment are tree height, DBH, canopy density, and drought resistance. Planning for the physical tree planting should consider bare root and balled-and burlapped (B&B) trees. When deciding upon bare root or B&B, species, age, street traffic intensity, site type, wound presence, and dimensions of sidewalk pit cuts should be examined. Taking into account bare root and B&B trees along with the above criteria are beneficial in the physical aspects of establishing street trees.