Urban Site Considerations
During the site assessment you must be able to determine the features that will most directly affect the establishment and long-term health of a tree. If the site cannot be modified, then you must select species that will work. While this will limit your species palette, it is best to know now rather than to lose precious time and investment. For a tree to maximize its environmental and economic benefits, it must grow well and maintain health into middle age, which may be 30–50 years, depending on the species. While many resources indicate that the average lifespan of a street tree is 7 years, this may simply be due to a lack of proper plant selection, site preparation, insufficient soil volume, or incorrect species-to-site matching.
In the tree-selector database, you will note various delineations with regard to where the tree(s) will be planted. The designated site types include tree pit (square or rectangular hole in hardscape with limited rooting area), tree lawn (either 4–6′ or 6–8′ wide), and park or open space. Each site offers varying degrees of limitation to growth. Even in a park or open space, there may be issues such as compaction, flooding, or periods of drought. You must consider the type of soil texture and its structure. Soil texture is the proportion of sand, silt, and clay in a soil. Texture determines water and nutrient-holding capacity, ease of cultivation, and what a soil can be used for (building or growing plants). Structure refers to how individual soil granules clump or bind together and aggregate. This arrangement affects continuity of soil pores and, therefore, influences water and air movement, biological activity, root growth, and seedling emergence. Therefore, a compacted soil has fewer large pores and reduced continuity of pores (meaning a reduction in air and water movement.) (See Figure 1). Compacted soil reduces root growth and, thereby, above-ground growth.
Figure 1. The compacted soil around this tree reduces water infiltration and air-filled pore space and prevents roots from penetrating the entire soil volume available.
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