To figure out why trees suffer in the urban environment it is necessary to understand what they require to be successful. One of the most important, but overlooked, factors for successful tree growth is the provision of appropriate growing space for roots. The roots of a mature tree, in a natural unobstructed setting, can spread one-and-half to two times the radius of the crown. Providing enough space for root growth is a major challenge when establishing trees in dense populated areas, dominated by hard surfaces.
A tree is always seeking to maintain a balance between the area of canopy and the roots. Although the canopy provides the important role of carbohydrate production, the root system is as vital for its function of anchorage (i.e. supporting the tree), absorption (i.e. the uptake of water and minerals), conduction (i.e. the transfer of water and minerals to the stem and canopy), and storage (i.e. the retention of food during the dormant season for early spring use).
There is a direct correlation between soil volume and the mature size and optimum health a tree will achieve. To reach a 200-mm (7.8-in.) stem diameter (measured at 1.4 m [4.5 ft from grade]), a tree requires approximately 11 m3 (388 cf) of soil. To reach maturity or 0.5 m (1.6 ft) in diameter, a tree requires approximately 34 m3 (1200 cf) of soil.
Due to the expanse of hard surfaces and a network of underground utilities, it is often a major challenge to provide enough space for 34 m3 (1200 cf) of soil. Three innovative methods are now being implemented to improve soil volumes and reduce heavy soil compaction. The use of CU-Structural Soil Urban Tree Planting Mix (a soil and aggregate mix), soil trenches (suspended concrete slab over topsoil-filled trench) and Silva Cells (modular suspended pavement system) allow for the creation of soil volume, while providing support for surface paving within the root growth area. Each method has its appropriate use, but they have all demonstrated the ability to improve the environment in which root development may occur.
Respiration is the process by which the sugars (i.e. carbohydrates), along with oxygen, are used by the tree to fuel actual growth. The byproducts of this process are carbon dioxide and water. This process occurs day and night.
Photosynthesis is the process by which the chlorophyll within a tree’s leaf uses the energy from sunlight to convert carbon dioxide and water into sugars used for food. The waste product of this process is oxygen.