Arboriculture & Urban Forestry 40(4): July 2014 Arboriculture & Urban Forestry 2014. 40(4): 193–217 193 The Management of Tree Root Systems in Urban and Suburban Settings: A Review of Soil Influence on Root Growth Gary W. Watson, Angela M. Hewitt, Melissa Custic, and Marvin Lo Abstract. The physical, chemical, and biological constraints of urban soils oſten pose limitations for the growth of tree roots. An under- standing of the interrelationships of soil properties is important for proper management. As a result of the interdependence of soil properties, the status of one soil factor can have an effect on all others. Preventing soil damage is most effective and preferred. Cultural practices, such as cul- tivation and mulching, can be effective in improving soil properties. Soil additives, such as biostimulant products, have not proven to be consis- tently effective through research. The management challenge is to provide an urban environment that functions like the natural environment. Key Words. Biostimulants; Bulk Density; Cation Exchange Capacity; Mechanical Resistance; pH; Soil Oxygen; Soil pH; Soil Salt; Soil Water; Temperature. In urban and suburban areas, the soil environ- ment oſten creates numerous challenges for tree root growth. Urban soil has been defined as, “a soil material having a non-agricultural, manmade surface layer more than 50 cm thick that has been produced by mixing, filling, or by contamina- tion of land surface in urban and suburban areas” (Bockheim 1974). Urban soils are oſten highly altered from the natural state, and human activ- ity is the primary agent of the disturbance. They generally have high vertical and spatial variabil- ity, modified and compacted soil structure, an impermeable crust on the soil surface, restricted aeration and water drainage, interrupted nutrient cycling, altered soil organism activity, presence of anthropogenic materials and other contaminants, and altered temperatures (Craul 1985; Bullock and Gregory 1991; Scheyer and Hipple 2005). These physical, chemical, and biological constraints of urban soils pose limitations for the growth of tree roots. Early experience gained working with the urban soils in Washington, D.C., and other difficult urban sites, led to the projection that about 80% of urban tree problems can be attributed to a poor soil environment, leading to synergistic effects of other debilitating urban stress factors producing an over- all decline in plant vigor (Patterson et al. 1980). The resources provided by the soil environment for root growth include adequate oxygen, water, and nutri- ents, non-limiting penetration resistance, acceptable pH range, and robust biological activity. Presence of contaminants or pathogens can be harmful to roots. Any one of these factors can limit root growth and development, even if all others are in adequate supply. Urban environments are quite different from the natural environment to which trees are adapted, yet they must provide the same resources for growth if trees are to maintain a healthy balance between the crown (supplier and user of energy, user of nutrients and water) and root system (sup- plier of water and nutrients, user of energy). The management challenge is to provide an urban environment that functions like the natural envi- ronment, though its appearance may be different. Recent reviews have described root architec- ture and rhizosphere ecology in the urban envi- ronment (Day et al. 2010a; Day et al. 2010b) and serve as a foundation for this review of research summarizing our current understand- ing of soil management techniques for urban trees. ©2014 International Society of Arboriculture
July 2014
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