Arboriculture & Urban Forestry 36(4): July 2010 Arboriculture & Urban Forestry 2010. 36(4): 149–159 149 Contemporary Concepts of Root System Architecture of Urban Trees Susan D. Day, P. Eric Wiseman, Sarah B. Dickinson, and J. Roger Harris Abstract. Knowledge of the extent and distribution of tree root systems is essential for managing trees in the built environment. Despite recent ad- vances in root detection tools, published research on tree root architecture in urban settings has been limited and only partially synthesized. Root growth patterns of urban trees may differ considerably from similar species in forested or agricultural environments. This paper reviews literature documenting tree root growth in urban settings as well as literature addressing root architecture in nonurban settings that may contribute to present understanding of tree roots in built environments. Although tree species may have the genetic potential for generating deep root systems (>2 m), root- ing depth in urban situations is frequently restricted by impenetrable or inhospitable soil layers or by underground infrastructure. Lateral root extent is likewise subject to restriction by dense soils under hardscape or by absence of irrigation in dry areas. By combining results of numerous studies, the authors of this paper estimated the radius of an unrestricted root system initially increases at a rate of approximately 38 to 1, compared to trunk diameter; however, this ratio likely considerably declines as trees mature. Roots are often irregularly distributed around the tree and may be influ- enced by cardinal direction, terrain, tree lean, or obstacles in the built environment. Buttress roots, tap roots, and other root types are also discussed. Key Words. Root Depth; Root Extent; Root Restriction; Urban Forestry; Woody Plants. Tree roots are supply lines for water and minerals, play impor- tant roles in carbohydrate storage and hormonal signaling, and physically anchor trees in the ground (Kozlowski and Pallardy 1997). To perform these functions, roots must be able to explore their environment and maintain their health. A root system has the potential to draw water and mineral resources from the soil it explores, so how the root explores its environment affects poten- tial resource acquisition. In urban settings, the belowground envi- ronment is often inhospitable and restrictive to tree root growth. Impediments to a healthy root system are frequently pointed out as the underlying cause for a wide array of tree growth and health problems (e.g., Patterson 1977; Hawver and Bassuk 2006). Thus one seeks to understand how root systems develop and respond in this environment. This knowledge is essential for comprehending how trees grow in urban and landscape settings and how belowground features of the built environment, includ- ing landform, structures, and urban soils, interact with tree roots. The study of trees is a vast area of scientific inquiry, yet the study of urban trees represents only a fraction of published re- search. In this review, the term “urban trees” refers to trees grow- ing amongst buildings or other structures for human use regard- less of overall land use. Thus the discussion may be germane to trees growing around a visitors’ center at a national park, but may not be relevant to trees in a forest fragment within a large city. Roots are always difficult to study, simply because they are under ground and respond to localized environmental changes. These difficulties are exacerbated in urban trees because of the variety of circumstances where trees are grown. Earlier reviews of urban tree root systems (e.g., Perry 1982; Gilman 1990) have helped shape arboriculture research and practice for many years. More recent reviews have focused on specific aspects of root system development (e.g., Crow 2005). The aim of this paper is to pres- ent a survey of literature that is relevant to urban tree root systems around the world as well as to take a fresh look at these earlier works about urban tree root systems in the context of recent re- search. In addition to readily available sources, the authors made a concerted effort to uncover research from underrepresented arenas, including reports from various geographical regions in the world and research investigating root systems of urban trees. The objectives here are to: * Critically evaluate and present the current state of knowl- edge on tree root architecture in urban and landscape set- tings including depth and extent of root systems in a way that is useful to both researchers and practitioners. * Identify knowledge gaps in this arena. * Based on these knowledge gaps and the utility of past re- search results, propose areas where further research is a priority. TREE ROOT STRUCTURE AND FUNCTION Terminology used to describe tree roots is very diverse and not standardized. In their compendium of root system terminology, Sutton and Tinus (1983) defined more than 2,200 root terms, il- lustrating the wide variety of ways to describe tree roots. Clas- sification of roots has historically been based on their anatomical or functional characteristics (Sutton and Tinus 1983; Kozlowski ©2010 International Society of Arboriculture
July 2010
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