182 Day et al.: Deep Structural Roots in Urban Trees Arboriculture & Urban Forestry 2009. 35(4): 182–191 Causes and Consequences of Deep Structural Roots in Urban Trees: From Nursery Production to Landscape Establishment Susan D. Day, Gary Watson, P. Eric Wiseman, and J. Roger Harris Abstract. Recent research has improved our understanding of how structural roots of landscape trees respond to being located abnormally deep in the soil profile. This condition is widespread among landscape trees and may originate during nursery produc- tion, at transplanting into the landscape, or when construction fill or sediment deposits bury root systems of established trees. Deep structural roots sometimes hinder successful establishment of trees, occasionally enhance establishment, and often have little or no effect on growth or survival. When trees respond to deep structural roots, effects are sometimes observed when root collars are as little as 7.5 cm (3 in) deep. In some cases, deep structural roots are implicated in girdling root formation, but research in this area is quite limited. This review describes scientific progress in our understanding of deep structural roots and encompasses their history, causes, and significance, as well as interdisciplinary efforts to address deep planting and tree response during estab- lishment to deep structural roots. A theoretical model of short-term tree response to deep structural roots is presented that helps explain these conflicting outcomes and provides a decision framework for practitioners evaluating trees with deep structural roots. Key Words. Buried Roots; Deep Planting; Girdling Roots; Propagation; Root Architecture; Root Morphology; Urban Forestry. Excess soil over root systems of landscape and urban trees has long been recognized as a hindrance for successful transplant es- tablishment and long-term tree health. For example, reduced soil aeration in the root zone of established shade trees was attributed to compacted clay fill soil over forty-five years ago (Yelenosky 1963), and the recognition of decline in recently planted street trees with deep roots was first reported by Berrang et al. (1985). The large woody roots giving characteristic form to the root system are commonly referred to as structural roots, skeletal roots, or scaffold roots (Sutton and Tinus 1983). For both newly planted and established trees, covering these roots with excess soil can al- ter the rhizosphere environment. In soils buried by compacted fill, oxygen concentration may decrease and carbon dioxide increase; however, these effects may be temporary and influenced by both soil moisture and time of year (Yelenosky 1963). Additional soil covering the roots can also intercept rainfall or irrigation, reduc- ing soil water content in the root zone, particularly when fill is also covered with mulch (Arnold et al. 2005; Arnold et al. 2007; Gilman and Grabosky 2004), or is compacted (Day et al. 2001). Planting trees too deep with their root collars below soil grade can thus inhibit tree establishment and growth (Arnold et al. 2005; Arnold et al. 2007) or have no effect (Day and Harris 2008). In addition, deep planting has been shown to increase formation of girdling roots, a term generally used for roots in tight tangential contact with trunks (Giblin et al 2005; Wells et al. 2006; Day and Harris 2008); and trunk-soil contact due to deep planting or fill soil may increase the risk of disease infection. However, evi- dence of increased disease infection is limited (Smiley 1992) or contrary (Day et al. 2005). Finally, excess soil depth may be as- sociated with increased risk of tree failure. Post-hurricane studies ©2009 International Society of Arboriculture have found that trees with root-related failures were more likely to also have buried root collars than similar trees that did not fail (33% versus 8%) (Smiley 2005). However, it cannot be deter- mined whether this was caused by burial of the root collar, or by an associated activity such as root severance during grading. Although tree decline has often been attributed to deep or bur- ied roots, relatively few published reports have actually evaluated the impact of excess soil on root systems. Tree response to buried root systems is likely dependent upon numerous interacting fac- tors, including species, size, age, nursery production practices, planting practices, site conditions, soil properties, cultural prac- tices, and tree stress from other sources such as pests. As a result, it is no surprise that making prognoses for trees with deep struc- tural roots and prescribing effective treatments are so challenging. New research has been published in the last five years, especially in the area of transplant and establishment response to deep struc- tural roots, which may change our perspective on this challenge. This paper focuses on deep structural roots in landscape trees during the period from nursery production through full establish- ment. The objective is to synthesize published research to provide a better understanding of the issues at play in nursery produc- tion, transplanting, and landscape establishment of trees to better serve prevention efforts and management of deep structural roots. DEEP STRUCTURAL ROOTS: HISTORY AND PREVALENCE Deep Structural Roots in Nursery Production Deep structural roots sometimes originate during nursery pro- duction. Maynard (1995) reported trees leaving nurseries with
July 2009
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