152 Day et al.: Contemporary Concepts of Root System Architecture of Urban Trees Stone and Kalisz (1991) found the shallowest maximum root- ing depth of any tree surveyed to be 1 m, and the deepest exemplar surveyed was 61 m. Although open-grown or “horticultural” trees were included in this survey, results were not categorized by for- est versus urban growing sites, and most of the horticultural ex- amples were in orchards or other production agriculture settings. Although urban soils are heterogeneous and can defy generaliza- tion, it is common to find impenetrable horizons relatively near the surface; examples include buried asphalt, subsoils compacted by construction activity, and poorly drained horizons. Analogous conditions in forest settings (e.g., bedrock, hardpans, shallow wa- ter tables) result in shallower root systems than occur for the same species on less restrictive sites (Lyford and Wilson 1964; Stone and Kalisz 1991). Soil compaction is very common in urban areas and can result in severe root restriction (Alberty et al. 1984; Day et al. 2000). Species interaction with the environment plays a role here as well. Certainly there are instances of deep-rooted urban trees where conditions allow. For example, tree roots on the high- ly urbanized campus of the University of Costa Rica (San Jose, Costa Rica) were observed to penetrate several meters deep (per- sonal observation of the authors). Similarly, roots of Celtis laevi- gata (hackberry) and Ulmus americana (American elm), com- mon urban species in the U.S., have been found in natural settings at 6 m and 7 m depths, respectively (Jackson 1999), and young Populus tomentosa (Chinese white poplar) up to 14-years-old in Hebei Province in China were found to have root systems extend- ing as much as 4.5 m deep in a sandy soil (Wong et al. 1997). These studies and others (e.g., Stone and Kalisz 1991), indi- cate that some tree species commonly used in urban settings have the potential for rapid development of deep root systems. Do these species realize this genetic potential for exploration of deeper soil regions when planted in urban and landscape settings? Deep root systems have the potential to both exploit groundwater (Dawson 1996), and redistribute groundwater stores through hydraulic lift (Dawson 1993; Burgess et al. 1998), a process to which is attrib- uted the ability of stands of young Acer saccharum (sugar maple) to obtain as much as 17% of their water supply from groundwater (rather than soil water originating from rainfall), during extended dry conditions. Urban trees frequently experience drought, but whether conditions can be created where urban trees can ac- cess deep groundwater stores has yet to be explored, and no in- stances of hydraulic lift in urban settings have been documented. Root Spread Many rules of thumb have been offered for estimating root spread in urban trees. Ratios of height, trunk diameter (typically “diam- eter at breast height” or approximately 1.3 m), and canopy diam- eter may be used for root system spread estimation (e.g., Smith 1964; Gilman 1988; Gerhold and Johnson 2003), but the accuracy of these methods can depend upon the species or cultivar (Gilman 1988), tree vigor (Balasubramanyan and Manivannan 2008), and the rooting environment (Gerhold and Johnson 2003). Moreover, estimates of root spread generally assume there are few physical impediments to root extent. This is rarely the case in very urbanized settings. For example, root system spread may be halted within approximately 10 cm after penetrating beneath roadways or side- walks (Gerhold and Johnson 2003). Even where soil conditions are homogeneous, roots may not be uniformly distributed around the tree (Tubbs 1977; Watson Himelick 1982; Ghani et al. 2009). ©2010 International Society of Arboriculture Root spread studies must also be interpreted with caution due to the potential methodological discrepancies as previously described. Tree height as a predictor of root spread How reliable is tree height for estimating root spread of urban and landscape trees? Open-grown trees have been documented to have wider root spread than forest-grown trees of the same spe- cies when considered as a function of tree height (Smith 1964). Drier sites have in some cases been observed to result in wider spreading root systems (Smith 1964; Belsky 1994). For the current study, the authors combined available published data (Appendix), including several examples from urban sites, to analyze the pre- dictive capacity of tree height for root system radius using regres- sion analysis (SigmaPlot v. 9.01, Systat Software, Inc., Chicago, IL) (Figure 1). Of the studies analyzed, tree height explained only 36% of the variation in root spread; however, the power of this analysis is limited by the dearth of published data. Yet, even with large numbers of trees of the same species and in the same region, Smith (1964) found that only 33%–50% of the variation in root spread could be explained by height. Although the relationship es- tablished in Figure 1 is approximately 1:1, it is instructive that al- most none of the data points fall within the 95% confidence inter- val; thus for an individual tree, there is no assurance that any root estimate based on height will be accurate. In summary, tree height is a poor predictor of root spread in urban and landscape settings. Figure 1. The relationship between tree height and maximum root radius from summarized literature. R2 Dashed lines represent 95% confidence interval. When data for conifers and deciduous species were analyzed separately, data was transformed to achieve a more constant variance and re- lationships were as follows: Deciduous: p = 0.25 and R2 Conifers: R2 average, see Appendix for data sources and N values. = 0.359 and p = 0.002. = 0.09 = 0.28 p = 0.18. Each data point represents a study
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