Arboriculture & Urban Forestry 36(4): July 2010 to the soil surface and adopt a more typical depth distribution as they extend from the tree (e.g., Day and Harris 2008). In addition to limiting soil conditions below, the presence of turfgrass has been associated with reduced tree fine root development in upper soil regions (Watson and Himelick 1982), perhaps limiting root development from above. Roots are opportunistic and will grow wherever environmental conditions permit. Species may differ in their foraging strategies, either proliferating in nutrient-rich pockets, or extending widely to explore the largest soil volume possible (Mordelet et al. 1996; Mou et al. 1997; Huante et al. 1998). Mordelet et al. (1996) found that mature palms (Borassus aethiopum) extended roots as far as 20 m before encountering a nutrient-rich soil patch where root proliferation was ten times that in ordinary soil. The roots of palms are likely not representative of hardwoods or conifers, but the same localized proliferation has been observed in Liquidambar styraciflua (sweetgum) (Mou et al. 1997). An analogous foraging opportunity presented in an urban environment might be when a broken seal in a sewer pipe creates a soil patch rich in water and nutrients—a common occurrence in cities (Rolf 1991; Randrup et al. 2001; see Schroeder 2005 for photographic documentation of such an instance). Not surpris- ingly, however, the ecology of root foraging has not been stud- ied systematically in urban environments. For urban trees, three root depth issues are of particular interest: (1) Can root depth be influenced by species selection—is it under genetic control? (2) How deep can urban tree roots reach? (3) What role do deep roots play relative to surface roots in terms of resource acquisition? Rooting depth varies among species in similar conditions (Watson and Himelick 1982; Jackson 1999); whether there is genetic control over root depth, independent of species’ environ- mental tolerances is less clear. This is of considerable interest for urban forestry. For example, if rooting depth can be controlled genetically, then deep-rooting trees could be selected to minimize conflicts with pavement. There is evidence for this genetic con- trol, but tolerances of soil conditions such as moisture (Hosner 1960; Hook and Brown 1973) and pH (Martin and Marks 2006) vary—even within a species—and it may be difficult or impos- sible to separate the influence of genetics on root architecture from the influence of genetics on tolerance of soil conditions, since these conditions also have a tendency to vary with depth. Species differences in rooting depth within the same environ- ment have been documented. For example, a study in Texas, U.S., linked roots penetrating underground caverns to surface vegetation using DNA sequence variation (Jackson 1999). Roots of Quercus fusiformis (Texas live oak) were consistently present in the deep- est caves, with water uptake by roots verified at 25 m depth. On one site, Jackson (1999) found Q. fusiformis was the only species with roots that penetrated to 14 m, even though surface vegetation included other species, such as Q. stellata (post oak), with similar environmental tolerances (Stransky 1990). Whether the ability of Q. fusiformis to grow extremely deep roots in these environments reflects genetic control of geotropic response (i.e., directional growth in response to gravity), or simply genetic control of toler- ance for soil environmental conditions is not known. Burger and Prager (2008) explored this question in a recent study addressing whether root architecture could be preserved in clones created through vegetative propagation. One species, Pistacia chinensis (Chinese pistache), clearly formed deeper root systems than two other species, Fraxinus uhdei (shamel ash) and Zelkova serrata (Japanese zelkova), when planted in a 2 m deep Yolo loam. How- 151 ever, when shallow- and deep-rooted genotypes from within the same species were selected and propagated vegetatively, their depth-of-rooting characteristic was not conveyed to their clones. Suspecting differences in geotropic response among root types, Burger and Prager (2008) surmised that the effect of vegetative propagation on root architecture may have obscured any genetic control of rooting depth. Vegetative propagation by cuttings de- pends upon adventitious roots being generated from the cut stem, which in some cases has been linked to shallower root systems (Yamashita et al. 1997; Mulatya et al. 2002). When the orientation of clonal tea plants (Camellia sinensis) grown in windowed box- es was altered, seminal roots displayed more pronounced geotro- pic response than adventitious and lateral roots (Yamashita et al. 1997). This behavior was linked to a more pronounced presence of amyloplast particles in the root cap of seminal roots. However, instances of deeply rooted vegetatively propagated trees have also been recorded. For example, tap roots of clonal Pinus taeda (lob- lolly pine) propagated by rooting cuttings, penetrated downward more than 2 m in a sandy clay loam soil (Fairview series) in the Piedmont region of Virginia, U.S. (Jeremy Stovall, pers. comm.). P. taeda typically forms tap roots, so there is a genetic propensity for such root architecture (Baker and Langdon 1990). Nursery production, regardless of propagation technique, alters root sys- tem architecture in various ways (see Day et al. 2009; Hewitt and Watson 2009). However, whether the tendency toward shallower root systems persists in mature urban trees has not been studied, and the relative influence of propagation and production factors in relation to soil environmental conditions remains unknown. How deep are tree roots of urban and landscape trees? Several surveys documenting tree root depth have been published. Each review, however, has a different scope and intent, and results must be considered in such a light (e.g., Stone and Kalisz 1991; Schenk 2002). Stone and Kalisz (1991), for example, conducted a comprehensive survey of literature and observations reporting maximum rooting depth for more than 1,000 trees from dozens of species of different ages in hundreds of different settings, but summarized studies are almost entirely from forest or orchard settings. In addition, the methods of the collected research vary dramatically with many only entailing partial sampling or exca- vations. This is understandable because excavating tree roots is extremely laborious, and if the research question at hand can be answered with limited excavation (e.g., to 60 cm), then such exca- vating will prove to be the appropriate technique. Thus, in all but few cases (e.g., Lyford and Wilson 1964), root depth and distribu- tion research on larger trees must be interpreted with caution, as it is generally impossible to follow every tree root to its tip. Indeed, although Lyford and Wilson (1964) excavated entire roots of Acer rubrum to their tips and documented all breaks where the tip was not found, the natural result is that only two trees were success- fully excavated. Thus, literature reviews by necessity combine results from many different types of studies. Occasionally, a special occurrence, such as a storm that uproots trees, allows a methodologically consistent survey of root systems, but generally only a portion of the root system may be studied (e.g., Glasson and Cutler 1990). Interpretation of potential tree root spread is subject to the same limitations as root depth. Nonetheless, sum- mary analyses provide a sense of the range of rooting depths across environments and are helpful for understanding the poten- tial for soil exploration and infrastructure invasion by tree roots. ©2010 International Society of Arboriculture
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