86 polyvinyl chloride planter boxes, nylon fabric, copper screen, Typar fabric (DuPont, Summerville, SC), and coarse gravel. Although these options are not purpose-built, some have proven successful, in particular coarse gravel (Gilman 2006). SUMMARY OF ROOT BARRIER RESEARCH After initial root barrier studies during the early 1980s, publica- tions increased substantially during the mid to late 1990s, mir- roring their increased popularity. Despite over 20 years of study, relatively few articles describing root barriers have been pub- lished to date, likely the consequence of the high costs and long timeframes associated with belowground experiments. Despite this, the published studies have successfully described root bar- riers and their uses, contrasted the practical application and ef- fectiveness of different classes and types of barriers, and exposed some important limitations to their use. Introductory articles on the topic of root barriers are abundant (e.g., Hamilton 1984; Urban 1995; Nicoll and Coutts 1997; Coder 1998; Randrup et al. 2001). These articles are geared toward practitioners or managers looking for a solution to the conflicts between sidewalks and roots. The articles provide the reader with a description of the need for root barriers, the dif- ferent classes of barriers, the benefits and drawbacks of their use, and their potential applications. In addition to these overviews, numerous experiments have been conducted to test the effectiveness of root barriers under controlled conditions. Most have tested barriers on newly planted trees (e.g., Wagar 1985; Knight et al. 1992; Barker 1995a, 1995b; Gilman 1996; Costello et al. 1997; Peper 1998; Peper and Mori 1999; Gilman 2006), but some have tested the effectiveness of barriers on established trees (e.g., Wagar and Barker 1993). The results generally support the assertion made by Wagar (1985) and supported by others (Urban 1995; Gilman 2006) that barriers are perhaps least effective where they are most needed. This sentiment implies that barriers are effective in well-drained, noncompacted soils, which are virtually nonexis- tent in roadside urban areas. Interestingly, the experiments undertaken have resulted in dif- fering and often opposing root responses to barriers, thus high- lighting the variability of root growth and its sensitivity to soil conditions. Although some studies have detailed the effective- ness of root barriers, others have drawn attention to their inad- equacy. A closer look at the measurements conducted, the bar- riers tested, and the site conditions elucidate some of the reasons for the reported discrepancies. In all experiments, root response quantification has been lim- ited to measurements of root biomass, number, diameter, and depth. Of the four root measurement parameters, only root dry weight was consistently lower for trees treated with barriers (Wagar 1985; Barker 1995a, 1995b; Peper 1998; Peper and Mori 1999). Interestingly, two studies on the same site, using the same barriers (DeepRoot [Deep Root Partners LP, San Francisco, CA] and Tree Root Planter [Bumble Bee Products, Signal Hill, CA]), reported different results with respect to dry root weight. Peper (1998) found that the dry root weight of mulberry (Morus alba) decreased as affected by the root barriers, whereas Peper and Mori (1999) reported that the change in dry root weight of hackberry (Celtis sinensis) was insignificant when compared with controls. This points to a species-specific response to root barrier use, a phenomenon noted by Wagar (1985) and Costello et al. (1997). ©2008 International Society of Arboriculture Morgenroth: Root Barrier Research Only extreme conditions resulted in decreased root diameter for trees treated by root barriers. For example, root barriers 60 cm (24 in) in depth significantly reduced mean root diameter relative to controls; however, 30 cm (12 in) barriers had no effect (Peper 1998). Mean root diameter was also significantly less than controls, but only when rootballs were encased in a tight polyethylene tube measuring only 18 cm (7.2 in) in diameter as compared with the 61 cm (24.4 in) diameter of DeepRoot or Tree Root Planter (Peper and Mori 1999). Another experiment detected root diameter decrease for trees treated with gravel root barriers, but not for any two-dimensional barriers relative to controls (Gilman 2006). Apart from these examples, mean root diameter has been found not to change relative to controls by numerous authors (e.g., Costello et al. 1997; Peper 1998; Peper and Mori 1999; Gilman 2006). Perhaps the take-home message from these studies is that to affect significant changes in root diameter, typical two-dimensional, 30 cm (12 in) deep barriers are insufficient. Quantifying root number yielded highly variable results influ- enced by species, barrier type, and soil type. A species effect was evident when the mean number of ash (Fraxinus oxycarpa) roots did not vary with respect to controls; however, poplar (Populus nigra ‘Italica’) roots were reduced significantly (Costello et al. 1997). A barrier-type effect occurred when the root penetration of Red maple (Acer rubrum) and American sycamore (Platanus occidentalis) treated by an inhibitor were significantly lower than controls, but roots of the same species did not differ from controls when treated with a trap (Knight et al. 1992). Finally, a soil type effect was noted by Gilman (2006) who observed simi- lar root numbers were observed across treatments in well- drained soils but significantly decreased root numbers in poorly drained soils. The highly variable nature of root number quan- tification suggests that this parameter is more highly influenced by species, barrier type, or soil type than the presence or absence of a barrier. The effect of root barriers on mean root depth is inconclusive. The outcome of the experiments assessing this parameter were split with some finding increased mean root depth associated with barrier use (Peper 1998; Peper and Mori 1999; Gilman 2006), but others reporting no significant difference (Costello et al. 1997; Peper 1998; Gilman 2006). Interestingly, some studies found barrier-specific dependencies. For instance, 30 cm (12 in) deep barriers did not affect root depth; however, 60 cm (24 in) deep barriers did (Peper 1998). Three-dimensional barriers (gravel) did increase mean root depth, but two-dimensional bar- riers (polyethylene, DeepRoot) did not (Gilman 2006). One aspect of this relationship that is less contentious is that roots tend toward upward growth once they have passed under two- dimensional barriers (Gilman 1995, 1996; Costello et al. 1997; Peper 1998; Peper and Mori 1999; Gilman 2006). Barker (1995a) observed no discernible return of roots toward the sur- face after downward deflection, but admitted that “continued later growth of these deepened roots eventually may be into shallower soil.” No two-dimensional barrier class (trap, deflector, inhibitor) or brand (DeepRoot, Biobarrier [Fiberweb plc, Hickory, TN], Root Block [Mann Made Products, Redwood City, CA], Tree Root Planter, Vespro [Vespro Inc., San Rafael, CA], polyeth- ylene, nylon fabric, copper screen) consistently outperforms oth- ers. Clearly, soil environment, barrier design, and tree species interact to influence the outcome of root barrier experiments.
March 2008
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