Arboriculture & Urban Forestry 35(1): January 2009 Arboriculture & Urban Forestry 2009. 35(1):41– 46. 41 Six-Year Evaluation of Circular Root Barriers on Two Tree Species Dennis Pittenger and Donald Hodel Abstract. The influence of four circular root barriers on surface root development and tree growth was evaluated on Liquidambar styraciflua (American sweetgum) and Ficus microcarpa (Indian laurel fig) in southern California, U.S. Six years after installation, root barriers had reduced the total number of roots growing in the surface 15 cm (6 in) of soil and nearly eliminated large (diameter 5 cm [2 in] or greater) surface tree roots within a 120 cm (48 in) radius of the trunk. Various sizes and types of low-cost noncom- mercial barriers, including a container-grown tree’s nursery container with the bottom removed, were equally effective in reducing the number of large-diameter surface roots. However, roots grew below barriers and returned to the surface soil when soil texture, bulk density, and water content were near optimum for root growth at the bottom of the barrier. Many small roots 1.25 cm (0.5 in) £ diameter less than 2.5 cm (0.5 in £ diameter to less than 1 in) were found growing in the upper 15 cm (6 in) of soil just beyond bar- riers within 18 to 60 cm (7 to 24 in) of the trunk. No barrier treatment reduced the number of small roots of either species beyond 60 cm (24 in) radius from the trunk. Keeping pavement at least 120 cm (48 in) away from trees would be as effective as a root bar- rier in reducing the possibility of damage from large surface roots. A #15 nursery container serving as a root barrier reduced caliper increase of both species, whereas this treatment and the DeepRoot ® barrier treatment reduced height increase of Liquidambar . Key Words. Landscape trees; root diameter; root systems; surface roots; tree growth. When too little urban space is allocated for tree root systems to grow and develop, they often damage nearby infrastructure. In many urban areas of the United States and other countries, tree root damage to infrastructure is a considerable problem (Costello and Jones 2003). For example, McPherson (2000) estimated that in California, U.S. alone, over $70 million was spent annu- ally to repair sidewalks, curbs, pavement, and other hardscape broken or lifted by tree roots and that this amount represented only a portion of the repairs that are actually needed. Installation of circular root barriers when planting street or parking lot trees is often specified as a means of preventing or delaying damage to hardscape by future root growth (Randrup et al. 2001). Research with several tree species has shown root barriers have variable effects on root distribution (Costello and Jones 2003). All of the studies except Gilman (2006) were relatively short-term, evaluating barrier effects approximately 3 years after treatments were established. An early study by Wilson (1967) under controlled conditions demonstrated that laterally growing, very small 1 to 2 mm (0.04 to 0.08 in) diameter roots of red oak turned downward when pre- sented with a rigid barrier placed perpendicular to their growth path. The roots then grew to the bottom of the barrier, recurved, and continued growing. More recent field research experiments using various physical and chemical barrier treatments with sev- eral tree species found roots grew down and under barriers and returned to the soil surface a short distance beyond the barrier (Wagar 1985; Urban 1994; Gilman 1996, 2006; Costello et al. 1997; Peper 1998; Peper and Mori 1999; Smiley 2005; Smiley et al. 2000 ). Roots that grew down and out of barriers were delayed in reaching the surface soil and occurred there in fewer numbers compared with controls. Sometimes roots were found in the upper 30 cm (12 in) of soil within 30 cm (12 in) of the barrier (Costello et al. 1997; Peper 1998; Peper and Mori 1999), whereas other times, they were found in the upper 15 cm (6 in.) of soil at distances from 30 cm (12 in) to 150 cm (60 in) from the barrier. The distance outside a barrier that roots would likely return to the surface soil and continue growing was not narrowly defined in these studies. With the exceptions of Gilman (2006) and Smiley (2005), previous studies were conducted with barriers that com- pletely circled the tree root system. Many studies concluded that tree species and soil properties are the key factors determining the number of roots that return near the soil surface, the depth that they grow, and the distance from the trunk at which they return to the surface (Barker 1995a, 1995b; Costello et al. 1997; Peper 1998; Peper and Mori 1999; Randrup et al. 2001; Gilman 2006). The objectives of this study were to: 1) compare the effective- ness of three simple, low-cost physical barrier materials and a commercial physical circular root barrier in preventing surface root development of two commonly used landscape tree species several years after planting; 2) determine the influence of these circular physical root barriers on tree growth; and 3) define the distance beyond a barrier that surface roots occur. MATERIALS AND METHODS Two commonly planted street, parking lot, and landscape tree species observed to develop extensive surface roots (Warriner 1999; Costello and Jones 2003), Liquidambar styraciflua L. (American sweetgum) and Ficus microcarpa L.f. (Indian laurel fig), were transplanted from #5 containers [12.6 l (3.3 gal.)] into a field experiment in June 1992 at the University of California in Riverside, California. Trees were planted at a spacing of 6.1 m × 6.1 m (20 ft × 20 ft) in planting pits approximately 1.2 m × 0.9 m wide × 0.8 m deep (4 ft × 3 ft × 2.5 ft). The following five circu- lar physical root barrier treatments were established at planting ©2009 International Society of Arboriculture
January 2009
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