Arboriculture & Urban Forestry 35(1): January 2009 Table 3. Effects of five root barrier treatments on mean number of roots 1.25 cm (0.5 in) or greater diameter for two tree species averaged over five radial distance zones from the trunk 6 years after planting, Riverside, CA, 1998. Barrier Check Polyethylene sleeve #5 container DeepRoot ® #15 container Species Liquidambar Ficus Least significant difference ( P = 0.05) Statistical effects z Species (S) Barrier (B) S × B z Mean no. of roots 4.3 a 2.3 b 2.2 b 1.1 c 1.0 c 2.6 a 1.9 b 0.5 * *** NS NS, *, **, *** = not significant and significant at P £ 0.05, 0.01, and 0.001, respectively. Means followed by the same letter are not significantly different. the distance beyond each barrier treatment that surface roots appeared and grew. Roots with diameter 1.25 cm or greater were counted in each distance zone they grew in within the surface 15 cm of soil. In each zone, counted roots included those emerg- ing from deeper than 15 cm in the soil within the zone as well as those growing into the zone after emerging into the surface 15 cm of soil in a different zone. Roots were classified into the following diameter size classes: 1) small, 1.25 cm £ diam- eter to less than 2.5 cm (0.5 in £ diameter to less than 1 in); 2) medium, 2.5 cm £ diameter to less than 5 cm (1 in £ diameter < 2 in); and 3) large, diameter 5 cm or greater (2 in or greater). RESULTS Surface Root Development Overall, barrier treatments reduced the total number of roots 1.25 cm or greater in diameter growing in the excavated area for both species with the DeepRoot and #15 container barri- ers allowing the fewest ( Table 3 ). Liquidambar developed more measurable roots than Ficus in the excavated area regardless of 43 barrier treatment, and root systems of Liquidambar appeared more branched than those of Ficus . Reduced numbers of small roots were unearthed in the surface 15 cm of soil a short distance outside of each barrier treatment in both species ( Tables 4 and 5 ). Thus, small roots were found within Zone 1 in #5 container barri- ers, Zone 2 in the #15 container and polyethylene sleeve barriers, and Zone 3 cm in the DeepRoot barrier. In both species, control and barrier-treated trees had produced few medium or large roots beyond a 120 cm radius (Zone 4) from the trunk. In Ficus , barrier treatments equally reduced small-diameter roots to a radius of 60 cm (Zones 1 to 3) from the trunk and medium- and large-diameter roots to a radius of 120 cm (Zones 1 to 4) from the trunk ( Table 4 ). Occasionally, Ficus roots breached the #5 container or polyethylene sleeve barriers. In Liquidambar , the #15 container and DeepRoot treatments equally reduced small-diameter roots to a radius of 60 cm from the trunk, but the #5 container and polyethylene sleeve treatments failed to reduce small roots within this radius ( Table 5 ). Barrier treatments were equally effective in eliminating large Liquidambar roots to a radius of 60 cm from the trunk, and all barriers except the polyethylene sleeve reduced medium-diameter roots to a radius of 180 cm (Zones 1 to 5) from the trunk. Liquidambar roots of all size classes were often found growing out through breaches in the sides of the #5 container and polyethylene sleeve barriers. Trunks of both species grew large enough in caliper during the 6-year study period for their bases to fill and frequently split the #5 container barriers, whereas the taped seams of the poly- ethylene sleeve barriers repeatedly failed. Also, the exposed rim of the polyethylene sleeve degraded with time and occasional small roots of both species were observed growing over this barrier. Because Liquidambar generally produced more roots of all sizes and more roots closer to the trunk than Ficus , the structural failures of these two barrier treatments made them ineffective at reducing small- and medium-sized roots of Liquidambar . Tree Growth Cumulative increase in trunk caliper of both species 6 years after planting was reduced only by the #15 container barrier, and Ficus had greater cumulative increase in trunk caliper than Liquidambar ( Table 6 ). There was an interaction between tree species and barriers on cumulative height increase ( Table 7 ). Ficus height growth was unaffected by barrier treatment, but height increase of Liquidambar was reduced in the DeepRoot and #15 container Table 4. Effects of root barriers on mean numbers of small, medium, and large roots of Ficus microcarpa present within 15 cm (6 in) of the soil surface in five radial distance zones from the trunk 6 years after planting, Riverside, CA, 1998. z Zone 1 (3 cm < radius £ 18 cm) Barrier Check #5 container Zone 2 (18 cm < radius £ 30 cm) Zone 3 (30 cm < radius £ 60 cm) 4.2 a 2.9 a 3.1 a 3.6 a 5.3 a 3.7 a Zone 4 (60 cm < radius £ 120 cm) 2.5 a 9.5 5.2 a Zone 5 (120 cm < radius £ 180 cm) Small Medium Large Small Medium Large Small Medium Large Small Medium Large Small Medium Large 1.6 a 2.9 a 1.0 a 6.8 2.4 0.6 b 0.3 b 0.6 b 1.3 b 0.5 b 0.6 bc 2.1 b 1.9 b 0.5 b 8.2 1.9 b 0.0 b 5.6 0.7 Polyethylene sleeve — — — 0.7 b 0.9 b 0.9 c 2.4 b 1.9 b 1.0 b 5.7 2.8 b 0.6 ab 6.7 2.5 #15 container — — — 0.4 b 0.2 b 0.0 c 2.3 b 0.6 c 0.2 b 5.4 1.1 b 0.2 b 5.4 1.2 DeepRoot ® Least significant difference y 0.8 1.4 1.2 0.8 1.9 1.2 1.0 NS 1.9 ( P = 0.05) diameter £ 5 cm (diameter £ 2 in). y z Root diameter size classes: small, 1.25 cm £ diameter < 2.5 cm (0.5 in £ diameter < 1 in); medium, 2.5 cm £ diameter < 5 cm (1 in £ diameter < 2 in); large, NS = not significant; means within columns followed by the same letter are not significantly different. ©2009 International Society of Arboriculture — — — — — — 1.5 b 0.8 bc 0.1 b 6.5 1.7 b 0.0 b 7.1 1.4 0.8 0.9 0.1 0.0 0.1 0.0 0.0 0.6 NS NS NS
January 2009
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