42 ( Table 1 ): 1) control, no barrier; 2) DeepRoot ® barrier (model UB 24-2; Deep Root Partners, LP, San Francisco, CA) with dimen- sions of 0.8 m (2.5 ft.) diameter × 0.6 m (2 ft) deep; 3) #15 plastic nursery container with the bottom removed, having a top diame- ter 35.6 cm (14 in), height 43.2 cm (17 in), and volume 38.6 l (10 gal); 4) original #5 plastic nursery container used to produce the tree with the bottom removed and having a top diameter 25.4 cm, height 30.5 cm, and volume 12.6 l (3.3 gal); and 5) black poly- ethylene, 0.3 mm (0.012 in) thick, formed into a sleeve the same dimensions as the #15 container. The sleeve was manufactured on-site by wrapping the polyethylene from a roll around a #15 container and applying vinyl adhesive tape to hold the seam. The study was a 2 × 5 factor (two tree species × five root barrier treatments) experiment with 10 replicates arranged in a fully randomized 10 × 10 Latin square design of 100 trees. Tree growth, root size, and root location data were subjected to analy- sis of variance using the GLM procedure in SAS (SAS Institute, Inc., Cary, NC). Planting pits were partially filled with soil and watered to settle the soil before setting the barriers. Backfill consisted of the native soil excavated from the planting pits. Barriers were set so that their top edge was 5 cm (2 in) above the surrounding grade, and trees were transplanted into barriers so that the surface of their root balls was even with the surrounding grade. Control trees and trees retained in their #5 plastic nursery container with the bottom removed were planted so that their root balls were even with the surrounding soil. Trees were not staked because their trunks were of sufficient caliper to support them. The entire plot was thoroughly irrigated before transplanting and trees were thoroughly hand-watered immediately after final backfilling of the planting pits and barriers. The soil at the site was a Hanford coarse sandy loam (coarse- loamy, mixed, thermic Typic Durizeralf) with pH of 7.2, electri- cal conductivity 1.0 dS/m −1 , and bulk density of 1.4 to 1.5 g/cm –3 remeasured and averaged 1.5 g/cm –3 . Its characteristics were uniform to a depth of at least 60 cm. The entire field was deep tilled before planting which reduced the bulk density to 1.2 g/cm –3 . At the end of the study, the bulk density was or less to a depth of 60 cm. Irrigation was managed during the study so that soil water content was not limiting below the bottom of the root barriers. Soil water content was monitored by gravimetric soil moisture determinations and a pair of tensiometers placed approximately 15 cm outside the barrier at one replicate of each barrier-tree species treatment. One tensiometer was set 30 cm and one 60 cm deep at each location. Irrigation was scheduled to maintain soil moisture content at 70% or greater of field capacity at the 60 cm depth, which assured nonlimiting soil moisture for root development below the deepest root barrier treatment. As a result of the Mediterranean climate in Riverside, irrigation was sched- uled regularly and frequently (two to four times per week) from spring through fall and irregularly and infrequently during the Table 1. Dimensions and interior surface features of circular root barrier treatments, Riverside, CA. Barrier #5 container Ht (cm) 30.5 Polyethylene sleeve 43.2 #15 container DeepRoot ® 43.2 UB 24-2 61.0 Diameter of top (cm) Volume (L) Interior surface 25.4 35.6 35.6 61.0 ©2009 International Society of Arboriculture 12.6 38.6 38.6 Smooth Smooth Smooth 163.4 Vertical ribs Pittenger and Hodel: Evaluation of Circular Root Barriers winter. The planting was irrigated for the first 11 months with drip irrigation in which an emitter was placed inside and immediately outside of each barrier to maintain the desired level of soil mois- ture in and below the barrier. From Month 12 to the end of the experiment, the planting was irrigated with a minisprinkler sys- tem that applied water to the entire planted area. Trees were fer- tilized annually in the spring or early summer the first 3 years of the study, and no fertilizer was applied in succeeding years. In Years 1 and 2, each tree received 37 g (1.3 oz) nitrogen (N) from urea applied within the dripline; in Year 3, each tree received 71 g (2.5 oz) N from 21-0-0 applied also applied within the dripline. The field was kept weed-free in Years 1 through 3 by the combi- nation of hand-weeding and pre-emergent herbicides (oxadiazon 2% and oryzalin 4AS at minimum label rates) applied each spring and fall. Weed management was achieved in succeeding years with hand-weeding and spot-spray applications of glyphosate. Tree height and trunk caliper 15 cm above the soil line were measured annually. Six years after planting, trees were cut 60 cm above the soil, their canopies discarded, and the root zone of each tree was excavated using the tines on the bucket of a backhoe in combination with hand-raking to remove the upper 15 cm of soil in a radius of 180 cm (72 in) from the trunk. Excavation with the backhoe bucket tines began at the outer edge of a bar- rier (at the bases of the trunk in control trees), and soil was pro- gressively loosened away from the trunk to a radius of 180 cm. The backhoe portion of the procedure was monitored directly by the authors and involved tedious, careful, and methodic use of the tines to loosen the upper 8 to 10 cm of soil and tease out the roots of each tree that were 1.25 cm (0.5 in) or greater in diam- eter. Hand-raking immediately followed the backhoe operation to remove the remaining soil to a final depth of 15 cm and to clearly expose the principal root system found within the excavated area. Caution was taken during excavation to avoid breaking off roots 1.25 cm or greater in diameter. In the rare instances when a root was broken off, the location it was growing in was marked so that it could be replaced and recorded during data collection. The excavated area was divided into five concentric radial dis- tance zones from the center of the trunk as described in Table 2 . These zones enabled us to compare the barriers’ effectiveness in reducing surface root development and to narrowly delineate Table 2. The five concentric radial distance zones from the center of each tree trunk in which roots to a depth of 15 cm were quantified in a root barrier study, Riverside, CA. Zone Distance from trunk 1 13 cm < radius £ 18 cm (5 in < radius £ 7 in) 2 18 cm < radius £ 30 cm (7 in < radius £ 12 in) 3 30 cm < radius £ 60 cm (12 in < radius £ 24 in) 4 60 cm < radius £ 120 cm (24 in < radius £ 48 in) 5 120 cm < radius £ 180 cm (48 in < radius £ 72 in) Distance represented From the perimeter of the #5 container barrier out to the perimeter of the #15 container and polyethylene barriers From the perimeter of the #15 container and polyethylene barriers out to the perimeter of the DeepRoot barrier From the perimeter of the DeepRoot barrier out to 30 cm beyond the barrier From 60 cm to 120 cm from trunk From 120 cm to 180 from trunk
January 2009
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
