Arboriculture & Urban Forestry 32(3): May 2006 115 were growing in limited spaces and had well-established mulch over the root zone. Nine of the trees were treated with PBZ. The PBZ con- centrate was diluted with water per label instructions at 12.5: 1 for a basal drench application with 0.8 g active ingredient (a.i.)/cm (2 g/in) on 11 April 1995, the same rate used in a previous study that increased root density of white oaks (Wat- son 1996). The product label calls for reapplication every 3 to 4 years. Due to a lack of response from the first treatment, all trees were retreated with a higher rate of 1.2 g a.i/cm (3 g/in) on 8 October 1998. Sampling began in 1996. Each year in June, fine root de- velopment was measured using root density cores. One 20 cm (8 in) deep, 7 cm (2.8 in) diameter core was taken 1.5 m (5 ft) from the base of each tree. Cores were stored at 4°C (39°F) until processing. Soil was washed from the roots, and oak roots were separated from other roots and debris by hand. Length of fine roots [<2 mm (0.08 in) diameter] was mea- sured and converted to fine root density with a WinRhizo system (Regent Instruments, Montre ´al, Que ´bec). June 2001 fine root samples were also used for assessment of mycorrhizal colonization. An additional set of core samples were collected in October 2001. Mycorrhizal colo- nization was assessed by staining fine roots with Ponceau S stain for 15 min at room temperature, destained with 10% acetic acid, and rinsed in deionized water. Root tips in ten randomly selected squares of a 625 square grid [2.5 mm (0.1 in) between lines] were examined under 20× magnification. Stained (red) root tips were recorded as ectomycorrhizal. Woody roots 0.5 to 1.0 cm (0.2 to 0.4 in) diameter were collected from each tree for visual starch assessment in early June and early October, when starch reserves were expected to be at the low and high points in the normal annual cycle, respectively. At least two roots from each tree were used each season, more if they could be found without excessive dis- turbance. Cross sections were stained with I2-KI solution and each sample was rated from 1 to 4 (devoid, low, medium, and high, respectively) (Wargo 1975). Twig growth and leaf area were measured in August each year. An aerial lift was used to cut two samples per tree, from halfway up the crown on opposite sides. The distance from the budscale scars to terminal budtip was recorded in centi- meters. Area of three fully expanded leaves was measured on a Delta-T (video) Area Meter (Delta-T Devices, Burwell, Cambridge, U.K.). T-tests were used to compare twig growth, leaf area, and root densities each year (P < 0.05) using SigmaStat Version 3.0 for Windows. RESULTS AND DISCUSSION PBZ produced no reduction in leaf size or twig growth of the white oaks at any time during the 7 years of the study (Tables 1 and 2). High growth regulation was not desired because it Table 1. Twig growth (cm) of white oaks following PBZ treatments in April 1995 and October 1998. Year 1996 1997 1998 1999 2000 2001 2002 PBZ 7.2 8.0 6.1 5.1 10.4 7.4 7.6 Control 6.7 7.5 7.8 5.0 11.0 5.6 8.5 There was no significant difference between treated and control trees in any year. could have resulted in a reduction, rather than stimulation, of root growth (Watson 2001, 2004). The 0.8 g a.i./cm (2 g/in) DBH rate used in the first treatment was chosen because it did result in increased fine root density in a previous study with white oaks (Watson 1996), but most of those trees had re- duced, declining crowns. Trees in this study had fuller crowns, and the 0.8 g a.i./cm (2 g/in) DBH rate did not produce any measurable effects above or below ground. Choosing an appropriate rate is more difficult than it might seem. According to the product label, rate is determined by trunk diameter. Label rates for the 2SC formulation of PBZ can be traced back to the use for reducing aggressive re- growth of shoots after pruning near utility wires. Line- clearance pruning reduces crown size and over many years can result in smaller crown/trunk diameter ratios, similar to the declining trees in the earlier study (Watson 1996). The 0.8 g a.i./cm (2 g/in) dbh rate used for the first application, though current at the time, was lower than the rate recom- mended on subsequent revisions of the label. Given the higher crown/trunk diameter ratios of the trees in this study not repeatedly pruned for utility line clearance, or declining, the lack of growth regulation is understandable. The lack of measurable growth regulation after the second treatment at a higher rate is more puzzling. Greener leaves (observation only), especially on chlorotic trees, were evi- dence that PBZ was affecting the crowns. Most studies on Table 2. Leaf area (cm2 ) of white oaks following PBZ treatments in April 1995 and October 1998. Year 1997 PBZ 58.2 1998 1999 2000 2001 2002 45.2 51.6 41.6 37.9 45.8 Control 52.9 51.1 47.0 43.1 41.0 45.1 There was no significant difference between treated and control trees in any year. ©2006 International Society of Arboriculture
May 2006
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