Arboriculture & Urban Forestry 43(1): January 2017 was mowed only two or three times each grow- ing season and clippings were not collected. No supplemental irrigation was provided. Thirty ‘Summit’ green ash (Fraxinus pennsyl- vanica ‘Summit’), 14.8 cm average caliper and 12 ‘Green Column’ black maples (Acer nigrum ‘Green Column’), 15.6 cm average caliper, likely graſted on sugar maple (Acer saccharum) root stock, but not confirmed, were selected from a larger group. Ash were fertilized by liquid soil injection and broadcast applications (15 trees each). Maples were fertilized by liquid soil injection application only due to the limited number of trees available. In order to measure whole root system response, root systems were isolated by trenching at the midpoint between trees (2.4 m from each tree) and placing plywood barriers into the trench before backfilling. This midpoint was typically less than 30 cm beyond the drip line. The trenches cut very few tree roots over 6 mm diameter, so the total root system of each tree was not sub- stantially disturbed or reduced by the trenching. Nu-Gro Nitroform fertilizer (38-0-0) was applied in granular form for broadcast applica- tion, and the powder form dissolved in water for subsurface injection application. Both are a urea formaldehyde nitrogen source, with 71% of the nitrogen from a water-insoluble fraction. Nitro- gen was applied for two consecutive years at 0.96 or 2.88 kg N/100 m2 , the lower and upper limits of the standard recommended range (ANSI 2011). For broadcast applications, the granular fertilizer was spread evenly over the entire isolated root system in the first week of June. Liquid injections were arranged in a 2.4 m square pattern, locating the holes halfway between the trunk and the root barrier. For the liquid injections, the fertil- izer powder was dissolved in the same amount of water (94.6 L/tree) for both rates and distrib- uted equally among injection points (2.6 L at each) approximately 30 cm apart, 15 cm deep. In the first year, the liquid fertilizer application was delayed until late June and applied in two smaller applications because of wet soil conditions. The second year, drier soil conditions allowed all of the fertilizer to be applied in a single application at the same time as the broadcast application. Two terminal twig samples per tree were pruned from halfway up the crown on opposite 41 sides. No samples were taken the same year as the first N application since the N fertilizer was applied aſter growth for the year was essentially complete. Samples were collected in September of the second year, capturing growth from the first season aſter the initial N application, and again the following June aſter terminal buds had set. The final harvest was destructive, and 10 sam- ples per tree were collected. The distance from the bud scale scars to the terminal bud tip was recorded on each terminal twig sample. The area of three fully expanded leaves per twig sample was measured on a Delta-T (video) Area Meter (Delta-T Devices, Burwell, Cambridge, England). Relative chlorophyll content was measured twice on each of the leaves used for leaf area measurements with a Minolta SPAD-502 meter. Fine-root development was measured using root density cores in June of the second year, a few days aſter the tops were harvested. One 30 cm deep, 7 cm diameter core was taken 0.6, 1.2 (soil injec- tion location), and 1.8 cm from the trunk in east and west directions. Cores were stored at 4°C until processed. Soil was washed from the roots, and ash and maple roots were separated from other roots and debris by hand. Length of fine roots (<2 mm diameter) was measured and converted to fine- root density with a WinRHIZO™ image analysis system (Regent Instruments, Quebec, Canada). One-way ANOVA (P ≤ 0.05, Normality, P > 0.05), with separation of means by the Holm-Sidak Method (SigmaStat 3.0, SPSS Science), was used to compare fine-root density, caliper increase, twig growth, leaf area, and SPAD measurements from each year. RESULTS & DISCUSSION Mature Oak Experiment Neither the broadcast N fertilization nor basal drench PBZ treatments, separately or combined, affected fine-root density of mature oaks growing with turf- grass competition in the root zone (Figure 1, white oak data not shown). The vertical distribution was similar to other species in a similar soil type (Watson 2006a; Watson and Kelsey 2006), with maximum fine-root development within 10 cm of soil surface, diminishing rapidly between 10 and 20 cm, and lower throughout the remainder of the soil profile. Turfgrass competition and/or more extreme fluctu- ©2017 International Society of Arboriculture
January 2017
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