40 Watson and Hewitt: Fine Root Growth Response to Soil-Applied Nitrogen and Paclobutrazol and PBZ application on mature, native oaks. Turfgrass was present around the trees on both sites. Total soil nitrogen in the upper 15 cm of soil before application of nitrogen fertilizer was 0.21% on both sites, in the middle third of the normal range of 0.06%–0.5% (Bremner 1965). Mature Oak Experiment Thirty-six mature bur oaks (Quercus macrocarpa), and 18 mature white oaks (Quercus alba), grow- ing among a larger group of trees with overlap- ping root systems were used. The trees were aging, but healthy and showing no major signs of decline. The study trees averaged 66.0 cm dbh. No fertilization or maintenance other than mowing was performed for many years previ- ous to the initiation of the fertilization study. The soil was an undisturbed, somewhat poorly drained Nappanee silt loam (NRCS 2016). Soil tests showed low phosphorous levels in the soil in both fertilized and control trees. Triple super phosphate (0-45-0) was applied at 0.96 kg N/100 m2 /year, during the first three years until levels increased to normal range. Soil organic mat- ter was 4%–6% in the upper 15 cm of topsoil. The crowns of the trees formed a forest-like closed canopy and the root systems likely overlapped some- what. To avoid exposing a portion of a tree’s root system to another treatment, fertilizer treatments were applied to trees in groups. Treatment areas were based on 12 cm radius/cm dbh. The overlap of portions of the root system adjacent to other trees in the group would result in a smaller “shared dose” of fertilizer, but would likely be balanced by a smaller crown, also resulting from the closer spacing. PBZ (Cambistat 2SC, Rainbow Treecare Sci- entific Advancements, Minnetonka, Minnesota, U.S.) was applied once at the beginning of the study and according to the label instructions (rate of 1.2 g a.i. /cm dbh for oaks) as a basal drench in a shallow furrow around the base of the tree. Granular 38-0-0 fertilizer (Nu-Gro Nitroform®), composed of a urea formaldehyde nitrogen source with 71% of the nitrogen from a water-insoluble fraction, was applied annu- ally for four years on fertilized trees. Application rates were 0, 0.96, or 2.88 kg N/100 m2 , the lower and upper limits of the standard recommended range (ANSI 2011). The lower-rate treatment was ©2017 International Society of Arboriculture applied in May of each year. The higher-rate treat- ment was applied half in May and half in August. Twig growth and leaf area were measured in August each year. An aerial liſt was used to remove three twig samples per tree from the upper half of the crown, equally spaced from around the crown. The distance from the bud scale scars to terminal bud tip was recorded in cm. The area of the first three fully-expanded leaves was measured on a Delta-T Area Meter (Delta-T Devices, Burwell, Cambridge, England). Relative chlorophyll content was measured on each of the leaves used for leaf area with a Minolta SPAD-502 meter. Five measurements were taken on different lobes of each leaf and averaged. Foliar N was determined using the Dumas method (AOAC 2012) on an Elementar Rapid N analyzer (A&L Great Lakes Laboratories, Ft. Wayne, Indiana, U.S.). Determination of total soil nitrogen in the soils was by way of the Kjeldahl method (Bremner 1965). Soil samples were col- lected with a 19 mm diameter soil sampler and cores were divided into 0–15 cm and 15–30 cm sec- tions for processing. Two cores were taken around each tree and the cores were combined into the same groups by which the fertilizer was applied. Fine-root development was measured near the end of the fourth season of fertilization using root density cores. One 42 cm deep, 7 cm diameter core was taken 1.5 m from the base of each tree. Cores were stored at 4°C until processing. Soil was washed from the roots, and oak roots were sepa- rated 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 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 Sci- ence) was used to compare fine-root density, dbh increase, twig growth, leaf area, leaf N, and SPAD measurements from each year. Young Tree Experiment A existing plot of trees at The Morton Arbore- tum, Lisle, Illinois, U.S., growing at 4.8 m spac- ing on an Ozaukee silt loam soil (NRCS 2016) was utilized. Trees and color exhibited typical for their growth species and age. The turf
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