©2023 International Society of Arboriculture Arboriculture & Urban Forestry 49(5): September 2023 215 to reduce the chance of spurious significance from the multiple comparison analyses. Shade Effects on Winter Color Change In 2021 August, 29 boxwood with greater than 40% winter color change in the outer canopy representing 8 cultivars/selections were chosen for use in the shad- ing experiment. The selected cultivars had a 79% mean and 75% median color change in outer canopy in the 2021 ratings. These cultivars/selections were ‘American,’ ‘Buddy,’ ‘Dee Runk,’ ‘Fastigiata,’ ‘Green Mound,’ ‘Green Velvet,’ ‘Piney Mountain,’ and ‘Vardar Valley.’ Within each cultivar, individuals were randomly selected to receive the shade treatment (n = 16) or full- sun control (n = 13). Shade tents were constructed using tee posts and 70% shade cloth (Joepen, Guang- zhou City, China) rated for 65% to 75% shade. Shade cloth was cut to size to drape the entire height of each plant, and the cloth was affixed to tee posts with zip- ties, ensuring the cloth was not in contact with the foliage. The nonshaded treatments were left as is in full sun. On September 13 at 3:36 p.m. and Septem- ber 14 at 1:30 p.m., light intensity was measured with a Dr. Meter LX1330B Digital Illuminance Light Meter (Dr. Meter, Shenzhen, China) on the south-facing side of each plant to gauge the difference in light inten- sity measured in lux. Shade tents were removed on 2022 March 9, and plants were rated as previously for percent outer can- opy leaves with winter color change. These were assessed visually, and a representative plant from each cultivar-treatment combination was photographed. To capture the difference in color change between sun exposure levels and between years, the average difference in winter color change between 2021 and 2022 ratings for each individual were analyzed with a mixed model ANOVA in JMP 16, where shade treat- ment was a fixed effect and cultivar was a random effect in the model. Further, a regression analysis between the differences in winter color change between 2021 and 2022 and light intensity was ana- lyzed in JMP 16. Fertilization Experiment On 2021 April 21, 12 of each Buxus sempervirens ‘American,’ B. sempervirens ‘Rotundifolia,’ and B. microphylla ‘Winter Gem’ were planted as described previously in a full-sun field in a randomized block with leaves showing nongreen coloration. At each of these periods, digital photographs of each individual cultivar were taken with a Canon Rebel T3i camera (Canon, Tokyo, Japan). In 2022, visual ratings and photographs were captured similarly on 2022 March 9. Of the boxwood rated in 2021, 16 individuals of 8 cultivars were used in the experiment investigating the effect of shade on winter color change. Ratings of these 16 plants were included in the 2021 but not the 2022 analysis of percent outer canopy showing color change. Means of the percent of outer canopy leaves with winter color change were analyzed by analysis of variance (ANOVA) for boxwood cultivar and spe- cies and separated by Tukey’s Honest Significant Dif- ference (HSD) test in JMP 16 (SAS Institute, Cary, North Carolina, USA). Each visual rating date was analyzed independently. Soil and Foliar Nutrient Analyses On 2021 March 22, one to two soil cores to a depth of 15 cm were collected from each plant and submitted to Waypoint Analytical Laboratories (Memphis, Ten- nessee, USA) for nutrient analysis. The S3M Mehlich 3 soil extraction was used by Waypoint Analytical Laboratories to calculate general soil properties (pH, organic matter, cation exchange capacity, nitrogen, phosphorus, potassium, magnesium, calcium, boron, sulfur, iron, manganese, copper, zinc, and sodium). Additionally, on 2021 March 2 and July 26, foliar samples were collected by cutting 5 to 10 13-cm twigs from the exterior foliage of each plant. The samples were submitted to Waypoint Analytical Laboratories for the micronutrient and macronutrient (PT2) analysis. The foliar nutrient sample collection dates were cho- sen based on peak winter color change (2021 March 2) and complete color reversion (2021 July 26). Multivariate analysis was conducted in JMP 16, and correlations were reported between each soil nutrient and percent canopy with winter color change as recorded on 2021 February 21 (peak color change) and the associated nonparametric P-value from a Spearman’s P-test. Similarly, foliar nutrients were analyzed by multivariate analysis with default set- tings in JMP 16, and correlations between foliar nutri- ent and percent canopy with winter color change as recorded on 2021 February 21 were reported with the associated nonparametric P-value from a Spearman’s P-test. For both soil and foliar nutrient analyses, the P-values were adjusted using Bonferroni correction
September 2023
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