Arboriculture & Urban Forestry 41(4): July 2015 minute as part of the Data Dictionary (TerraSync™ v.2.4) generated within the Trimble GeoXM. Data collected and entered into the GPS’s Data Diction- ary were start time, date, tree number, run num- ber, crew initials, status, species, field land use, dbh (up to six stems), total height, height to live top, crown base, percent crown missing, crown dieback, crown light exposure, crown width (north-south), crown width (east-west), tree site, additional com- ments, and stop time (i-Tree 2010b; i-Tree 2010c). Statistical Analyses Regression analyses were employed to calculate the relationship between tree sample time and dbh for the three crew sizes. Likelihood ratio F tests were then applied to determine whether the relationships were significantly different (P < 0.05) between the crew sizes by testing differences among slopes and intercepts for the crew size–dbh relation- ships. Aſter removing outliers from the analysis due to crewmember error (e.g., forgot to record start and/or stop times), N = 67, 69, 67 trees for one-, two-, and three-person crews, respectively. RESULTS The trees sampled in this study included 35 species, with loblolly pine (Pinus taeda), willow oak (Quercus phellos), and Chinese elm (Ulmus parvifolia) com- prising the most specimens, with 14, 7, and 5 trees, respectively. All other trees sampled contained fewer than four trees per species. Tree dbh ranged from 3.6–98.6 cm with an average of 30.5 cm. Total height averaged 12.1 m, with a range from 2.7–34.5 m. Average crown spread was 7.6 m, ranging 1.8–32.4 m. Likelihood ratio tests were used to compare slopes and intercepts of the three regression equa- Full model 225 tions (full model). In the current study, the like- lihood ratio test was used in a similar manner to a comparison test in an ANOVA for discrete data. The full model would have six parameters (i.e., a slope and intercept for each crew size). Several comparisons were made (Table 1). The null hypoth- esis of the first test states all the slopes and all the intercepts are equal to each other; the reduced model has a single common slope and a single common intercept. The slopes and intercepts were significantly different (F-ratio = 50.73, P < 0.0001); therefore, the hypothesis was rejected. Under the null hypothesis, this reduced model would have a single common slope and separate intercepts for a total of four parameters. This hypothesis was also rejected (F-ratio = 3.54, P = 0.031). However, when researchers used a null hypothesis of a single slope for a one- and two-person crew giving a reduced model, with two slopes and three intercepts for five parameters, the null hypothesis was not rejected, indicating it was not significantly different than the full model with six parameters (F-ratio = 1.23, P = 0.27), meaning the slope for a one- and two- person crews were not significantly different from each other. The next test used the simpler model, with four parameters as the new model and a null hypothesis of a single intercept (Table 1). The null hypothesis was rejected (F-ratio = 58.20, P < 0.0001), so the intercept cannot be combined for all crew sizes. However, when only the inter- cepts for two- and three-person crews were com- bined, the null hypothesis was not rejected (F-ratio = 0.57, P = 0.45); therefore, the intercepts were not significantly different. The simplest model (least parameters) is a model with four parameters: a common slope for the one- and two-person crews, Table 1. Likelihood ratio tests and P values comparing different models to determine differences in crew sizez Model Reduced model 1 2 3 4 5 Independent slope and intercept for each crew size Independent slope and intercept for each crew size Independent slope and intercept for each crew size One- and two-person crews with common slope, but independent intercepts One- and two-person crews with common slope, but independent intercepts z N = 67, 69, 67 trees, for one-, two-, and three-person crews, respectively. Common slope and intercept for all crew sizes Common slope, but independent intercepts One- and two-person crews with common slope, but independent intercepts One- and two-person crews with common slope and intercept One- and two-person crews with common slope and two- and three-person crew with common intercept 0.0001 0.4526 . P-value 0.0001 0.0308 0.2678 ©2015 International Society of Arboriculture
July 2015
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