298 Salisbury et al: Long-Term Highway Tree Growth Table 2. Full, final reduced (Red), and null model results for context variables in the 1988, 1997, and 2008 planting cohorts. 1988 Red Full 26.0 21.9 Null Fixed effects – Coefficient (95% confidence interval) Intercept 22.1 Distance Elevation Slope Northing Easting Stem type (single) (–0.1, 0) –0.3 (–1.0, 0.3) –0.2 (–0.5, 0.2) –3.4 (–0.8, 3.6) –1.0 (–4.1, 2.5) Random effects – Variance Planting site Species Residual Model performance Model χ2 AIC ICC conditional ICC adjusted R2 R2 conditional marginal 33.5 34.6 23.5 P-value 0.103 1187 0.67 0.74 0.77 0.10 33.7 32.8 23.7 0.037 1183 0.71 0.74 0.75 0.03 27.7 38.9 24.7 –3.3 (–6.5, –0.1) (–6.5, –0.2) 1.4 Full 15.7 (19.7, 32.2) (18.1, 25.6) (18.2, 25.9) (9.2, 23.0) 0 0 (0, 0) 0.7 (0.4, 1.1) 0.3 (0.1, 0.5) 0.2 (–1.0, 1.7) 0.3 (–0.7, 1.0) –1.5 (–4.2, 1.4) 0.5 22.5 10.6 1186 0.73 0.73 0.023 455 0.62 0.68 0.71 0.09 remaining species was lower than would be expected based on the allometric equations. In the 1997 cohort, Gymnocladus dioicus (L.) K. Koch DBH was similar to predicted values, though this species’ DBH was smaller than expected in the 1988 and 2008 cohorts (Figure 4). For the 2008 cohort, Q. macrocarpa and Pinus nigra ‘Arnold Sentinel’ were the only 2 species whose DBH was within the range of predicted values out of the 10 species with predicted DBH (Figure 5). Notably, DBH was generally more variable in the 1988 cohort compared to the 2008 cohort with the standard deviations for each species ranging from 0.6 ©2022 International Society of Arboriculture 0.5 10.6 10.6 0.002 450 0.45 0.51 0.57 0.11 4.5 10.8 10.9 0.8 (0.5, 1.1) 0.3 (0.1, 0.5) 1997 Red 13.6 Null 17.2 (8.8, 18.1) (15.2, 19.4) Full 11.9 (0, 0) 0.2 (0, 0.3) 0 (–0.1, 0) 0.3 (0, 0.5) 0 (–0.2, 0.3) –1.5 4.4 9.1 2.4 458 0.58 0.58 < 0.001 3181 0.84 0.85 0.85 0.01 0.3 (0, 0.5) –1.5 (–2.2, –0.8) (–2.2, –0.8) 4.5 8.3 2.4 < 0.001 3179 0.84 0.84 0.84 0.01 4.4 8.3 2.5 2008 Red 11.9 Null 10.3 (10.1, 13.8) (10.2, 13.6) (8.8, 11.9) 0 3196 0.84 0.84 to 10.6 for 1988 and from 0.4 to 4.8 for 2008. Multi- stem trees tended to have larger DBH (Equation 1) compared to single-stem trees within a given species, with the exceptions of Celtis occidentalis L. (1988), Populus × robusta (1988), and G. triacanthos ‘Sky- line’ (1997). Median per-tree estimated carbon sequestration and estimated carbon storage were fairly similar between the 22- and 31-year-old cohorts, while median per-tree estimated pollution removal and esti- mated runoff removal were higher in the 30-year-old cohort (Figure 6). The ranges of per-tree estimated
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