282 Gilman and Grabosky: Growth Partitioning Three Years Following Structural Pruning The smaller of the codominant stems (“the codominant”) aver- aged 47 mm (1.9 in) in diameter (SD = 13 mm), and was pruned according to the targeted severity. The larger of the codominant stems (“the leader”) averaged 62 mm (2.4 in) in diameter (SD = 16 mm), and was not pruned. The smaller stem was chosen for pruning to eliminate any possible effect of diameter ratio on growth response. Both stems were marked close to the union so they could be measured in subsequent years at the same position. Pruning was applied by removing branches on the pruned codominant stem as one of four visual targeted pruning severities (TPS): 0% (control), 25%, 50%, or 75% of total foliage removed. Severities were visual estimates of the percentage of foliage re- moved from the pruned codominant stem. Visual estimates were made by two assessors standing next to the tree as stems were re- moved. Both assessors had to agree on the severity before pruning was considered complete. To quantify amount of biomass removed, a cross-sectional area (CSA) of each pruning cut (outside bark) was calculated using a diameter tape and summed for each pruned tree. Ratio of CSA removed to CSA at base of the codominant was termed CSAR. Pruning severities were arranged in a randomized complete block design with 12 blocks and four trees per block. One tree in each block randomly received one of four severities. One to five pruning cuts (mean 2.9) were made on each pruned stem to attain the TPS; most cuts were reduction cuts and some were removal cuts (Gilman and Lilly 2008). The largest diameter, most upright portion of the codominant stem closest to the unpruned leader was removed first; others were removed, typically toward the end of the codominant stem, as needed to attain targeted sever- ity. This structural pruning type is described in detail in Gilman and Lilly (2008). In September 2006, June 2007, and May 2008, diameters of pruned codominant stem and unpruned leader on each tree were measured in the same position as described above. The relationship between branch or stem diameter and fresh foliage mass was calculated by choosing three trees at random from the source nursery field. Six branch and 6 stem segments were paired (for a total of 12) by equal diameter for the following diameters: 0.8 cm (0.3 in), 1.5 cm (0.6 in), 2.3 cm (0.9 in), 3.0 cm (1.2 in), 3.8 cm (1.5 in), and 4.6 cm (1.8 in); these represented or- thogonal values within the range of pruning cut diameters. Diam- eter was measured with a diameter tape at the base of each branch or stem 5 cm beyond any swelling associated with the union. Branches were less than one-third diameter of trunk measured just above the union, and stems were at least three times branch diameter at the union. All leaves were carefully pulled from the segment distal to the diameter measurement and weighed fresh. Fresh weight was used because arborists work with live trees. A least squares regression line was fitted to stem or branch diameter and foliar fresh weight separately, then data combined for stems and branches because slopes of both relationships were statis- tically similar (t-test, P > 0.05; slope and intercept P > 0.05). The percent increase in CSA growth of the codominant stem was calculated as [(CSA end of project – CSA at initial pruning) ÷ CSA at initial pruning] x 100. Each year, CSA growth of the pruned codominant stem and unpruned leader stem was calculat- ed as CSA current – CSA previous measurement. Stem diameter was measured with a diameter tape at the base of each pruned and unpruned stem 5 cm beyond any swelling associated with the union. Trunk diameters were measured at the beginning of project and annually with a diameter tape at 30 cm (12 in) from ground. CSAR was regressed onto TPS. Fresh leaf mass was re- gressed onto CSA of branches from which leaves were removed. The percent increase in CSA growth of the codominant stem was regressed onto CSAR. Linear relationships were analyzed with Minitab version 14 (Minitab Inc., State College, PA) and considered significant at P < 0.05. CSA on the codominant and leader, and ratio of diameter of the codominant to the di- ameter of the leader stem were analyzed by repeated measures two-way analysis of variance within a randomized complete block design using GLM in SAS 9.1.3 (SAS Institute, Cary, NC) with P < 0.05. Main effects were pruning severity and time. Trunk diameter was analyzed by repeated measures one- way analysis of variance with pruning severity as the main ef- fect. Means were compared using Tukey’s multiple range test. RESULTS The number of pruning cuts required to achieve TPS increased with pruning severity from 2.7 for 25% severity to 2.9 for 50% severity, and 3.6 for 75% severity. Pruning cut diameter ranged from 8 mm (0.3 in) to 48 mm (1.9 in) across trees in all treatments. CSAR correlated well with TPS but varied widely within each TPS (Figure 1). This large range is attributed to variation in the visual estimate of foliage removed because there was an excellent relationship between CSA removed and actual foliage mass re- moved (Figure 2). Since slope in Figure 1 was close to one (0.99), mass of foliage removed from a live oak stem might be predict- able visually by a trained assessor, although Smiley and Kane (2006) also found considerable variability in visual estimates. The CSAR of one pruned stem exceeded the CSA of the stem base and on two other pruned stems was close to 100% CSAR (Figure 3). Since these were all 75% pruning severity treatments, there was a visual 25% of foliage remaining on these three stems. Increasing pruning severity slowed percent increase in CSA growth in proportion to amount of foliage removed on the pruned codominant stem (Figure 3). CSA growth on pruned codomi- nant stems at 25, 50, and 75% TPS was less than on codomi- nant stems on unpruned control trees in the first growing season after pruning (Figure 4A). This indicated that pruned codomi- Figure 1. Relationship between percentage of cross-sectional area removed on the codominant (CSAR) and targeted pruning severity (TPS). CSAR = 0.99 (TPS) + 8.18. r2 intercept P = 0.19. The dashed lines represent a 95% confidence interval for the regression equation. = 0.69, slope P < 0.001, ©2009 International Society of Arboriculture
November 2009
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