Arboriculture & Urban Forestry 39(1): January 2013 the nearest 0.1 g to obtain green weights (Gw). The discs were dried in an oven at 104°C for 24 hours and reweighed to obtain dry weights (Dw). The oven dry volume (VD ) of each disc was determined by first dipping the entire 2.54 cm wood sections in molten paraffin wax and allowed to drip dry to ensure only a thin wax seal remained (the ratio of the volume of wax to that of the wood was considered negligible and is one limitation of this protocol). The wood discs were then totally submerged in a graduated (cm3 water represents the oven dry volume (VDcm3 ) container of water the volume of the displaced ) of the wood disc. Two evaluations were carried out as per standard wood mea- surement protocol (USDA; Forest Products Laboratory 1987; also Farrell 2003) on disc samples extracted within the ZF: (i) Percent moisture (Pmoist ) = [(Gw - Dw) / Dw] × 100 (ii) Spec. Grav. (oven dry basis) (SGDw) = [oven dry weight DW (g) / VD (cm3)] / 1 g/ cm3 (density of water) Total Linear Cracks Cracks present on the surface of the cross section of the wood disc samples that faced the fracture point were measured after first dusting with talc powder and shaking excess powder off. Visible cracks were then measured using a divider and add- ed cumulatively to produce a total linear value (cm) (TLC). Statistical Analysis Moments and bending stresses at the fracture point, % (LFU Moisture content Pmoist , specific gravity SGDw , and TLC were LB compared across treatments using ANOVA. Means were sepa- rated using the Scheffe Grouping, SAS statistical software (SAS Institute, Cary, North Carolina, U.S.). Percentage data for (% LFU LB ) was square root transformed before analysis. Study Part 2: Evaluation of the Resistance of Stem Tissue at Two Drill Heights in Two Groups of Ash Trees Ash trees ranging in diameter at breast height from 30 cm to 50 cm were selected in recreation park environments in and around the city of Perrysburg, Ohio. Two groups were selected for this trial similar to Group I and Group III trees in Part 1 of this study. Each group consisted of five trees. Trees used in Study Part 1 were not used in Study Part 2. An F400-S model (IML, Atlan- ta, Georgia, U.S.) was used to evaluate the resistance to drill- ing in the stems of the two ash tree groups. Measurements were taken in two directions (north to south and east to west) at the base and at 1 m height of stems of all tree replicates. An air- spade (155 cfm) was used to excavate around the base to provide clear access if needed. Stem diameter at each drill height was measured using an arborist diameter tape. The height resistance readings (from chart printouts) were read in millimeters at 1.27 cm increments into the stem (reflected as a percentage of the stem diameter at successive increments). Resistance (mm) val- ues obtained for drill points (NS or EW) at the base or at the 1 m height for trees in Group I were plotted separately against the corresponding incremental increase in percentage stem di- ameter. Separate scatterplots were thus obtained for each height (and direction) of drill point for all trees in Group I. This was repeated for resistance (mm) values obtained for drill points at the two stem heights of the trees assessed in Group III. ), Statistical Analysis Regression analyses with height of drill point as an indicator variable were conducted for each group of trees using SAS statistical software. RESULTS AND DISCUSSION Study Part 1 Moments and Bending Stresses Moments and bending stresses calculated at branch frac- ture were not significantly different (F = 0.47, DF = 3,38, P = 0.70 and F = 0.74, DF = 3,38, P = 0.53, respectively) among the three groups of ash trees tested (Table 1). These data are further discussed within the context Pmoist and TLC. Comparison of Percentage Distance from Fracture Point to Union to Entire Branch Length (%LFU The %LFU LB est in Group I trees and lowest in Group III trees which may suggest a possible shift in fracturing towards the union. Good- fellow (2009) identified a critical zone of failure within twen- ty percent of the branch length to the union with the main stem. The frequency of branch breakage also peaked in this region in this study (Figure 2). Of the 45 branches broken in total, six fractures or 13.3 % occurred at the point of union with the stem (LFU cantly (F = 63.00, DF = 3, 18, P < 0.0001) different among the groups (Table 1). The mean ± SE %LFU LB comparisons from broken branches were signifi- LB values were high- ) Across Groups 13 = o), these branches were all within a range of 3 to 7 cm in diameter. Interestingly three branches were from Group II trees and three originated from Group III trees. The Zone of Fracture for Pmoist, SGDW Pmoist , and Wood Strength calculated for wood disc samples taken from the ZF of the three groups of ash trees was not significantly dif- ferent among groups (F = 0.73, DF = 3, 37, P = 0.54) (Table 1). While specific gravity of wood may be correlated with strength in trees (Zoebel and van Buijtenen 1989; Niklas 1997), other studies, such as Lilly and Sydnor (1995), found that although Norway maple (Acer platanoides L.) had a sig- nificantly higher specific gravity than silver maple (Acer saccharinum L.), no significant variation was observed in bend- ing stresses at fracture point between the two maple species. trees in Groups I and II, which were statistically homogenous with each other (Table 1). Despite variable wood moisture content this did not directly affect maximum stress at failure or strength upon static loading. Farell (2003) reported similar findings between variable moisture content with harvest date but found similar strength at breaking was observed in sawtooth oak (Quer- cus acutissima Carruthers) and red maple (Acer rubrum) crotch- es. Wood moisture decreases have generally been more aligned with increase in strength of harvested wood in the traditional sense. In the context of living trees, drier wood may not necessar- ily constitute a stronger tree. Significantly lower wood moisture coupled with rotational forces from wind loading or incidental loading from snow or ice could increase risk of wood failure. SGDW ©2013 International Society of Arboriculture was significantly (F= 10.80, DF= 3, 38, P < 0.001) lower in Group III trees compared to Pmoist from sample discs cut from
January 2013
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