Arboriculture & Urban Forestry 38(2): March 2012 each core (Blake and Hartge 1986). These values were verified after all testing was complete by oven drying and then weighing the dried cores to calculate ρb intact cores in the laboratory following Klute and Dirksen (1986). Tree Measurements tent, air-filled porosity (AP) was determined. Water content at the permanent wilting point and available volumetric water content (AVWC) were determined using a tension table and pressure plate extractors (Soil Moisture Corp. Santa Barbara, California, U.S.), ranging from 0.5 MPa to 1.5 MPa suction, following methodol- ogy of Klute (1986), for the 2002 and 2003 cores. These values were used for development of a water-retention assessment (data not shown). AVWC was calculated by subtracting the volumet- ric water content values at field capacity (-0.03 MPa) from those determined at permanent wilting point (-1.5 MPa). Due to labo- ratory time restrictions, these measurements were not taken in 2004. Saturated hydraulic conductivity (Ks . From ρb Soil Measurements Mulch and other organic matter were scraped from the surface, and nine intact cores (7.6 cm length × 3.8 cm radius) were taken from random locations within each whole plot (soil treatment) each year of the study, using a slide hammer according to meth- odology described by Campbell and Henshall (1991). Soil was sampled outside of the root zone of the trees, from the top 6 cm, which is where the majority of the absorbing roots are found (Per- ry 1982). Researchers weighed cores to obtain wet bulk density, and preserved the intact cores for hydraulic conductivity testing. Extra soil, obtained at sampling, was used to measure the gravi- metric water content (as described earlier) and to calculate ρb of and known water con- 67 ing the Li-Cor meter and dried/weighed as before. The remaining portion of the leaf canopy was dried/weighed separately. An esti- mated LA for the entire tree canopy was calculated by multiply- ing the area of the sample by the dry weight of the remaining por- tion of the canopy and dividing this value by the dry weight of the sample. In 2004, the same trees used for leaf measures were also used to determine stem dry weight (SDW). Stems were harvested at ground level, air-dried in an unventilated, dry polyhouse for one month, and then weighed. Air-drying was done due to limited space in the drying ovens and the large volume of stem materials. In 2004, upon completion of the study, a small sample (n = 7, 2 C1, 3 NC, and 2 C2 trees) of tree roots were completely excavated using an air trenching tool (Air-Spade® , Chicopee, Massachusetts, U.S.). Although provided with an insufficient number for statistical analysis, the study authors documented these root systems with photos and will discuss them anecdotally. Experimental design and data analysis ) was measured for the The study authors measured trees at the beginning and end of each growing season from 2002 through 2004. Caliper was measured at 15.2 cm above the ground at the beginning and end of each growing season. Two measurements of diameter were taken at a 90 degree angle from each other at the same height on the trunk, and averaged to determine caliper. Caliper growth was calculated based on beginning and end of season caliper measurements. In addition, caliper growth was calculated for the entire study period based on caliper at the beginning of the 2002 growing season and the end of the study in 2004. Cross-sectional area was determined as the area of a circle. As maples often have more than one domi- nant leader, total height was determined by averaging the mea- sured height (from ground level) of the three dominant leaders on each tree. Height growth was calculated from beginning and end- of-season height measurements for each year. Three trees were randomly selected from each sub sub-plot (cultivar), from each whole plot (soil) and sub-plot (N treatment) to assess leaf area (LA) and leaf dry weight (LDW). In 2004, only four cultivars were sampled to accommodate limited field time in the final year of the study. The following cultivars were selected because they were more commonly used in urban landscapes in Ohio: ‘Cel- zam,’ ‘Morgan,’ ‘Fairview Flame,’ and ‘Frank’s Red.’ All foliage was harvested from each tree prior to leaf drop each year. In 2002, all the leaves were used to determine LA, using a Li-Cor 3100 Area Meter (Li-Cor, Inc., Lincoln, Nebraska, U.S.). Leaves were oven-dried at 82°C for two to three days, and then weighed to determine LDW. In 2003 and 2004, random samples totaling 100 and 500 leaves, respectively, were used from the entire canopy of each tree harvested. The area of the samples was measured us- A split-split plot design was used. The three soil treatments were randomized into whole plots in each of three replicates. Each whole plot was split into two sub-plots with trees treated with low or high N grouped separately. Cultivars were randomized throughout each fertilizer sub-plot into sub sub-plots. Data were analyzed using SAS's general linear model procedure (PROC GLM) to determine significant differences between soil, fertilizer and cultivar treatments and interactions between each of these and all three (SAS Institute, Inc., Cary, North Carolina, U.S.). Com- parisons were made between these treatments using Tukey's hon- estly significant difference (HSD). Treatment differences were considered significant if P values were equal to or less than 0.05. The AP and Ks measurements were found to be well described dures. Mean values for soil parameters and aboveground growth were analyzed both within each year and from year to year. by a log-normal distribution. The logarithmic-transformed AP and Ks values were then analyzed using GLM and Tukey’s proce- RESULTS The Effect of Compaction Treatments on Soil Physical Characteristics The mean ρb over the three-year study was 1.42 g·cm-3 for crease bulk density over C1. Values for soil measurements did not change significantly from year to year (data not shown). In all years of the study, percent AP was significantly low- er in the compacted soils than in the NC soils with the excep- tion of 2004, when there was only a difference between NC and C1 soils. On average, NC soils had an AP of 11%, while the C1 and C2 soils had values of 5% and 4%, respectively. In all study years, NC plots had significantly higher mean Ks ments (Table 1). This reflects a 12% increase over the mean pre-existing ρb the NC soils and 1.59 g·cm-3 for the two compaction treat- . The C2 treatment did not significantly in- than soils in either compaction treatment, although these did not significantly differ from one another (Table 1). Ks val- ues were highly variable across all samples likely due to the exceedingly heterogeneous nature of field soils. This is com- mon with such measurements (Coutadeur et al. 2002). Despite the variability, all NC sample Ks and log(Ks ) values were above ©2012 International Society of Arboriculture
March 2012
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