2 Morgenroth and Visser: Aboveground Growth Response to Porous Pavements deposit of sand and gravel, remnant from the alluvial outwash deposited by an ancient glacier (Brown and Weeber 1992). The climate is temperate, and mean daily maximum tem- peratures range from c. 10°C in July to 21°C in January (Mc- Gann 1983). Occasional dry northwesterly winds occur dur- ing spring and summer, when temperatures can reach 30°C and relative humidity can drop to 20%–40% (McGann 1983). Rainfall ranges from 600–700 mm annually and is gener- ally evenly distributed throughout the year, with a tendency for slightly higher early winter precipitation (McGann 1983). Experimental Design Chosen due to their wide-spread use as street trees, 50 one-year old, bare root oriental plane (Platanus orientalis) seedlings were randomly assigned to plots in an augmented factorial experiment consisting of controls and four treatments; trees were split evenly among treatments, such that ten replicates existed per treatment. Herbicide applications were used, as necessary, to limit weed competition in control plots, which were characterized by an ex- posed soil surface. In early August 2007, seedlings were planted at grade in undisturbed soil in plot centers and, in the case of treat- ed trees, surrounded by concrete pavement pads, which had been installed in early July 2007. The pavement treatments, measuring 2.3 m × 2.3 m (with a 30 cm diameter circular cutout in the cen- ter), were based on the combination of pavement type (2 levels: porous, impervious) and pavement profile design (2 levels: +/- subgrade compaction and gravel base). The resulting four treat- ments were impervious concrete pavement (IP), impervious con- crete pavement with compacted subgrade and gravel base (IP+), porous concrete pavement (PP), and porous concrete pavement with compacted subgrade and gravel base (PP+). The distinction between the two levels of pavement profile design is related to the preparation of the profile below the pavement surface course. In IP and PP plots, profile preparation was limited to leveling the topsoil with a 500 kg roller. In contrast, in IP+ and PP+ plots, top- soil was removed to a depth of 20 cm, exposing the parent materi- al which was termed the subgrade. Then, a 20 cm deep base layer of washed, uniformly graded, 20–40 mm gravel was placed in the hole and leveled with a 500 kg roller. The difference between the two levels of pavement profile design are thus related to the inclusion (or exclusion) of a gravel base and the soil strength of the subgrade. Soil penetration resistance was measured via a soil compaction meter (Spectrum Technologies, Inc., Plainfield, IL, U.S.) in accordance with ASAE Standard EP542 (2002). Penetra- tion resistance values for the uppermost 30 cm of soil are mea- sured by a load sensor and displayed in kPa. Mean values differed significantly among treatments (P < 0.001) and were 892 kPa, 874 kPa, 808 kPa, 2458 kPa, and 2363 kPa for control, IP, PP, IP+ and PP+, respectively. Finally, IP and IP+ plots were overlaid by a standard impervious concrete, while PP and PP+ were overlaid by a pervious concrete designed specifically for this experiment. Data Collection Initial height and diameter were measured at the time of planting, in August 2007, prior to the first growing season; subsequent mea- surements occurred at the end of spring in the first growing season (December 2007), as well as the end of the first (March 2008) and second (March 2009) complete growing seasons. Tree height was measured as the distance between the soil surface and the tip of ©2011 International Society of Arboriculture Figure 1. The effect of pavement type and profile design on mean height growth of Platanus orientalis relative to control plots char- acterized by bare soil. Values represent total growth following two growing seasons. Error bars represent one standard error. the apical bud on the leader of each tree, while the diameter was calculated as the average of two measurements taken perpendicu- lar to one another 10 cm above the soil surface. Mean initial tree height was 62 cm and initial trunk diameter was 6.7 mm. Neither initial height (P = 0.663), nor diameter (P = 0.961) differed signif- icantly among treatments. Height and diameter growth were mea- sured as the absolute growth occurring over the duration of the ex- periment. On March 12, 2009, all trees were harvested at ground level. This plant material comprised aboveground biomass and was dried in a kiln at 70°C to constant weight (Nicholson 1984). Statistical Analysis One IP+ tree died between the first and second growing seasons and was thus excluded in all analyses. Stem height, diameter, and aboveground biomass were compared via one-way analysis of variance (ANOVA), using orthogonal, a priori, single degree- of-freedom contrasts to examine treatment effects, as well as in- teractions of interest (Marini 2003). Significant differences are reported for P < 0.05. Analyses were performed using the R sta- tistical package, version 2.8.1 (R Development Core Team 2008). RESULTS Height Growth While height growth was dependent upon treatment (P = 0.022), mean height growth of control trees was equivalent to all other treatments (Table 1, contrast 1), thus implying differ- ences among the four pavement treatments. Alone, the pave- ment profile design had no effect on tree height, as the mean height growth of all PP and IP trees did not differ from all PP+ and IP+ trees (Table 1, contrast 2). Nevertheless, height was significantly affected by the interaction between pavement type and profile design (Table 1, contrast 4). Further investigation showed that without subgrade compaction or a gravel base, trees surrounded by porous paving grew approximately 205 cm, while those surrounded by impervious paving grew only 160 cm. However, in plots with a compacted subgrade and grav- el base, a difference of less than 2 cm existed between trees surrounded by porous and impervious pavement (Figure 1).
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