46 Morgenroth: Root Growth Response of Platanus orientalis to Porous Pavements (P < 0.001); mean values were 892 kPa, 874 kPa, 808 kPa, 2458 kPa, and 2363 kPa for control, IP, PP, IP+, and PP+, respectively. Data Collection Root growth and distribution was quantified by measuring total root biomass, as well as categorizing the abundance of roots by depth and diameter. Following concrete pavement removal in March 2009, a square trench was dug around each tree using an Air-Spade® (Concept Engineering Group, Inc., Verona, PA, U.S.) to expose roots. This technique allowed roots, ranging in size from fine through coarse, to be exposed, counted, and measured (Nadezhdina and Čermák 2003). Trenches measured 20 cm wide × 50 cm deep, and the distance between the tree stem and the nearest point on the inside wall of each trench was 100 cm. Root abundance in the trenches was categorized into three discrete root diameter classes (fine: < 2 mm, medium: 2–5 mm, and coarse: > 5 mm), and six depth classes each comprising a 5 cm deep soil layer. Following collection of abundance data, the excavation tool was used to remove all remaining soil surrounding each tree, allowing excavation of entire root systems. Whole root systems were placed in a kiln and dried at 70°C to constant weight (Nicholson 1984). The cumulative proportion of roots from the soil surface was calculated for all trees. Follow- ing Gale and Grigal (1987), an asymptotic nonline- ar model was used to describe vertical root distribution: [1] Y = 1 - βd where Y is the cumulative proportion of roots counted be- tween the soil surface and depth d in centimeters, and β is the estimated parameter. β was used as a relative index of the vertical root distribution across treatments. High values of β are associated with relatively deep root systems, where- as low values indicate proportionally shallow root systems. Statistical Analysis One IP+ tree died between the first and second growing seasons and thus was excluded in all analyses. No roots were found be- low 30 cm soil depth, so analyses were limited to the uppermost 30 cm. Root abundance data were analyzed using a generalized- linear model with a quasi-poisson distribution. Treatment dif- ferences were determined via analysis of deviance (Crawley 2007). Estimated β coefficients (Equation 1) and belowground 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 interactions of interest (Marini 2003). All significant differences are report- ed for P < 0.05. Analyses were performed using the R statisti- cal package, version 2.8.1 (R Development Core Team 2008). RESULTS Root Biomass Belowground biomass depended on treatment (p = 0.002). Mean root biomass of control plots did not differ from pavement treated trees, however, there were differences related to pave- ment type and profile design, as well as their interaction (Table 1). In IP+ and PP+ plots, root biomass was unaffected by pave- ©2011 International Society of Arboriculture ment type, but in the absence of a compacted subgrade and gravel base, root biomass beneath porous pavements significantly ex- ceeded root biomass beneath impervious pavements (Figure 1). Figure 1. The effect of pavement type and profile design on mean root biomass of Platanus orientalis. Error bars represent one standard error. Vertical Root Distribution Root allocation was greatest at shallower depths with over 90% of roots, irrespective of treatment, growing in the uppermost 20 cm of soil (Figure 2). The index used to measure vertical root distribution, β, ranged from 0.900 to 0.937 (Figure 2), where higher values signify relatively deeper root distribution (Gale and Grigal 1987). Control plots had comparatively higher β values than paved plots (Table 1, contrast 1), indicating that proportionally more roots grew deeper than in paved plots. Figure 2 illustrates this well; only c. 53% of roots from control trees grew in the uppermost 15 cm of soil, where- as the percentage of roots growing in this same 15 cm soil layer for pavement-treated trees was greater, ranging from c. 75% to 84%. Changes in pavement profile design also affected vertical root distribution (Table 1, contrast 2); roots grew relatively deeper un- der pavements without a compacted subgrade and gravel base, thereby resulting in a relatively higher β values for IP and PP plots (Figure 2). Pavement profile design differences were most prevalent in the uppermost 10 cm, where c. 42% of roots grew in IP and PP plots, in contrast to c. 56% of roots from IP+ and PP+ plots (Figure 2). No pavement-type effect existed, imply- ing that vertical root distribution beneath porous and impervious pavements was equivalent, regardless of pavement profile design. Figure 2. The effect of pavement type and profile design on cumulative root abundance (Y) with increasing soil depth (d). Mean β values (1 s.e.) indicating vertical root distribution were derived from Y = 1 - βd .
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