48 Table 2. Morgenroth: Root Growth Response of Platanus orientalis to Porous Pavements -values for single degree-of-freedom contrasts comparing the effect of pavement type and profile design on root abundance within root diameter and soil depth classes. 0-5 Contrast 1. Control versus all other treatments 2. Main effect (pavement profile design) 3. Main effect (pavement type) 4. Interaction 0.475 0.782 0.503 0.813 0.602 0.523 0.922 0.691 0.305 0.844 0.381 0.426 0.713 0.480 0.994 0.868 0.995 0.999 0.117 0.823 0.831 0.046 0.968 0.523 0.005 0.849 0.546 0.086 0.216 0.426 0.404 0.621 1.0 (pavement profile design × pavement type) DISCUSSION The results support the hypothesis that root biomass differs be- neath porous and impervious pavements but only in the absence of a compacted subgrade and gravel base (Figure 1). Given that coarse roots contribute more to total root biomass than fine or medium fractions (Misra et al. 1998), it was believed that treat- ment differences would also arise in coarse root abundance. Within individual depth classes, this was rarely true, possibly due to low overall frequencies and high within-treatment variation (Figure 3). Nevertheless, a closer look was warranted, given the propensity for coarse roots to contribute to conflicts with pave- ments (Nicoll and Armstrong 1998). Further inspection revealed that coarse root abundance, like root biomass, increased beneath porous pavement, but only in plots without a compacted sub- grade and gravel base. The mean number of coarse roots found beneath PP treatments was 12.2, compared with only 6 for IP treatments. Mean values for control (3.9), PP+ (5.6), and IP+ (5) treatments were all comparable to IP. Of particular interest, data revealed that in the uppermost 10 cm of soil in control plots, no coarse roots were present, while paved plots had mean abun- dances ranging from 2.6–3.9 coarse roots per plot depending on treatment. This is noteworthy because coarse roots at shal- low depths have the potential to conflict with overlying pave- ments by deforming adjacent soil during radial growth (Nicoll and Armstrong 1998). It’s likely that deeper root distribution for control plots is a response to diurnal temperature variation (Hillel 1998) and fluctuating moisture (Morgenroth and Bu- chan 2009) which readily occur at shallow depths. In contrast, coarse roots did grow in shallow soil beneath pavements, where temperature and soil moisture were presumably more stable. Though high within-treatment variation compromised the sta- tistical significance of the analysis, it is reasonable to suggest that coarse root abundance and biomass indicate the potential for pavements and, more specifically, porous pavements without a compacted subgrade and gravel base to result in more, larger roots with the propensity to conflict with overlying pavements. With greater than 90% of roots in the uppermost 20 cm of soil (Figure 2), the root systems studied in this experiment were consistent with other root systems (Gilman 1990). It was in this 20 cm soil layer that all treatment-related differences in root abundance existed. Results showed that root abundance ©2011 International Society of Arboriculture was greatest in control plots at 20 cm, whereas maximum val- ues were reached in paved plots between 10–15 cm. This differ- ence is likely, indirectly or directly, in response to available soil moisture. As part of a larger experiment, Morgenroth and Buchan (2009) monitored soil moisture for all treatments in this experi- ment; they showed that, in control plots, soil moisture increased with depth. Root branching and growth are known to increase under optimal soil moisture conditions (Ruark et al. 1983), pre- sumably to take full advantage of the water resource, but also because soil strength is reduced at greater soil moisture, preclud- ing physical obstructions to root growth. Thus, it’s likely that the uppermost soil layer in control plots, which was highly prone to moisture fluctuations and likely prone to temperature variation (Hillel 1998), dissuaded root growth, while the deeper layers with more stable soil moisture and temperature promoted root growth. In contrast to control plots, vertical root distribution beneath pavements was concentrated higher in the soil profile. Much as it did in control plots, high soil moisture may have promoted root growth in paved plots, beginning at shallower levels. This is because in paved plots, there was no dry zone, instead high soil moisture was found directly beneath pavements and extended deep into the soil profile (Morgenroth and Buchan 2009). Another explanation for shallow root growth beneath pavements also pertains to soil moisture, but in an indirect manner. The high soil moisture be- neath pavements may have acted as a barrier to oxygen diffusion, leading to the relatively anaerobic conditions found deeper in the soil profile (Morgenroth and Buchan 2009). Since one response of roots to anaerobic soils is to remain near the soil surface (Dittert et al. 2006), it’s possible that shallow root growth in this experi- ment was in response to anaerobic conditions present in the deep- er soil layers beneath paved surfaces. Tree species with different tolerances to soil anaerobiosis would certainly have differed in their response as has been seen in other studies (Day et al. 2000). Root abundance and distribution were also affected by pave- ment profile design. Trees exhibited relatively shallow root growth in paved plots designed with a compacted subgrade and gravel base (Figure 2). Differences in root abundance were significant between 10–20 cm, where abundance was greater in IP and PP plots (Table 2, contrast 2). To explain these dif- ferences, we must consider how pavement profile design af- fected soil physical conditions, in particular, soil compaction. 0.568 0.995 0.999 <2 2-5 >5 <2 5-10 2-5 >5 <2 Soil depth (cm) 10-15 Root diameter (mm) 2-5 >5 <2 15-20 2-5 >5 <2 20-25 2-5 >5 <2 25-30 2-5 >5 0.001 0.020 0.995 0.003 0.004 0.994 0.388 0.326 0.046 0.005 <0.001 0.013 0.621 0.895 0.994 0.325 0.995 0.999 0.924 0.155 0.722 0.232 0.632 0.946 <0.001 0.002 0.024 0.006 0.001 0.025 0.475 0.367 1.0 0.084 0.995 0.999 P
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