Arboriculture & Urban Forestry 35(3): May 2009 already wet soil to retain additional infiltration lead to similar soil moisture contents beneath pervious and impervious pavements. Given that ments, it was expected that soil oxygen may have been limited. The presence of smooth black or matte gray oxidation on buried steel rods was an indicator of anaerobic conditions. Rods exhib- iting these corrosion patterns received higher anaerobic scores. While anaerobic scores were distinctly lower in unpaved soils than in paved soils (Figure 5), large variation in the data resulted in only a few statistically significant differences (Table 3). The data confirmed that aeration within treatments was independent of soil depth; anaerobic scores were statistically similar throughout the soil profile within all treatments. However, soil aeration did differ among treatments within depth classes. In the uppermost soil layer, anaerobic scores were significantly lower in control plots than in either paved treatment. The only other statistically significant difference occurred in the deepest soil layer where im- pervious pavement resulted in lower aeration than control plots. The similarities in aeration beneath both pavement treatments mirrors the observed similarities in soil was consistently above FC beneath paved treat- inverse relationship between air and water occupying the soil pore space. Because oxygen diffuses through air c. 7500 times faster than through water (Feng et al. 2002), this was consistent with the lower degree of corrosion on rods buried beneath pavements. While paved soils are evidently more poorly aerated, the ques- soil (Figure 1), confirming the CONCLUSION tion of whether or not their aeration is adequate for root respira- tion is inconclusive. In paved soils, rods were covered by a varie- gated combination of bright orange and dark corrosion, a pattern representing oxygen concentrations between 2%–5% (Owens et al. 2008), well below the critical limit of 10% for root function (Glinski and Stepniewski 1985). This suggests that aeration of vir- Soil water content beneath both pavement treatments was ex- ceptionally high throughout the growing season, never drop- ping below the soil’s field capacity. Also, soil aeration beneath pavements was much lower than under unpaved soils. In addi- tion to better aeration, the soil moisture in unpaved soils was within the bounds of the LLWR. Soil moisture increased pre- dictably with depth in the unpaved plots; however, both pave- ment types modified this distribution. Under impervious pave- ments, the wettest soil was often in the uppermost 10 cm, while under pervious paving the deepest layer was wettest, followed by soil at 5 cm, then 10 cm depth. Daily fluctuations in also differed between paved and unpaved soils. Whereas un- paved soils are subject to large daily fluctuations, paved soils are buffered from change, especially under impervious paving. The observed differences in soil paved soils are likely driven by the distillation process and the buffering effects of paving. During daytime, pavements present an evaporation barrier, whilst at night, cooling sur- face temperatures result in upward distillation of moisture be- neath paved treatments. Together, these mechanisms result in relatively high soil moisture at shallow depths beneath paving. It was expected that the physical differences between pervious soil between paved and un- and impervious pavement would lead to differences in underly- ing soil moisture and aeration. However, this was generally not the case. Exceptions were better soil aeration at depths 39–45 cm (15.4–17.7 in) below the soil surface under pervious paving; and small, temporary increases in ments , the result of isolated rainfall events. However, aeration were broadly similar beneath both pavement treatments. In summary, under the conditions of this experiment, the hy- soil beneath pervious pave- soil and Figure 5. Evaluation of soil aeration. The mean anaerobic score (n = 5) for all treatments stratified by depth beneath the soil surface. Greater anaerobic score corresponds to decreased soil oxygen. Control plots exhibit greater aeration than soil beneath pavement. pothesis that soil moisture and aeration differ beneath pervious and impervious paving is not supported. However, soil physical conditions beneath both types of paving do differ markedly from unpaved soils. If pervious paving does benefit urban trees relative to impervious paving, as alluded to by other authors, it is unlikely that the benefits arise from improved water or aeration dynamics. 139 Table 3. Mean anaerobic scores and standard errors during summer 2008. Significant differences resulting from treatment are noted by different symbols following the mean value. (P = 0.05) Treatment Control Impervious Pavement 0-12 Depth below ground (cm) 12-24 24-36 Porous Pavement 5.45 (1.85)b 6.55 (2.58)a 36-48 0.15 (0.22)a 0.55 (0.82)a 1.70 (1.87)a 1.15 (1.42)a 6.80 (2.67)a 6.30 (2.51)ab 4.75 (2.22)b 5.00 (2.66)a 5.80 (2.63)a 9.30 (1.98)b tually the entire soil profile beneath paved treatments limits root function. Conversely, Watson (2006) determined that fine root density decreased consistently only when rusting occurred on less than 25% of the rod. This occurred only in deep layers [36–48 cm (14.2–18.9 in)] under impervious paving, implying that only the deepest soils beneath impervious surfaces would limit root func- tion. This conflicting information prevents conclusions on wheth- er soil aeration below pavements is adequate for plant function. ©2009 International Society of Arboriculture θ θ θ θ θ θ
May 2009
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