182 Kämäräinen et al.: A Case Study of Street Tree Soil Aeration in Two Different Soil Types In contradiction with the second hypothesis, soil aeration was not seriously interrupted or consider- ably slowed down by the cobblestone paving laid on macadam with wide joints. The study results differ from those of Weltecke and Gaertig (2012), who reported no significant differences between relative diffusion coefficients obtained for cal urban soil cover types: asphalt, flagstones, and typi- ficient of the sand that was used for the joints. The results of Koolen et al. (2000) suggested that brick (20 cm × 5 cm × 10 cm) paving with a 7.7% relative area of joints caused several magnitudes lower resis- tance to belowground gas exchange than asphalt. The D/D0 is an index of soil aeration and usually var- ies between 0.003 and 0.2, reaching 0.5 in loose soils (Glinski and Stepniewski 1985). Researchers mea- sured D/D0 with simultaneous upsurges in the soil water con- tent. These weeks were associated with heavy rain- fall. Because the surrounding pavements conducted storm water toward the planting baskets, it is pos- sible that the conventional soil reached a water- ing water content (Hillel 1998). During weeks 21 and 33 of the latter growing season, decreases in the conventional soil O2 The diffusion of O2 saturated state on these occasions. Similar low O2 concentrations were not observed at the structural- soil site during these periods, indicating that struc- tural soils may be better suited to sites prone to water-logging, such as stormwater infiltration areas. The growth and oxygen consumption of increase with rising tree roots temperature (Glin- parison to conventional tree soil at the study sites. ©2018 International Society of Arboriculture ski and Stepniewski 1985). Hence, it is probable that the lower temperature of the structural soil promoted its higher O2 concentrations in com- values in this range at both studied sites. in soil is retarded by increas- concentration were detected, cobblestone-coating. However, the numerical D/D0 values determined in this study cannot be compared with the absolute values reported by Weltecke and Gaertig (2012). Vertical and horizontal gas move- ment through washed macadam, used in laying the cobblestones and in the wide joints between them, could explain the differences between the findings of the current study and previous research. Koolen et al. (2000) presented gas diffusion coefficient val- ues for a range of paving materials and concrete ele- ments. They estimated the gas exchange through jointed paving by measuring the O2 diffusion coef- Acknowledgments. The research sites were established in collabo- ration with the City of Helsinki Public Works Department. This work was partly financed by the Academy of Finland’s Centre of Excellence (grant no. 272041). We thank Janne Järvinen and Mari Mäki for their help with field data collection, technical assistance, and instrument maintenance. LITERATURE CITED Arnold, H.F. 1993. Trees in Urban Design, second edition. Van Nortstrand Reinhold, New York, New York, U.S. 197 pp. Bartholomeus R.P., J.-P. Witte, P.M. van Bodegom, J.C. van Dam, and R. Aerts. 2008. Critical soil conditions for oxygen stress to plant roots: Substituting the Feddes-function by a process- based model. Journal of Hydrology 360(1–4):147–165. Becket, K.P., P. Freer-Smith, and G. Taylor. 2000. Effective tree spe- cies for local air-quality management. Journal of Arboriculture 26:12–19. Bühler, O., M. Ingerslev, S. Skov, E. Schou, I.M. Thomsen, and P. Kristoffersen. 2017. Tree development in structural soil–an empirical below-ground in situ study of urban trees in Copen- hagen, Denmark. Plant and Soil 413:29–44. Bühler, O., P. Kristoffersen, and S.U. Larsen. 2007. Growth of trees in Copenhagen with emphasis on the effect of different estab- lishment concepts. Arboriculture & Urban Forestry 33:330–337. Costello, L.R., J.D. MacDonald, and K.A. Jakobs. 1991. Soil aeration and tree health: Correlating soil oxygen measurements with the decline of established oaks. pp. 295–299. In: R.B. Standiford (Ed.). Proceedings of the Symposium on Oak woodlands and hardwood rangeland management. USDA Forest Service Tech- nical Report PSW, Davis, California, U.S. Craul, P. 1985. A description of urban soils and their desired char- acteristics. Journal of Arboriculture 11:330–339. Drew, M.C. 1983. Plant injury and adaptation to oxygen deficiency in the root environment: A review. Plant and Soil 75:179–199. However, the general effect of the temperature differences appears somewhat irrelevant due to the similar O2 differences during the cooler peri- ods. Additionally, oxygen consumption in the soil depends on soil organic matter content, which was relatively low on both sites overall, but in soils with high organic matter, soil O2 consumption may be very significant (Glinski and Stepniewski 1985). The intrinsic limitation of this study arises from measurements being executed at only two sites. Hence, the results should only be extrapolated with caution. Further research is needed to evaluate the effects of environmental factors on the air compo- sition in structural soil. Nevertheless, researchers conclude that structural soil is able to maintain similar or higher soil air oxygen concentrations under a pavement than an unsealed conventional tree soil, and recommend considering the use of structural soils for hypoxy-sensitive species.
July 2018
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