Arboriculture & Urban Forestry 44(4): July 2018 enced by soil respiration (i.e., the biological oxida- tion of organic compounds by microbes and plant roots that is a function of soil water content and the quality and quantity of easily decomposable organic matter in soil). Soil respiration is usually most intense during the late summer months, and ceases toward the end of the growing season. In frozen soil, fluctuations in the soil oxygen concen- tration are minor (Glinski and Stepniewski 1985). Depending on the tree species, roots per- 1985). The O2 form best at oxygen levels above 10% (Kozlowski 1985). Minimum thresholds for tree root survival and growth are estimated to be 3% and 5%–10%, respectively (Kozlowski and Davies 1975). Poor aeration leading to inadequate O2 concentrations hampers root respiration (McDowell et al. 1999; Gaertig et al. 2002), diminishes water and min- eral uptake, and retards protoplasm synthesis and cell membrane maintenance (Kozlowski 1999). In hypoxic conditions, the loss of fine roots (Kozlowski and Pallardy 2010) can restrict water and nutrient intake, resulting in poor tree growth and appearance (Krizek and Dubik 1987). While evidence on soil oxygen status effects on urban tree health remain unclear (Costello et al. 1991; Wat- son and Kelsey 2006), such effects are well-known from many other environments (e.g., Drew 1983; Glinski and Stepniewski 1985; Simojoki 2001). Besides the supply of water, nutrients, and air to tree roots, urban soils are expected to fulfill numerous other functions. Urban soil characteristics important for root growth have been reviewed by Craul (1985) and Watson et al. (2014), among others. While serving as a suitable rooting space for urban trees, the same material may be required to sustain the weight of the aboveground structures, paving, and traffic. Increasing the topsoil strength by mechani- cal tamping decreases macro-porosity and hinders plant growth via restricted air and water movement (Kozlowski 1999). Moreover, soil compaction limits root penetration, as the mechanical resistance of the soil increases (Grabosky et al. 2002). Consequently, trees at paved sites oſten suffer from chronic nutri- ent deficiencies and water stress (Flueckiger and Brown 1999), both conditions being detrimen- tal to tree growth (Kozlowski and Pallardy 2010). Increased attention to the growing conditions and value of urban trees has contributed to the concentration in soil air is influ- 175 development of landscaping applications, such as structural soil (Roberts et al. 2006). In this paper, researchers use the term structural soil as a shorthand for the designed load-bearing urban soil, although the term structural soil as used in soil science has a wider meaning, referring more generally to the variability in the spatial arrange- ment of soil particles and pores. The first structural tree soil experiments were initiated in the 1990s (Grabosky and Bassuk 1995; Kristoffersen 1998). In structural soils, the gravel fraction provides a skeletal stone structure that transfers surface loads to the subsoil, whereas the fine material between the stones serves as the actual rooting space. Follow-up studies have demonstrated that roots exploit the rooting space expeditiously (Grabosky and Bassuk 1996; Kristoffersen 1999; Grabosky and Bassuk 2016). The total tree growth in struc- tural soils has been demonstrated to be compara- ble with the growth in native topsoil (Kristoffersen 1999; Bühler et al. 2007), but some issues remain, such as the possibility of nutrient depletion (Loh et al. 2003; Smiley et al. 2006; Bühler et al. 2017). This study compared the soil air O2 concentra- tions between conventional and structural tree soils at two sites in Helsinki, Finland, which is in the hemiboreal bioclimatic zone. Furthermore, the impacts of urban tree soil cover on soil aeration were evaluated by comparing the relative gas dif- fusion coefficients. Researchers hypothesized: 1) the average O2 concentration is higher in structural centrations in different street tree establishments. MATERIALS AND METHODS than in conventional soil, and 2) cobblestone pav- ing impairs root-zone ventilation compared to cast- iron tree grating. The main objective was to gain empirical knowledge of fluctuations in soil O2 con- Study Sites Soil aeration was examined at two street tree sites in Helsinki, Finland. Soil air O2 concentra- conducted in July and August 2014. The con- ventional-soil site (60°10’26”N, 24°57’40”E) was a main road (Pohjoisranta) planted in the early 1990s with a 700-m-long row of approximately 70 lime trees (Tilia × vulgaris Hayne). The trees tions were recorded during the growing seasons of 2012 and 2013. CO2 ©2018 International Society of Arboriculture flux measurements were
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