Arboriculture & Urban Forestry 45(6): November 2019 Arboriculture & Urban Forestry 2019. 45(6):239–253 URBAN FORESTRY ARBORICULTURE Scientific Journal of the International Society of Arboriculture & Soil Compaction Affects the Growth and Establishment of Street Trees in Urban Australia By Gregory M. Moore, Alicia Fitzgerald, and Peter B. May Abstract. Growing conditions for street tree roots are generally harsh with restricted space and soils compacted from streetscape infrastructure. Allocasuarina littoralis, Corymbia maculata, Cupressus sempervirens var. stricta, Eucalyptus polyanthemos, Lophostemon confertus, Olea europaea, Quercus palustris, and Waterhousea floribunda were grown in compacted and uncompacted soils for 20 months in experimental blocks. The bulk density and penetrative resistance of the soils and height, canopy spread, trunk diameter, leaf area, and chlorophyll fluores- cence were measured regularly. Root and shoot biomass were determined after harvesting. Since the bulk density of compacted compared to uncompacted soil was root growth limiting, it was hypothesised that species would have reduced growth in compacted soils. However, C. mac- ulata and E. polyanthemos grew better, C. sempervirens, Q. palustris, and W. floribunda grew well, and A. littoralis, L. confertus, and O. euro- paea were the worst performing in compacted soil. E. polyanthemos, L. confertus, and Q. palustris had higher canopy:root ratios in compacted soil. Q. palustris had greater mass below ground than above, which has implications for its use in confined sites. In a field study, C. maculata, E. polyanthemos, L. confertus, O. europaea, and Q. palustris growing as street trees were surveyed to determine their rates of establishment and growth under urban conditions. In addition to the soil and tree parameters mentioned above, a Visual Tree Assessment (VTA) was undertaken. E. polyanthemos had the largest trunk diameter, height and canopy spread, indicating its potential for rapid establishment in streets. It was the only species with a larger mean leaf area in compacted soil. E. polyanthemos and O. europaea were the only species classed as healthy from chlorophyll fluorescence but there was no significant difference in fluorescence between compacted and uncompacted soils. VTA showed that C. maculata and O. europaea performed best and that E. polyanthemos, L. confertus, and Q. palustris had reduced but acceptable growth in compacted soil. Soils ranged from non-saline to moderately saline and were slightly to strongly acidic. All soils were compacted to some degree and penetrative resistance was at root limiting levels. The results suggest that careful species selection and soil amelioration for species prone to the effects of compaction would facilitate street tree establishment. Keywords. Australian Street Trees; Soil Compaction; Street Tree Establishment; Street Tree Growth. 239 INTRODUCTION Street trees are vital to the urban environment but can be severely site-constrained by urban development (Trowbridge and Bassuk 2004). Urban soils are cre- ated by intense human activity and are commonly of poor quality due to chemical and physical constraints with increased bulk density, soil strength, and pene- trative resistance usually due to compaction (Craul and Craul 2006; Roberts et al. 2006; Hazelton and Murphy 2011; Fitzgerald 2012). Soil compaction can have a major impact on tree growth and establishment by altering the pore alignment from vertical to parallel to the soil surface (Gregory et al. 2007). Roots either grow through existing soil pores or move particles aside but can only grow through a rigid soil pore with a diameter greater than the root. When roots encounter pores smaller than their diameter and cannot exert enough pressure to expand the pore, they are deflected (Kozlowski 1999). When a site is relatively undisturbed, local species may prove superior for use in urban landscapes (Smith and Moore 1996), but species from harsher climates often out-perform native species in urban sites (Watson and Himelick 1997). Understanding adaptations to drought, waterlogging, and poor soils can be useful in selecting native trees for urban use where tree establishment can be problematic (Moore 2003; Sæbø et al. 2003; Jutras et al. 2010; Moore 2013). Degraded city soil conditions make it difficult to find different species tolerant of urban conditions (Phillips 2008; Urban 2008) and so a diverse group of species was chosen for this research. ©2019 International Society of Arboriculture
November 2019
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