250 tree. However, having the longest roots of any species in both compacted and uncompacted soil, E. polyan- themos also has potential for damaging surrounding infrastructure earlier than other species. O. europaea is regarded as being a resilient and adaptable urban tree in Melbourne, with this research confirming that it grows well in both compacted and uncompacted soils. However, the data show that it grows best and would establish more rapidly in uncompacted soils, so if rapid establishment and growth are desired, then it would be wise to amelio- rate compacted sites. Similarly, the performance of L. confertus as a street tree will be enhanced if it is planted into uncompacted or ameliorated soils. If slower growth and establishment are tolerable, plant- ing into compacted soils will provide a satisfactory outcome. In compacted soil, A. littoralis was shorter with a wider canopy, which may be desirable in some land- scapes. In such situations, soil compaction could be used to manipulate tree growth and development, which warrants further research. A. littoralis had the shortest, shallowest roots of the eight species in com- pacted and uncompacted soil, and as a consequence of this root architecture may cause less damage to surrounding infrastructure (Moore 2013). C. maculata, C. sempervirens, and E. polyanthe- mos had longer roots growing in compacted soils compared to uncompacted soils, demonstrating that they were suitable choices for hostile soil conditions. E. polyanthemos, L. confertus, and Q. palustris had higher canopy:root ratios in compacted soil, meaning these species put on more canopy growth in com- pacted soil. For these and for other species which showed greater growth in compacted soils, such as C. maculata, A. littoralis, and E. polyanthemos, the increase in aboveground growth did not appear to come at the expense of root system establishment, allaying concerns that increased above-growth may mean that root systems were compromised by the retention of photosynthate in the aboveground parts of the plant. However, after 20 months, all trees were growing well and were in good condition. C. maculata, C. sempervirens, and E. polyanthe- mos had longer roots in compacted soil, while A. lit- toralis, L. confertus, O. europaea, Q. palustris, and W. floribunda had longer roots in uncompacted soil, suggesting that they have reduced root growth in compacted soil. Roots were significantly longer in the ©2019 International Society of Arboriculture Moore et al.: Growth and Establishment of Australian Street Trees northerly direction, probably due to higher soil tem- peratures from a lack of self-shading. It is often assumed that trees will have smaller leaves if they are grown under stressful conditions. However, there was no difference between leaf sizes for trees growing in compacted and uncompacted soils. Indeed, trees growing in the compacted soils of Hume had larger leaves on average (for O. europaea, twice the size) than trees growing in the better quality and uncompacted soils of the Burnley field station, suggesting that leaf size is not a good indicator of environmental stress. Most of the trees had their larg- est leaves on the southern side of the canopy, which was exposed to lower levels of sunlight. Bulk density data were collected outside the mulch placed around trees, as under the mulch the profile had been disturbed at planting, and lower bulk den- sity readings were anticipated. To gain an accurate measure of site bulk density, data were collected from the undisturbed profile, which is where new root growth would occur during tree establishment. The bulk density and penetrative resistance data verified the significantly different levels of compaction for the compacted and uncompacted soils, but over the 20 months, the bulk density of the uncompacted soil remained constant, while that of the compacted soil reduced. It is possible that root penetration and rip- ping reduced bulk density, but it is well known that mulch can be a cost-effective remedy to soil compac- tion over the medium to long term (Scharenbroch et al. 2005; Urban 2008). This study supports this approach, but over a shorter period of 20 months. Despite the soils of the survey being classed as compact to extremely compact (1.3 to 1.7Mg/m-3 ) with often root growth limiting levels of penetrative resistance (1.3 to 4.9 MPa)(Roberts et al. 2006), the street trees were rated as being in outstanding or very good condition. Furthermore, most trees were sym- metrical in canopy form, upright, and without trunk lean—all desirable characteristics of a good urban street tree. The data showing that there were consid- erable differences in compaction and penetrative resistance on different sides of the one street tree sug- gests that planting technique needs to be reviewed. The insertion and levering of the mechanical spade in creating the planting holes seemed to compact the soil on one side. Specifying that the sides of the planting holes need to be decompacted after planting should be written into future street tree planting contracts.
November 2019
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