Arboriculture & Urban Forestry 43(4): July 2017 Platanus × acerifolia, Acer × freemanii, and Taxo- dium distichum. Similar results on 10 urban street species were also reported by Jack-Scott (2012). B&B trees potted in containers (irrespective to HA treatment) had no mortality during the hard- ening-off period, and grew better than BR trees. Therefore, these trees (termed as CG trees) were used in the second experiment in order to com- pare their post-transplant growth and survival with freshly dug FG trees (B&B or BR). It is nota- ble that no significant root defects were detected in CG trees, whereas a substantial amount of fine roots were produced. This is probably due to the bottomless containers with slits embedded in the walls used in this study, which can stimulate lateral air pruning and reduce the incidence of deformed roots. Several studies have demonstrated that con- tainer type can affect the root-system architecture and long-term problems on plant growth (Marshall and Gilman 1998; Amoroso et al. 2010; Gilman et al. 2010). Comparison between hardened-off CG trees and FG trees showed CG trees had greater percent survival and growth responses by the end of the first year following transplanting. This can be attributed to the less stress of hardened-off CG trees, resulting from their adaptation to the planting site environment and new root regeneration. It has been shown that container trees are better able to meet the transpirational needs of the plant imme- diately aſter transplanting due to the production of many fine roots along the outside of the root ball that can rapidly penetrate into the surrounding soil and absorb water from it (Harris and Gilman 1993). In confirming the results of the first experiment, BR trees suffered from transplant stress more than other methods, suggesting the susceptibility of ori- ental thuja to the bare-rooted transplanting method. In a similar study, Mexal et al. (2002) compared post-transplant growth and survival of container- ized Spanish cedar (Cedrela odorata L.) to BR seed- lings. Containerized seedlings had 82% survival (compared to 10% for BR) and nearly twice the growth aſter 28 months. Aſter 57 months, all the BR seedlings had died, while 50% of the contain- erized seedlings survived. In another study, Harris and Gilman (1993) showed that freshly dug field- grown B&B East Palatka holly (Ilex × attenuata ‘East Palatka’) trees were more stressed and more likely to die than trees planted from plastic con- 151 tainers if they were not regularly irrigated aſter transplanting due to the sudden loss of roots. How- ever, with regular irrigation, laurel oak (Quercus laurifolia) and East Palatka holly trees (Gilman and Beeson 1996) and live oak (Gilman 2001), from either B&B or container production method, experienced similar post-transplant water stress. By the end of the second year of the current study, the significant differences observed between B&B and CG trees disappeared for all measured variables. It was also found in some previous studies that first year effects of transplanting method may not persist in the following years (Vanstone and Ronald 1981; Buckstrup and Bassuk 2000; Etemadi et al. 2013). This observation is of significant impor- tance, as it shows freshly dug oriental thuja trees can be successfully transplanted using the preva- lent B&B method without the hardening-off pro- cess, which requires more time, labor, and expense. Humic Acid Based on the results of the first and second experi- ments, HA application (across production methods) was not able to substantially improve growth and vitality of transplanted trees, except for BR trees. HA-treated BR trees possessed relatively higher survival percentage and greater canopy color rate, shoot extension, and lower canopy desiccation com- pared to non HA-treated BR trees. These observa- tions may be related to the supposed influence of HAs on stimulating drought tolerance (Russo and Berlyn 1990; Farahat et al. 2012) and increase in the synthesis of the chlorophyll and photosynthesis rate that consequently promote plant growth (Chen et al. 2004; Nikbakht et al. 2008; Ferrara and Bru- netti 2010). An increase in absorption of mineral nutrients and the possible hormone-like activity of the HAs (i.e., auxin-, gibberellin-, and cytokinin- like activity) can also promote growth of BR trees and ameliorate the harmful effects of water stress (Mustafa and El-Shazly 2013). It has to be pointed out that despite better performance of HA-treated BR trees, this procedure would not be advisable for transplanting oriental thuja species due to the high mortality rate of BR trees (regardless of HA treat- ment) compared to the other transplanting methods. No significant differences were found between the HA-treated trees from B&B and CG methods and their control counterparts. B&B and CG trees ©2017 International Society of Arboriculture
July 2017
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