324 Moore et al.: Root penetration of PVC pipes roots, and again less chance of encountering cracks. A. palmatum had less numerous and finer roots than other species, which resulted in lower rates of penetration. Species root archi- tecture may be exploited where pipe penetra- tion is a problem. Although there were four different contents Figure 10. Percentage increase in height against hole configuration for trees irrigated by water and stormwater. Vertical bars show Least Significant Difference (P < 0.05). 2013). Because responses to phytohormones occur at a cellular level, consistent tissue responses are to be expected (Nakamura et al. 2012; Koepke and Dhingra 2013), which would suggest that the action of the roots tips of mature trees will be the same as those reported for seedlings. Experiment 1 Many roots penetrated the wider cracks, but only five penetrated the 0.04 mm cracks. Very fine roots devel- oped in pipes with the narrowest cracks, and longer and heavier roots grew in the pipes with medium and large crack widths. Fine cracks seemed to limit subsequent root growth, even if the crack was penetrated. There were significant differences between species in their capacity to penetrate cracks. Only three species pene- trated the narrowest cracks: C. salignus and single instances for E. leucoxylon and L. confertus. C. salignus (68%), A. littoralis (60%), L. confertus (58%), E. leu- coxylon (44%), and P. calleryana (43%) had higher rates of penetration compared to A. palmatum (18%). Differences in root growth and architecture explain some of these differences in crack penetration. From observations during harvesting, C. salignus, A. litto- ralis, and L. confertus had many fibrous roots distrib- uted through the pots and a high chance of encountering cracks in pipes. E. leucoxylon seedlings had long, woody tap roots which wound around the bottom of the pot with few laterals and so had a lower chance of encountering cracks. P. calleryana had sev- eral widely spaced woody roots growing from the stem and down the sides of the pots, but fewer roots in the middle of the container than species with fibrous ©2019 International Society of Arboriculture of the PVC pipes, the only significant difference (P < 0.05) was in the greater root penetration and growth for pipes filled with soil compared to those filled with stormwater soil. Tree roots are opportunistic, growing where their require- ments are provided (Perry 1982), and it is likely that they could proliferate in soil more easily than stormwater, as soil contains higher levels of oxygen. Experiment 2 The controls had the greatest root mass in the lower parts of the pots, as there were no plates to restrict growth. There was no significant difference in the per- centage of root penetration for different sized holes, and all four species could penetrate holes 0.5 mm or greater in diameter. It was surprising that roots of F. mac- rophylla could penetrate 0.5 mm holes, as their fine root diameter was larger than 0.5 mm, but their root tips passed through the holes and then thickened again. Significant differences between species were expected, as there are clear differences in root struc- ture and architecture. S. fragilis had the heaviest mean mass of roots in the lower parts of the pots, perhaps because their roots were already becoming lignified. F. macrophylla had heavy, rubbery roots, while M. ericifolia had fine roots. The roots of E. leucoxylon were very fine, apart from the tap root, none of which grew through holes in this experiment. Generally, plants irrigated with stormwater were heavier and taller than those irrigated with water, and towards the end of this experiment some of the water- irrigated plants were beginning to look less healthy than stormwater-irrigated plants. The stormwater, containing 6 mg/L of nitrogen and 0.6 mg/L of phos- phorous, was providing a source of nutrients (Den- man 2009; Denman et al. 2011). There was an interaction between hole configuration and species for percentage increase in height, with S. fragilis and M. ericifolia growing the tallest for controls and plates with mixed hole sizes and 0.5 mm diameter holes, but these are the fastest growing species.
November 2019
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