Arboriculture & Urban Forestry 45(6): November 2019 determine which of the species were most likely to penetrate the cracks, how crack size influenced root penetration, and whether the content of the pipes affected penetration and subsequent root growth. The second investigated the capacity of the roots of four tree species to grow through holes of different sizes drilled into polycarbonate laser-cut discs. Below the discs, media were irrigated with potable water or stormwater, and root and canopy growth were mea- sured. This experiment investigated whether the size and number of holes in a pipe influenced root pene- tration. In both experiments, plants were irrigated with water and stormwater to examine whether root penetration and proliferation were greater in pipes containing stormwater that contained higher levels of some nutrients. MATERIALS AND METHODS Experiment 1 Six species commonly grown as urban trees in Mel- bourne, Australia were grown at the University of Melbourne, Burnley College, 500 Yarra Blvd, Rich- mond: Acer palmatum Thunb (Japanese maple), Allo- casuarina littoralis (Salisb.) L.A.S. Johnson (black she-oak), Callistemon salignus (Sm.) DC (pink tips bottlebrush), Eucalyptus leucoxylon F. Muell. Subsp. megalocarpa Boland (yellow gum), Lophostemon confertus (R. Br.) P.G. Wilson and Waterhouse (brush- box), and Pyrus calleryana Decne (callery pear). Seedlings of all species were 350 +/- 100 mm tall at planting and were grown for ten months between late-January and mid-December. Plants were grown in soil purchased from Burdetts Nursery, 615 McClel- land Drive, Langwarrin, which consisted of 60% whitewashed sand, 25% screened soil, 5% chicken manure, and 10% organic compost by volume. The soil had a drainage rate of 6 mm/hr and a wettability rate of 60 mm/min (Australian Standard 4419-2003 using compacted soil and three replicates). Particle size of the soil was also determined according to Aus- tralian Standard 4419-2003 USGA sieve 1993 (Table 1). Soil pH and electricity conductivity were mea- sured in 1:5 extracts, and the data sheet provided by the supplier showed the pH to be moderately acidic (6.0 to 6.6) while the total salts were low (electrical conductivity < 0.6 dS/cm). Plants were irrigated by overhead sprinklers and fertilized with 12 mL Maxi- crop diluted in 4 L of water in May and then irrigated with 17.8 mL Peters liquid fertilizer (15N:11P:25K) 317 diluted in 37.8 L of water in mid-August and mid-October. PVC stormwater pipe (75 mm diameter) was cut into 150-mm lengths and had cracks of 100-mm length cut into them. The narrowest cracks were made by squeezing the pipes in a vice and cutting with a Stanley knife. These cracks averaged less than 0.04 mm wide, measured with an engineer’s feeler gauge, which measures the gap between two solid components, and the gap width was the average for ten pipes. The medium cracks were cut using a Dremel saw with a fine cutting wheel and averaged 0.66 mm wide, and the large cracks were cut using a circular saw and averaged 1.48 mm wide. Caps were welded onto the ends of the pipes using Plaskeen sol- vent cement containing methyl ethyl ketone, as rec- ommended by the manufacturer. The experiment was a randomized complete block design. The treatments were a combination of three factors (crack, media, species), each having multiple levels. There were three widths (small, medium, large), four different media (water, stormwater, soil, soil + stormwater), and six species. There were six replicates for each of these 72 treatments and a total of 432 plants. Positions of containers in the six blocks were chosen by using a random number generator for each block. Soil was inserted into the already cracked pipes with one cap glued on and the crack taped over with duct tape, and then 450 mL of water was added to a quarter of the pipes before welding the second cap. Similarly, 450 mL of stormwater was added to another quarter of the pipes. Another quarter of the pipes were filled with the same dry sandy soil used to grow the plants. The remaining pipes were filled with 180 mL of stormwater, which saturated the soil. Table 1. Particle size analysis of the soil used to grow plants and inserted into pipes. Particle size Fine gravel (2.0 mm) Fine sand (0.15 mm) Very coarse sand (1.0 mm) Very fine sand (0.05 mm) Coarse sand (0.5 mm) Silt and clay (< 0.05 mm) Medium sand (0.25 mm) Percentage 3 20 6 7 19 2 43 ©2019 International Society of Arboriculture
November 2019
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