Arboriculture & Urban Forestry 45(4): July 2019 would receive an estimate of 60%. To estimate the number of leaves per cm, four side branches were selected randomly on each tree. At the beginning and end of the study period, each branch’s length was measured and the number of leaves was counted. Pre-dawn leaf water potential, which indicates tree stress caused by water deficiency, was also measured for each tree near the end of the study period (week 11) to compare the effectiveness of the watering devices from a physiological standpoint. During that week, water potential measurements were recorded early morning one day after watering, four days after watering, and seven days after watering (immediately before the next water delivery). Water potentials were measured with a Model 600 Pressure Chamber, PMS Instrument Co. Statistical Analysis Main effects of watering devices on growth (stem heights and calipers) and ratings (leaf coverage and number of leaves per cm) variables as well as on water potential were analyzed with one-way analysis of variance (ANOVA) using generalized linear model (GLM) procedures. In the case of significant varia- tion (P ≤ 0.05), means were separated with Tukey’s multiple range test. Linear regression analyses were employed to examine relationships between growth and ratings variables and water potential as an approach to evaluating the overall benefits of water- ing devices in terms of enhanced growth and/or reduced water stress regardless of treatments. Only response variables having significant relationships with water potential are reported. The significance of relationships was determined by P ≤ 0.05. All analy- ses were conducted using R statistical package v. 3.4.2 (R Core Team 2017). RESULTS AND DISCUSSION Evaluating Watering Device Performances All watering devices were easy to install. The geom- etry of multi-stemmed trees made installation of the upright bags somewhat difficult, but this would not present a challenge for single-stemmed trees. Water- ing device empty weights ranged from 0.36 kg (0.79 lb) for the upright bags to 1.36 kg (2.9 lb) for the Bio- plex tub (Table 1). Depending on water holding capacity, each watering device was filled with 38 liters (10 gal) of water and was able to drain most of it (details in Materials and Methods). In terms of cost 113 per liter of water, devices ranged from $0.31 to $0.91 (Table 1), with ring bags and tubs being the least and most expensive, respectively. A lower cost is desir- able if watering devices have similar performance, especially when purchasing many watering devices for a large-scale urban tree planting program. Drainage times varied substantially among water- ing devices ranging from 2 minutes to 9.5 hours (Table 1). With the exception of ArborRain ring bags, which drained completely during the drainage test, all other devices had residual water, which could not drain from a device due to the configuration of drain- age holes or the device itself. Significant volumes of water remained in almost all devices, with ring bags and tubs having the least (1.0 liter) and the ArborRain upright bag having the most (3.3 liters) residual water (Table 1). While this study was not designed to inves- tigate the cause of this drainage variability, water- filled upright bags tend to collapse onto drain holes and block them, thereby reducing water release. The drain holes in the upright bags are located several inches above the base of the bag. Incomplete drain- age in devices with small drain holes could also be associated with reduced water pressure resulting from reduced water weight and also from water tension. For the devices with larger drain holes, such as Tree I.V. tubs, although drainage was faster, complete drainage was likely prevented by the lip around the “soil injector” placed in the bottom of the tub. Finally, differences in drainage times among devices are asso- ciated with the total number and size of drainage holes (water entry points), which ranged from 2 to 22 holes across all devices, with the Bioplex tub having the most water entry points into the root zone (22 holes, total drain hole area 3.3 cm2 ). Overall, this study demonstrates that indirect watering devices offer logistic advantages in many aspects such as ease of installing and handling (asso- ciated with light weights) as well as filling of water. We expect that these logistic benefits would render slow-release indirect watering devices favorable over fast-release direct watering devices such as hoses, which may not be readily available at the site and may be difficult to install and time-consuming to han- dle for watering newly transplanted trees. Our results also indicate that when choosing slow-release water- ing devices for an urban tree planting program, ring bags should be favored over other devices due to their lower cost and ability to drain water more completely. Ring bags, because they are enclosed, also have the ©2019 International Society of Arboriculture
July 2019
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