226 Chappelka et al.: Determination of Crew Size Efficiency Relative to Urban Tree Inventories separate slope for a three-person crew, and a sepa- rate intercept for a one-person crew, and a common intercept for two- and three-person crews (Table 1). As shown in Figure 1, the two-and three-person crews had the same fixed time (setup) as the dbh approaches zero (intercept), but a one-person crew has a higher fixed time (approx. 2×). The sampling times for one- and two-person crews increase at the same rate as dbh increases, but the time needed to sample a tree for a three- person crew increased at a slower rate (Figure 1). Using these equations (Figure 1) the relative time required to measure a tree with increasing diameters for each crew in 25.4 cm increments is shown in Table 2. For example, a 25 cm tree takes a one-person crew 4.6 minutes to measure, but a three-person crew 2.3 minutes to measure. Table 2. Time required (minutes) to measure trees (25.4 cm dbh increments) by crew size. Diameter (cm) 0 25 51 76 102 One 3.7 4.6 5.4 6.2 7.0 Crew size [person(s)]z Two 1.9 2.8 3.6 4.4 5.2 Three 1.9 2.3 2.6 3.0 3.3 z One-person crew: Time = 3.74158 + 0.08265 × dbh; two-person crew: Time = 1.94142 + 0.08265 × dbh; three-person crew: Time = 1.94142 + 0.034854 × dbh. N = 67, 69, 67 trees, for one-, two-, and three-person crews, respectively. DISCUSSION The overall objective of this project was to evaluate crew size efficiency of a 100% inventory. In this study, researchers used the i-Tree Eco protocols, which included measuring tree dbh, height, crown characteristics, dieback, and percent missing crown. However, the results are applicable to the vast majority of other tree inventories. Based on the results there was a higher fixed time (setup) spent by the one-person crew than the others, irrespective of tree dbh. It took 3.7 minutes for setup (fixed time) for the one-person crew compared to 1.9 minutes for the other crews. These times may vary depend- ing on the experience of the crews conducting the surveys, and on the site characteristics (e.g., topog- raphy). It stands to reason that it would take a one- person crew a longer time to setup the equipment than a larger crew size as reflected by the analysis. However, once this task was completed the rate of change (slopes) as dbh increased was the same for one- and two-person crews and increased at a slow- er rate for the three-person crew. Therefore, there was a time-saving per increasing dbh using a three- person crew. These results can be very helpful for managers who want to conduct tree inventories. The number of trees (and their relative sizes) play a major role in deciding the crew size. To illustrate this point, three hypothetical examples are included. The time to complete these tasks was estimated using the equations illustrated Figure 1. Sample time related to tree diameter for one-, two-, and three-person crew sizes. N = 67, 69, 67 trees, for one-, two-, and three-person crews, respectively. in Figure 1 and Table 2. First 100, 2.54 cm trees were to be inventoried. The time to complete the inventory was 6.4, 3.4, and 3.3 hours for one-, two-, and three-person crews, respectively. The cost was USD $64, $68, and $99 for one-, two-, and three-person crews, respectively, at $10/ hour per crewmember. Based on this cost alone a one- or two-person crew would be the most efficient. However, travel time, food, lodging (if traveling overnight), and fuel for a vehicle or vehicles, among other expenses, will influence the cost depending on the number of crewmem- bers, and need to be included in a total estimate. Second, for a manager with 1,000 trees to inven- tory with an average diameter of 25.4 cm, a one- person crew would require 77 hours, a two-person crew 47 hours, and a three-person crew 38 hours to complete the inventory. This would represent a sav- ing of 30 and 39 hours, respectively, for the two- and ©2015 International Society of Arboriculture
July 2015
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