Arboriculture & Urban Forestry 40(2): March 2014 rates, and condition ratings in neighborhoods with different tree maintenance characteristics. Statistical Analysis Most statistical analyses were performed using the individual tree as the unit of analysis with different types of neighborhoods serving to divide the trees into categories for comparison of growth, condi- tion, and survival rates. This allowed researchers to take advantage of the relatively large sample of trees (n = 1,462 trees), instead of deferring to the smaller sample of neighborhoods (n = 18). Analyses of collec- tive action pre- and post-planting were conducted at the neighborhood scale. All statistical analyses were performed with Statistical Analysis Soſtware (SAS Institute, Cary, South Carolina, U.S.). A χ2 test was used to test for significant differences in mortality rates (assignment to “Dead” or “Alive” category) and tree condition ratings (for Alive trees) across neigh- borhoods with different characteristics. T-tests and an ANOVA (for balanced samples) and a generalized linear model (for unbalanced samples) were used to test for significant differences in average growth rates between trees in neighborhoods with different char- acteristics (e.g., signed commitment to watering or no signed commitment). For all analyses, a P-value of less than 0.05 was considered statistically significant. RESULTS AND DISCUSSION Of the 6,366 trees planted in NeighborWoods projects by KIB between 2006 and 2009, 1,462 trees had been planted in the sampled neighbor- hoods; 1,304 (89%) of them were found to be alive, while 158 (11%) were found to be dead or missing upon sampling. Of the 1,304 living trees, growth rates were measured for 663 sample trees. Overall, average caliper growth rate was 1.11 cm per year (standard deviation = 0.61). Of the sampled living trees, 85.4% were observed to be in good condi- tion, while 11.2% were in fair condition, and only 2.5% were in poor condition (0.9% of living trees were in shrub form and condition was not assessed). Watering Strategy and Signed Water- ing Agreements The results suggest the potential importance of watering strategies to the survival and growth of neighborhood-planted trees. The findings demon- 89 strate that trees in neighborhoods with some col- lective watering were significantly more likely to be alive (n = 554, 93.7% alive) than trees in neigh- borhoods with no collective watering (n = 908, 86.5% alive) (Table 1). Although the magnitude of observed effects is small and researchers cannot make a causal claim about the mechanism through which survival and watering strategy are related given the lack of quantitative data on the consis- tency of tree watering by neighborhood, this finding suggests that some tree-relevant differences exist in watering strategies between neighborhoods. There is theoretical and empirical reason to con- sider that collective watering increases individual accountability to follow through with planned water- ing, given the physical presence of other neighbors at the specified watering time, a factor that could lead to more consistent watering and thus greater tree survival. The fact that neighbors can see one another watering may lead to the perception that one could be “caught cheating” (not watering) and face social sanctioning unless one is watering per the collective agreement (an institutional mechanism). Conse- quently, watering in these neighborhoods may be motivated by the neighborhood social network and the norms of reciprocity (an institutional mechanism) and trustworthiness that can arise from them. While qualitative in nature and limited in number, inter- viewee statements in collective watering neighbor- hoods empirically support this reasoning; for example, “[when] the dedicated few [are watering] . . . the others, out of guilt, will walk outside and start helping . . . [or] come back later.” For this neighborhood, and perhaps others in the sample, the collective watering strategy appears to be related to institutional confor- mance, which leads to consistency and frequency of tree-watering activities and thereby to tree survival. Although surviving, the living trees in neigh- borhoods where at least some collective watering occurred were oſten in worse overall condition (Table 1) than those in neighborhoods without col- lective watering. The authors speculate the higher rates of tree mortality in neighborhoods without collective watering resulted in better average tree condition, possibly because the trees in the poorest condition died and dropped out of the sample of living trees given condition ratings. Gilman (2004) found similar results from analysis of 210 trees in an experimental study; the only three trees that died ©2014 International Society of Arboriculture
March 2014
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