Arboriculture & Urban Forestry 39(4): July 2013 193 Table 3. Significant social and biophysical factors affecting percent live crown of street trees. Significance codes: 0 (***), 0.001 (**), 0.01 (*). Interaction effects are noted (+). Percent live crown regression coefficients Street tree percent live crown (Adjusted R-squared = 0.0431) Tree age Ornamental trees Group experience (# trees planted as of planting year) Neighborhood resident groups Social service/non-profit groups Yard tree percent live crown (Adjusted R-squared = 0.1398) Ornamental trees Group longevity (# years active as of planting year) Group size (# participants during planting year) Tree age + Group longevity Park tree percent live crown (Adjusted R-squared = 0.2992) Group experience (total # trees planted) Group longevity (total # years active) Group size (average # participants) Shade trees + Tree age Vacant lot tree percent live crown (Adjusted R-squared = 0.2004) Group experience (# trees planted as of planting year) Park groups Social service/non-profit groups groups), and in yard tree percent live crown. This finding sup- ports work by Pagdee et al. (2006) and Wagner and Fernandez- Gimenez (2009) indicating group size as a marker of stew- ardship success, at least within the context of urban forestry. Certain group types were also recurring factors affecting tree survival and growth. Street trees planted by neighborhood resident groups had significantly improved growth, although the opposite was true among vacant lot trees. Conversely, so- cial service/non-profit groups demonstrated improved growth among trees in vacant lots and yards, and significantly lower growth on streets. Park groups were effective in park plantings, with higher survival rates, but had low growth rates among vacant lot plantings. All of these findings support work by Millar (2003) that emphasizes the importance of participants feeling their work is in line with their mission so that they may be maximally effective. In support of Nowak’s (1990) findings, public housing groups had lower survival of their yard trees than did other groups. However, yard trees planted by apartment groups actually had significantly higher survival rates, disputing findings by Nowak (1990) in that same paper. Neighborhood characteristics, such as percent homeowner- ship, and existing canopy cover accounted for a small portion of the variability in survival and growth as regarded by social factors. Percent homeownership was only significant in exam- ining the survival of park and vacant lot trees, and findings were mixed on whether higher homeownership was a boon or a burden. Likewise, existing canopy cover was sometimes an indicator of improved mean annual growth increment (street trees) and in others diminished mean annual growth increment (ornamental park trees). Therefore the roles of neighborhood homeownership and existing canopy cover may require ad- ditional scrutiny in order to understand their contributions. Many of the findings in this study underscore Abbott’s (1995) assertion that factors affecting street tree mortal- ity and health across community groups are complex. Rarely does a single factor account for a substantial amount of the variability seen in street tree survival and growth. Indeed, a Estimate 0.0075 -0.0228 0.0019 0.0742 -0.0428 -0.0391 -0.0387 -0.0017 0.0037 -0.0113 0.0521 0.0191 0.0287 0.0093 -0.1997 0.2000 P-value <0.0001 *** 0.0384 * 0.0577 0.0093 ** 0.0405 * 0.0166 * 0.0004 *** 0.0135 * 0.0022 ** 0.0315 * <0.0001 *** 0.0005 *** 0.0506 0.0117 * 0.0181 * 0.0037 ** single factor (such as group type or percent homeowner- ship) can be a decidedly positive indicator in some situa- tions and an apparently negative one in others. Therefore, while easily defined measures, such as group experience, size, longevity, or type, can offer valuable guidance on how to extend street tree investments, they are limited in how they define community groups composed of many different types of individuals. Only through more in-depth research, including interviews, could more comprehensive data on the impact of community groups on tree survival and growth be ascertained. CONCLUSIONS The most interesting findings of this study are that (a) social factors account for a small, albeit significant and important compo- nent of urban tree health, (b) community group experience, longevity, and size are frequently positively associated with improved tree survival and growth, (c) group types were most effective when planting in areas most in line with their focuses, and (d) percent homeownership and existing percent canopy cover played a limited role in the survival and growth of com- munity-planted trees. Findings (b) and (c) supported the hypoth- esis and existing literature, while (d) went against predictions. The most valuable management implication from this study is its insight into just how nuanced social factors affecting street tree survival and growth can be. This study sup- ports existing literature indicating the importance of group experience, longevity, and size, but it also illustrates that no single social factor can guarantee successful stewardship. Acknowledgments. We would like to thank The Urban Resources Initiative of Yale University/City of New Haven, Connecticut, for provid- ing the original archived planting data, social assessment data, and for logistical guidance and advice in sampling. We thank The Hixon Center for Urban Ecology of the School of Forestry and Environmental Studies, Yale University for providing funding to carry out this study. ©2013 International Society of Arboriculture
July 2013
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