192 Jack-Scott et al.: How Community Group Dynamics Affect Street Tree Survival and Growth 0.0263). Neighborhood resident groups had higher mean annual growth in their street trees (p = 0.0063), but lower mean annual growth in their vacant lot trees (p = 0.0126). Social service/non- profit groups had higher yard tree mean annual growth incre- ment than other groups (p = 0.0017). Street trees in areas with higher existing percent canopy cover had higher mean annual growth increment rates (p = 0.0055), while ornamental park trees in areas of high existing canopy cover have lower (p < 0.0001). Percent live crown was similarly affected primarily by tree age, tree function, group experience, group size, group longevity, and group type (street tree Adjusted R-squared = 0.0431, yard tree Adjusted R-squared = 0.1398, park tree Adjusted R-squared = 0.2992, vacant lot tree Adjusted R-squared = 0.2004) (Table 3). Older trees had higher live crown percent- ages among street trees (p < 0.0001), yard trees (p = 0.0022), and park trees (p = 0.0506), but not among vacant lot trees. Ornamental trees had significantly lower percent live crown than shade trees in street (p = 0.0384), and yard (p = 0.0166) site types. When paired with tree age, shade trees in parks had higher percent live crown as well (p = 0.0506). Group planting experience was positively correlated with percent live crown among street (p = 0.0577) and vacant lot (p = 0.0117) trees, but negatively correlated in park trees (p = 0.0315). Group longevity was altogether negatively correlated with percent live crown among yard trees (p = 0.0004), but as trees aged group longevity proved positively correlated with percent live crown (p = 0.0022). Group longevity was also positively correlated with park tree percent live crown (p < 0.0001). Group size was negatively correlated with percent live crown in yard trees (p = 0.0135), but positively with that of park trees (p = 0.0005). Among street trees, neighborhood resident groups had higher percent live crown measures (p = 0.0093), while social service/non-profit had lower measures (p = 0.0405). Among vacant lots, on the other hand, social service/ non-profits planted trees with significantly higher percent live crown (p = 0.0037) than did park groups (p = 0.0181). DISCUSSION Overall, social factors have a relatively small impact on ur- ban tree survival and percent live crown with R-squared values ranging from 0.0924 (street trees) to 0.2568 (park trees), and from 0.0431 (street trees) to 0.2992 (park trees), respectively. Slightly more of the variability in mean annual growth increment could be accounted for by social factors with R-squared values from 0.2031 (street trees) to 0.4728 (park trees). And yet, even those R-squared values include tree age as well as tree function (shade versus ornamen- tal), which in some analyses were highly significant and influential factors. Additional investigation into the influ- ence of biophysical factors on tree survival and growth was beyond the scope of this study, but is encouraged. Neverthe- less, there are notable significant social factors that impact urban tree health, an understanding of which can offer in- sight into extending community investment in urban forests. Firstly, group experience played a prominent role in many regressions as a predictor for improved survival and percent live crown (for street, park, and vacant lot trees). Interestingly, group longevity did not impact tree survival as much as mean annual growth increment and percent live crown, with signifi- cantly improved growth in trees of all four site types (after accounting for colinearity with tree age). This validates work by Burch and Grove (1993) that pointed to the importance of group longevity. On the whole, groups with more participants also demonstrated improved tree survival and growth, with exceptions in park tree survival (when planted by non-park Table 2. Significant social and biophysical factors affecting variation in mean annual growth increment of sampled street trees. Significance codes: 0 (***), 0.001 (**), 0.01 (*). Interaction effects are noted (+). Mean annual growth increment regression coefficients Street tree mean annual increment (Adjusted R-squared = 0.2031) Tree age Shade trees Neighborhood resident groups Group size (# participants during planting year) Existing % canopy cover Tree age + Group longevity (total # years active) Yard tree mean annual increment (Adjusted R-squared = 0.3491) Tree age Shade trees Group longevity (# years active as of planting year) Group size (average # participants) Social service/non-profit groups Group longevity + Tree age Park tree mean annual increment (Adjusted R-squared = 0.4728) Tree age Ornamental trees Ornamental trees + Existing % canopy cover Vacant lot tree mean annual increment (Adjusted R-squared = 0.2702) Tree age Shade trees Group experience (# trees planted as of planting year) Group longevity (total # years active) Neighborhood resident groups ©2013 International Society of Arboriculture Estimate -0.04911 0.1025 0.2028 0.0058 0.0033 0.0031 -0.0654 0.3613 -0.1457 0.0231 0.3244 0.0170 -0.0658 0.5840 -0.0280 -0.0927 0.2407 0.0208 0.0571 -0.2650 P-value <0.0001 *** 0.0003 *** 0.0063 ** 0.0056 ** 0.0055 ** 0.0231 * 0.0002 *** <0.0001 *** 0.0038 ** <0.0001 *** 0.0017 ** 0.0031 ** <0.0001 *** 0.0004 *** <0.0001 *** <0.0001 *** 0.0080 ** 0.0263 * 0.0509 0.0126 *
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