Arboriculture & Urban Forestry 39(4): July 2013 struction or planted in unrecorded locations. These sites were ex- cluded from statistical analysis as no data was available for them. Survival Trees across all thirteen age groups had an average survival rate of 74% (1029 out of 1393 trees) and an average age of 9.2 years (post-planting). By site type, street trees had an average survival rate of 77.5% (average age 9 years), yard trees 58.6% (10.1 years), park trees 80.1% (7.9 years), and vacant lot trees 73.1% (9.8 years). Logistic regression revealed significant fac- tors affecting survival were tree age, group experience, group longevity, group size, group type, and percent homeownership (street tree Equivalent R-squared = 0.0924, yard tree Equiva- lent R-squared = 0.1364, park tree Equivalent R-squared = 0.2568, vacant lot tree Equivalent R-squared = 0.1402) (Table 1). Survival rates dropped with tree age for street (p = 0.0061), yard (p < 0.0001), and park trees (p = 0.0008). There was not, however, a significant change in survival rate among vacant lot trees with age. Group experience (# trees planted) was positively correlated with survival within all planting sites: street trees (p = 0.0088), yard trees (p = 0.0398 for apartment groups only), park trees (p = 0.0462 in areas with higher % homeownership), and vacant lot trees (p = 0.0108). Group size correlated positive- ly with survival for vacant lot trees, as well as park trees (p = 0.0290), but only for those planted by park groups; otherwise, group size actually had a significantly negative correlation with survival among park trees. Group longevity was only significant for street trees, with increasing survival the longer groups were active in the Emeritus stewardship program (p = 0.0418). The interaction effect between tree age and group longevity was actu- ally negatively correlated with survival for street trees (0.0233). Group type was also a significant factor affecting survival. Public housing (p = 0.0003) and social service/non-profit (p = 0.0457) groups had lower survival among their yard trees than did experienced apartment groups (p = 0.0398). As for trees 191 planted in parks, large park groups had higher survival than other group types (p = 0.0290). Finally, percent homeowner- ship within the census block group of each planting group proved significant in some instances. In general, park trees (p = 0.0278) and vacant lot trees (p = 0.0042) in areas with higher percent homeownership had lower survival. However, as groups gained more experience in areas with high percent homeown- ership, their park tree survival actually went up significantly (p = 0.0462). Shade versus ornamental, and existing percent canopy cover did not significantly correlate with tree survival. Growth Indices Growth was measured through mean annual growth in- crement and percent live crown. Multiple linear regres- sions indicated tree age, tree function (shade versus orna- mental), group planting experience, group longevity, group size, and group type significantly affected tree growth. Mean annual growth increment was primarily correlated with tree age, tree function, group type, group size, and group longevi- ty (street tree Adjusted R-squared = 0.2031, yard tree Adjusted R- squared = 0.3491, park tree Adjusted R-squared = 0.4728, vacant lot tree Adjusted R-squared = 0.2702) (Table 2). Among all site types, mean annual growth increment negatively correlated with tree age (street p < 0.0001, yard p = 0.0002, park p < 0.0001, and vacant lot p < 0.0001). Mean annual growth increment was higher in shade trees for street (p = 0.0003), yard (p < 0.0001), and va- cant lot (p = 0.0080) trees, but was actually lower among those in parks (p = 0.0004). After accounting for covariance with tree age, higher group longevity significantly correlated with higher mean annual growth increment among street trees (p = 0.0231), yard trees (p = 0.0031), and vacant lot trees (p = 0.0509). Groups with more participants also had higher mean annual growth in- crement among street (p = 0.0056) and yard (p < 0.0001) trees. Group experience planting trees had a significantly positive effect on mean annual growth increment only in vacant lot trees (p = Table 1. Significant social and physical factors affecting street tree survival. Significance codes: 0 (***), 0.001 (**), 0.01 (*). Interaction effects are noted (+). Survival regression coefficients Street tree survival (Equivalent R-squared = 0.0924, AIC = 820.77) Tree age Group experience (# trees planted during planting year) Group emeritus longevity (# years in Emeritus program) Group longevity (# years as of planting year) + Tree age Yard tree survival (Equivalent R-squared = 0.1364, AIC = 319.74) Tree age Public housing groups Social service/non-profit groups Apartment groups + Group experience (total trees planted) Park tree survival (Equivalent R-squared = 0.2568, AIC = 143.06) Tree age Group experience (total # trees planted) Group size (# participants during planting year) % Homeownership (in Census Block Group) Group experience + % Homeownership Group size + Park groups Vacant lot tree survival (Equivalent R-squared = 0.1402, AIC = 142.08) Group experience (# trees planted during planting year) Group size (# participants during planting year) % Homeownership (in Census Block Group) Estimate -0.1032 0.0542 0.1724 -0.0307 -0.1931 -1.5485 -0.7858 0.1550 -0.4799 -0.3669 -0.2040 -0.1896 0.0073 0.1797 0.1257 0.0817 -0.0568 P-value 0.0061 ** 0.0088 ** 0.0418 * 0.0233 * <0.0001 *** 0.0003 *** 0.0457 * 0.0398 * 0.0008 *** 0.0413 * 0.0089 ** 0.0278 * 0.0462 * 0.0290 * 0.0108 * 0.0257 * 0.0042 ** ©2013 International Society of Arboriculture
July 2013
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