156 Townsend et al.: Exploring the Relationship Between Trees and Human Stress negatively significant. Block Features remained positively significant. None of the additional vari- ables were significant. The explanatory value of the model increased slightly (Adjusted R2 = 0.63). Model 4 used only the variables that main- tained significance in the previous three models. The HAUS is regressed on Street Trees, Other Trees, Median Income, and Block Features. Like Model 2, Street Trees (P = 0.002) and Other Trees (P = 0.003) have negative significance. Block Fea- tures was positively significant (P = 0.004), while Median Income, still negative, was highly signifi- cant (P ≤ 0.001). The model explained 58% of the variance in the HAUS score. Based on the VIF, none of the final variables has issues of collinearity. In contrast to the PSS, the models for the HAUS in Table 8 show that trees (both Street Trees and Other Trees) contribute to a reduction of stress on the block. The reasoning behind the HAUS is that environmental factors both psychologi- cal and physical may contribute to stress when they are seen as challenging. Stress comes first from a subconscious appraisal and then from a conscious appraisal that some things or events may tax individuals beyond their abilities. In contrast to the PSS, the HAUS is very responsive to both Street Trees (P < 0.01) and Other Trees (P < 0.01) on the block. The Adjusted R2 shown in Table 8, Model 1 is 0.39. This means that Street Trees and Other Trees account for 39% of the change in the HAUS measure of stress. Like the PSS, the HAUS also shows a strong connection to Median Income (P < 0.001). Unlike the PSS, the HAUS is moderately impacted by Block Fea- tures in addition to trees (P < 0.01). While five of the seven other independent variables show significant impact on the HAUS when fitted individually (Education, Socialize, Employed, Home Ownership, and Reporting Police Visits) none of them have significance in the full model. DISCUSSION By means of a physical inventory and a survey of 80 randomly selected blocks in Wilmington, Delaware, it was possible to examine the relationship between the amount of tree cover on a block and the level of neighborhood stress. Both the inventory and the survey incorporated redundant instruments for measuring variables. There were three differ- ©2016 International Society of Arboriculture ent measures of tree cover, two different measures of stress, and two measures to validate the stress scores. Not every measure performed as intended, but the conclusion is undeniable that tree cover does have a significant impact on neighborhood stress. Regarding the best method for measuring urban tree cover, the physical inventory of eighty blocks in Wilmington produced three tree cover mea- sures: Total Trees, Street Trees, and Other Trees. Analysis of the data showed that Street Trees and Other Trees were moderately significant in their impact on neighborhood stress (HAUS) in a nega- tive direction. According to Table 3, Street Trees and Other Trees have very little correlation (0.009). They explain different things about the tree cover on the block. In future research, to determine the measure of tree cover on a block, the dual count of street trees and other trees is a robust measure to use. Together they capture the large picture and separately they provide detailed information. The distinction between Street Trees and Other Trees in calculating stress reduction is an impor- tant contribution of this research. In the HAUS models, Street Trees and Other Trees together account for as much as 39% of the variance in the reported level of block stress. Regressed individu- ally on the HAUS, Street Trees has an Adjusted R2 of 0.07 and Other Trees has an Adjusted R2 of 0.31 (models not shown). For city administrators and urban planners, the recognition that both types of tree cover contribute to the overall positive effect of tree canopy in a neighborhood has practical application. Street trees provide a significant con- tribution but private trees (Other Trees) substan- tially augment that relationship. Where planting street trees is not a viable option, encouraging private tree planting and tree maintenance is an alternative solution. In a 2005 study on enhancing Wilmington’s Brandywine Valley Scenic Byway, three examples of “borrowed trees” are men- tioned. Wilmington and Brandywine Cemetery, Trinity Church, and the Delaware Children’s Theatre, all on Delaware Avenue, provide: existing openspace housing trees that are critical to the tree canopy of Delaware . . . Even though these trees are growing on private land, the city should regard them as valuable resources and should play an active role in their stewardship. This
May 2016
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