Arboriculture & Urban Forestry 40(2): March 2014 2003; Hope et al. 2003; Heynen et al. 2006; Szantoi et al. 2012). If residents of affluent neighborhoods are also more likely to participate in tree-planting pro- grams than residents of less affluent neighborhoods, then these programs have the potential to exacerbate environmental injustice. Conversely, if residents of poorer neighborhoods are more likely to partici- pate in a tree-planting program, then the program could be used to reduce environmental injustice. Only two previous studies were found to have investigated the reasons why urban residents plant trees. Summit and McPherson (1998) administered a survey to 133 residents of Sacramento, California, U.S., about past tree planting and maintenance. In addition, they took detailed inventories of existing trees and identified the potential for new plant- ings. The authors found that residents were most likely to plant trees soon aſter they moved to a house. In addition, aesthetics and shade were iden- tified as the main reasons for tree planting. The authors collected detailed information on exist- ing trees as well as house and lot size. They did not, however, investigate the effect of demograph- ics (except age) on the tree-planting decision. In contrast to Summit and McPherson (1998), Greene et al. (2011) focused on how demograph- ics influenced participation in a tree-planting pro- gram in Toronto, Ontario, Canada. They found that the number of single detached dwellings, immigration status, income, house age, and per- cent of females employed all influenced participa- tion in the program, although results varied across neighborhoods. The authors did not have access to household-level data. Rather, their unit of obser- vation was a census tract (n = 490). The authors were careful to point out that their aggregate results may not apply at the household level. The coarse scale of the study also meant that they were unable to account for the effect of existing trees, planting spaces, house size, and lot size, among other criteria. The authors build on these two studies by inves- tigating why urban residents in Portland, Oregon, U.S., agree to participate in a tree planting program. Detailed household-level data were incorporated, as well as demographic data at the census block-group level, into a random-effects regression model. For- mally, the study authors hypothesize that attributes of a house, the number and size of existing trees, the number of planting spaces, characteristics of the 71 canvasser, and the demographic makeup of a neigh- borhood all influence the likelihood that a house- hold agrees to participate in a tree-planting program. DATA AND STUDY AREA Portland is located in northwest Oregon near the confluence of the Willamette and Columbia rivers. The city has a population of 584,000, while the Port- land metropolitan area has population of 2,260,000. Currently, 26% of the city is covered by tree cano- py, but the city wants to increase canopy cover to 33% (Karps 2007). This goal cannot be met solely by planting trees on public land; the city must also rely on private landowners to plant trees. As part of this process, the city has been canvassing residents about their willingness to plant a tree in the park- ing strip fronting their home. If residents agreed, then trees were provided at reduced cost and plant- ed during a neighborhood tree-planting day over- seen by a local non-profit called Friends of Trees. In Portland, parking strips are public land (i.e., right-of-way), but the adjacent property owner is responsible for planting and maintaining trees in the parking strip. In addition, a permit is required to plant or remove a tree in the parking strip. Between June 10, 2010, and August 10, 2011, the City of Portland canvassed 51,885 homes in Eastside Portland (east of the Willa- mette River). The city focused on Eastside Port- land because it has 20% canopy cover, whereas Westside Portland has 55% canopy cover (this includes the 2,000 hectares of Forest Park). In addition to promoting a tree-planting pro- gram, canvassers recorded data on the number of trees currently in the parking strip fronting a house, the number of unfilled planting spaces, width of the parking strip, and the presence of overhead power lines. Canvassers only visited houses that had at least one available planting space in the parking strip fronting the house. A priori, canvassers were not expected to be equally effective at convincing residents to participate in the tree-planting program. Therefore, 11 dummy vari- ables were used to identify canvassers. Ten of these dummy variables denoted an individual, while the eleventh denoted a group of canvassers who had individually visited only a handful of houses. In addi- tion to these canvasser dummy variables, a dummy variable was created to denote a canvasser’s gender. ©2014 International Society of Arboriculture
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