28 METHODS Current Species Composition The current species composition of Toronto’s urban forest was identified from two preexisting citywide inventories: the Toronto street tree inventory, and the 2008 i-Tree sample inventory. The street tree inventory was obtained through the City of Toronto’s open data website and contains spatial and botanical information (e.g., species, diameter at breast height) for Toronto’s city-owned trees located on road allow- ances. The data were acquired in December 2012, last updated by the city in March 2012. Users download- ing the data are warned that there may be inaccura- cies and that some data may be out of date as the city only updates the street trees that are worked on in a given year (City of Toronto 2012). Acknowledging these issues, the data set represents the most compre- hensive inventory of all the street trees in Toronto. The 2008 i-Tree data is based on a citywide sam- pling effort through the use of the i-Tree Eco model (formerly UFORE; www.itreetools.org). Toronto initially selected a total of 435 plots using a spatially stratified random sampling approach, 407 of which were ultimately sampled (the remaining 28 were not accessible) (City of Toronto 2011). Of all the plots sampled, 276 contained trees. The i-Tree data included text files with plot and botanical informa- tion, as well as shapefiles representing the geographic location of the center point of each sample plot. For both data sets, the information used for analysis in this research was species and geographic location. There was limited overlap between the data sets, occurring only where street trees were sampled in the i-Tree inventory. In the interest of minimizing overlap, street trees were removed from the i-Tree data (169 trees in total), which also resulted in the removal of 33 plots from the analysis. Diversity of the current composition was assessed in two ways. First, genera that exceeded 5% of the population were identified. The thresh- old of 5% was chosen because the City of Toronto uses a 5-10-20 rule to help guide species diversity in the city’s urban forest. Under this rule, no more than 5% of the population should be a single species of trees, no more than 10% a single genus, and no more than 20% a single family. While this analysis occurs at the genuslevel, 5% was used as the thresh- old for analysis in order to be conservative and ©2015 International Society of Arboriculture Vander Vecht and Conway: Comparing Species Composition and Planting Trends look for potential future concerns, not just genera that are already present at problematic levels. Second, the inverse of Simpson’s diversity index (inverse SDI) was calculated. Inverse SDI was chosen following the work of Sun (1992) because it gives a measure of diversity and even- ness that is easily compared to the 5-10-20 guideline. The formula for the inverse SDI is: [1] [ΣNi • (ΣNi where Ni – 1)] / [Σ Ni (Ni – 1)] is the number of individuals in the ith (1, 2, 3 . . . nth) group (genus) and N is the total num- ber of groups in a particular population (Sun 1992). Larger inverse SDI values indicate greater diversity. When interpreting these inverse SDI results in light of the 5-10-20 diversity guideline, an inverse SDI value of 10 or more would broadly indicate that the desired genera diversity has been reached. Spe- cifically this means that out of 10 random samples containing two trees, one or less is expected to have two trees belonging to the same genus (Sun 1992). Toronto’s Tree Planting Trends The planting-related activities of various actors were examined to represent a wide spectrum of indi- viduals, business, and organizations impacting the future composition of the urban forest. The goal was to assess as much of the planting occurring on public and private property as possible. Ultimately four types of actors were included: landscape architects, non-profit organizations (NPOs), garden centers and nurseries, and city urban forestry staff. Lists of the most commonly planted or sold trees were then compiled from the surveys and interview results of each actor. Landscape architects work at both large and small scales in Toronto, planning exteriors for indi- vidual residential properties as well as for large- scale developments. The expectation was that landscape architects oſten include trees in their design. As such, they would be a valuable group to survey to gain perspective on what trees are com- monly planted on various property types, including for private citizens and construction contractors. NPOs were included in the research because there are several organizations throughout the city actively engaged in increasing the quality and quan- tity of the urban forest, both citywide and within
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