62 King and Locke: Methods for Measuring Local Urban Tree Canopy Cover Arboriculture & Urban Forestry 2013. 39(2): 62–67 A Comparison of Three Methods for Measuring Local Urban Tree Canopy Cover Kristen L. King and Dexter H. Locke Abstract. Measurements of urban tree canopy cover are crucial for managing urban forests and required for the quantifica- tion of the benefits provided by trees. These types of data are increasingly used to secure funding and justify large-scale plant- ing programs in urban areas. Comparisons of tree canopy measurement methods have been conducted before, but a rapidly evolving set of new technologies and applications may leave urban foresters wondering, “Which method is most appropriate for my circumstances?” This analysis compares two well-established measures of local tree canopy and building cover with a third, relative- ly untested technique. Field-based visual estimations (using the USDA Forest Service’s i-Tree protocols), summaries of high- resolution land cover data using geographic information systems (GIS), and an analysis of skyward-oriented hemispherical photo- graphs at 215 roadside sites across the five diverse counties of New York City, New York, U.S., are the methods evaluated herein. The study authors found no statistically significant differences between the methods when compar- ing tree canopy; however, the hemispherical camera had a tendency to overestimate building coverage. It is concluded that hemispheric photo techniques are understudied in urban areas, and that the i-Tree and GIS-based approaches are complementary and reinforcing tools indispensable for both the urban forest management and research communities. Key Words. Forest Measurement; Gap Light Analyzer; GIS; Hemispheric Photos; i-Tree; Urban Land Cover; Urban Tree Canopy. Measurements of urban tree canopy cover are crucial for manag- ing urban forests and required for the quantification of the ben- efits provided by trees. These types of data are increasingly used to secure funding or justify large-scale planting programs in ur- ban areas. Although many municipalities undertake these types of analyses, there are multiple methods for doing so (Nowak et al. 1996), and given different available technologies and underlying assumptions, these tools will ultimately differ in their results and costs. Comparisons of canopy cover measurement methodologies in undeveloped areas are quite common (e.g., Ganey and Block 1994; Fiala et al. 2006), and previous research has compared oth- er methods of calculating urban tree cover (Nowak et al. 1996). But with the advent of new technologies and applications this problem is worth revisiting. This analysis compares three mea- sures of local tree canopy and building cover derived from field- based visual estimations, summaries of very high-resolution land cover data using geographic information systems (GIS), and an analysis of skyward-oriented hemispherical photographs at 215 sites across the five counties of New York City, New York, U.S. MATERIALS AND METHODS The majority of data (155 of 215 total sites) for this analysis were collected in 2009 in support of the New York City Com- munity Air Survey (NYCCAS). Of the 155 air quality monitoring sites, 120 were preferentially assigned to areas with high traffic density, high building density, or both. Because of this stratified ©2013 International Society of Arboriculture random sampling, plots represent a broad geographical range and diverse urban conditions while maintaining a sufficiently robust sample size (Figure 1). An additional 60 sites featured the same measurements in a more restricted area of New York City (including the northern Manhattan and Bronx boroughs, or administrative divisions) and these data were collected as part of a separate air quality study during the summer of 2010. Because of this, sampling intensity is greater in that geographic area. Two field-based measures of local tree canopy were taken at all 215 sites. Estimates of tree canopy and building coverage were collected in accordance with the USDA Forest Service’s Urban Forest Effects Model (UFORE) protocol, now called i-Tree Eco (see USDA Forest Service’s i-Tree Eco User’s Manual for additional details), within 0.08 ha circular plots surrounding street-side utility poles, which functioned as the plot’s center. The goals of the NYCCAS study, for which these data were originally intended, required that 0.08 ha plots were used instead of the smaller, standard 0.04 ha. Tree cover, as the amount of the plot covered by tree canopy, is gathered by visualizing the area of the plot that would be shaded by trees when the sun is directly over- head, ranging from 0% to 100%, in 5% increments. When trace amounts are present, 1%, 2%, 3%, etc., are recorded. Canopy from trees outside of the plot are included, so plots not actually containing trees can have measurable tree cover. Aerial images were used to assist the field crew’s estimations. Using the same incremental measurements, building coverage is determined by estimating what portion of the plot ground area is covered by
March 2013
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