66 King and Locke: Methods for Measuring Local Urban Tree Canopy Cover in this study misses much of this critical resource due to the roadside nature of the sampling locations. For estimates of localized building cover, the GLA methodology results in higher values than the other two methods. If the 15.24 cm land cover data and derived GIS-based measurement may be considered the baseline due to its high spatial resolution and categorical accu- racy, then the GLA-based method overestimates building cover – likely because the hemispheric photos are capable of capturing buildings outside the reaches of the 0.08 ha plot. Furthermore, near and tall objects such as buildings may dominate the field of view. To the knowledge of the authors, this study represents the first use of the GLA methodology and software within an urban setting for the purpose of enumerating tree and building cover, and although the method was acceptable for measuring local tree canopy cover, it is not advised that researchers use these methods for capturing building cover across an urban landscape. Intensive field-based methods, such as i-Tree and GLA, involve direct sampling, while the extensive GIS-based approach utilizes a census of land cover derived from remotely-sensed data. Each method has value outside of simple measurements of urban tree canopy cover. Other data collected when using the standard i-Tree methods can be used to model ecosystem service values. GLA data may provide further analytical pos- sibilities, including urban micro-meteorological modeling and human comfort analyses (i.e., Heisler et al. 2003; Heithecker and Halpern 2006). In addition to quantifying tree canopy and building cover, the GIS-based approach can also be used to ana- lyze patterns in the urban landscape or identify possible planting areas by enumerating the amount of vegetated, soil, and impervi- ous areas that are not buildings, roads, water, and existing trees within any ecological or administrative boundaries. The high spatial resolution (15.24 cm) also allows for parcel-scale analy- ses, which are important because households may be considered the fundamental decision-making unit in urban areas (Pickett et al 2011). Researchers conclude that GLA techniques are un- derstudied in urban areas, and that the i-Tree and GIS-based ap- proaches are complementary and reinforcing tools indispensable for both the urban forest management and research communities. Acknowledgements. The authors thank the USDA Forest Service, Northern Research Station, Dr. Lynne Westphal, Dr. Gordon Heisler, and the National Urban and Community Forestry Advisory Council. Addi- tionally, this work benefited from Kyle Kalwarski, Jacqueline W.T. Lu, Nancy Falxa-Raymond, Peter Tiso, Peter Stothart, Sharai Lewis-Gruss, and Rich Hallett. LITERATURE CITED Fiala, A.C.S., S.L. Garman, and A.N. Gray. 2006. Comparison of Five Canopy Cover Estimation Techniques in the Western Oregon Cascades. Forest Ecology and Management 232:188–197. Frazer, G.W., C.D. Canham, and K.P. Lertzman. 1999. Gap Light Ana- lyzer (GLA), Version 2.0: Imaging software to extract canopy struc- ture and gap light transmission indices from true-colour fisheye photographs, users’ manual and program documentation. Simon Fraser University, Burnaby, British Columbia, and the Institute of Ecosystem Studies, Millbrook, New York, U.S. Ganey, J.L., and W.M. Block. 1994. A Comparison of two techniques for measuring canopy closure. Western Journal of Applied Forestry 9:21–23. Heisler, G.M., R.H. Grant, D.J. Nowak, Wei Gao, D.E. Crane, J.T. Walton. 2003. Inclusion of an ultraviolet radiation transfer compo- nent in an urban forest effects model for predicting tree influences on potential below-canopy exposure to UVB radiation. In: J.R. Slusser, J.R. Herman, and Wei Gao (Eds.). Proceedings of SPIE Vol. 5156 Ultraviolet Ground- and Space-based Measurements, Models, and Effects III. SPIE: Bellingham, Washington, U.S. Heithecker, T.D., and C.B. Halpern. 2006. Variation in microclimate associated with dispersed-retention harvests in coniferous forests of western Washington. Forest Ecology and Management 226:60–71. MacFaden, S.W., J.P.M. O’Neil-Dunne, A.R. Royar, J.W.T. Lu, and A.G. Rundel 2012. High-resolution Tree Canopy Mapping for New York City using LiDAR and Object-based Image Analysis. Journal of Applied Remote Sensing. Myeong, S., D.J. Nowak, P.F. Hopkins, and R.H. Brock. 2001. Urban Cover Mapping Using Digital, High-spatial Resolution Aerial Imagery. Urban Ecosystems 5:243–256. Nowak, D.J., R.A. Rowntree, E.G. McPherson, S.M. Sisinni, E.R. Kerkmann, and J.C. Stevens. 1996. Measuring and Analyzing Urban Tree Cover. Landscape and Urban Planning 36:49–57. Pickett, S.T.A., M.L. Cadenasso, J.M. Grove, C.G. Boone, P.M. Groffman, E. Irwin, S.S. Kaushal, et al. 2011. Urban Ecological Systems: Scien- tific Foundations and a Decade of Progress. Journal of Environmental Management 92(3):331–362. The City of New York, Mayor Michael R. Bloomberg. 2007. PlaNYC: A Greener, Greater New York.
USDA Forest Service. 2002. High resolution 2001/2002 land cover data for New York City. USDA Forest Service. i-Tree Eco User’s Manual. Accessed 03/30/2012. Kristen L. King (corresponding author) New York City Department of Parks and Recreation Forestry, Horticulture, and Natural Resources Group 1234 Fifth Avenue New York, New York 10029, U.S. [email protected] Dexter H. Locke NYC Urban Field Station 431 Walter Reed Road Fort Totten Park Bayside, New York 11360, U.S. [email protected] ©2013 International Society of Arboriculture
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