106 Nielsen et al.: Review of Urban Tree Inventory Methods Used to Collect Data at Single-Tree Level Key Parameters and their Accuracy of Measurements In total, 15 parameter groups were applied in the studies reviewed. Studies focusing on tests of tree inventory methods, tree vitality, arboricultural management, and tree architecture and amenity val- ues in particular cover a wide range of parameters (11–13). This corresponds well with the number of parameters typically collected in city authority inventories. For example, Keller and Konijnendijk (2012) found that six major cities in North Amer- ica and Europe collected data on 8–20 parameters. However, the number and type of parameters var- ied greatly between studies with different focus areas, with only species information and the three size parameters (crown size/density, DBH, and tree size other than crown size and DBH) being consis- tently determined. These parameters also obtained a high rating in a recent Delphi study in which city officials, arborists, and academics evaluated the rel- ative importance of tree parameters for inclusion in large-scale urban inventories (Östberg et al. 2013). In addition, these parameters are essential input in the modeling and quantification of the ecosystem services that urban trees provide to the community and their economic value (i-Tree 2012). These four parameters therefore appear to be fundamental, and it is not surprising that size parameters have been the focus of tests of inventory methods that are less labor- intensive than field surveys (i.e., methods supported by satellite images, aerial photos, terrestrial laser scanning and mobile scanning, and on-the-ground digital photography methods). While satellite- and airplane-supported technical aids have found a wide range of applications in land-use and veg- etation inventories at coarser scales, current image resolution and laser equipment limit the accuracy of the measurements to the level of single trees. Ground scanning by use of terrestrial laser provided the highest level of accuracy of tree size measure- ments. However, Park et al. (2010) found that the data processing time varied from 39 to 87 minutes per tree, indicating that it is questionable whether the method is less labor-intensive than the direct measurements and visual inspections applied in field surveys. In contrast, the accuracy of species information data has scarcely been tested, and the test results for satellite-supported images show low lev- els of precision (Cavayas et al. 2012). In terms of tree ©2014 International Society of Arboriculture health status parameters (categorized in this study as ‘Damage’; ‘Hazard’; ‘Insects, pest, fungi’; and ‘Vital- ity’), the satellite- and airplane-supported methods and on-the-ground scanning methods again appear to have significant limitations, although their accuracy has not been tested in the papers reviewed. This constitutes an important barrier to applica- tion of these inventory methods by city authorities, for whom tree vitality assessment and hazard tree management are among the main reasons for inven- tory (Keller and Konijnendijk 2012; Thomsen 2012). LIMITATIONS OF THE REVIEW The four main types of tree inventory methods iden- tified can be combined to overcome the shortcom- ings discussed above. This has been done in several studies on emissions of VOC gases by urban trees (Geron et al. 1995; Benjamin et al. 1997; Drewitt et al. 1998; Karlik and Winer 2001; Zhihui et al. 2003; Wang et al. 2003; Wang et al. 2005), where aerial methods have been combined with field sur- veys to collect high-quality data for extrapolation. In this regard, satellite-supported methods appear to be the typology most frequently combined with another inventory method, primarily field surveys (Diem and Comrie 2000), but also historical docu- mentation (Mickler et al. 2002). While such ‘mixed inventory methods’ were beyond the scope of this review, they offer interesting perspectives that fu- ture research ought to explore (e.g., extrapolate inventories to larger areas). Furthermore, the review was restricted to articles written in English and pub- lished in peer-reviewed scientific journals. Howev- er, tree inventory methods and findings at the local or national level may also be published as reports, guidelines, and ‘gray’ literature, and thus a number of studies in which urban tree inventory methods are described and/or used as data sources may have been overlooked. An obvious example of this is the windshield method applied in practice (Pokorny 2003; Rooney et al. 2005; Escobedo and Andreu 2008), but not as part of the peer-reviewed research included in this review. With this exception, to the knowledge of the study authors, the scientific lit- erature provides a reliable profile of the current status of urban tree inventories. Thus this review provides a useful comparative analysis of the differ- ent methods used to collect data at single-tree level.
March 2014
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