98 the subject-specific focus of Nielsen et al.: Review of Urban Tree Inventory Methods Used to Collect Data at Single-Tree Level the study (hence- forth, ‘study focus’), e) the inventory method and type of technical aids applied, and f) the data parameters collected from each tree (e.g., diameter at breast height, tree height, species). In total, 152 parameters were identified across the 57 studies. However, many were redundant, like variations on the same type of measure- ment (e.g. crown width, canopy width, canopy drip line width, crown diameter, crown spread, width of crown). Aſter correcting for redundan- cies, a total of 59 parameters were grouped into 15 types of tree information parameters and three types of data-based parameters [e.g., vari- ous items of information on planting site in one group and various abiotic and biotic damage in another group (Appendix)]. Similarly, the inven- tory methods used to collect the data were clus- tered into overall typologies, reflecting differences with respect to terrestrial versus aerial methods, mode of transportation, and type of technical aid used to collect the data parameters from each tree. A comparative analysis was then made of the relationship between study focus, inventory method, and the 15 types of tree parameters col- lected. The data-based parameters were excluded from the analysis. Finally, the accuracy of the mea- surements obtained from each inventory method was evaluated for the 15 parameter groups. For each inventory method, the accuracy evaluation was restricted to the parameter groups collected by use of the method and/or tested by studies addressing the accuracy of the measurements. In the evaluation, the findings of studies specifically assessing the accuracy of measurements obtained by different inventory methods was the starting point. Due to the inclusion of both qualitative parameters (e.g., maintenance need) and mea- surable parameters (e.g., DBH and crown diam- eter), and the heterogeneity in studies in terms of methodology and reporting of data (some studies reporting deviation from field survey data (e.g., Park et al. 2010) and others omitting this infor- mation (e.g., Rutzinger et al. 2011), quantitative criteria for evaluation of accuracy levels were not deemed suitable. Thus the evaluation of accuracy comprised an interpretation of findings across the studies addressing the individual inventory methods, with the accuracy divided into a qualita- ©2014 International Society of Arboriculture tive scale with four levels: 0 = not possible or with very low precision; 1 = low precision; 2 = interme- diate precision; and 3 = high precision, meaning the accuracy corresponded to that obtained from direct measurements on site (field surveys). RESULTS Bibliographic Overview More than half the studies that met the inclusion criteria were conducted in North America (58%, n = 33), while articles originating from Asia accounted for 18% (n = 10), Europe 16% (n = 9), Australia 5% (n = 3), and South America 4% (n = 2). The search did not identify any publications from Africa. The studies were published in as many as 36 dif- ferent journals. Arboriculture & Urban Forestry (named Journal of Arboriculture until 2006) was the journal with the most publications, with 26% of the articles (n = 15), followed by Landscape and Urban Planning (n = 4), Urban Forestry & Urban Greening (n = 3), and Remote Sensing (n = 3). The remaining 32 journals were represented with one article each. Articles were published from 1984 onwards, and two breakpoints in the flow of publications could be distinguished. Up until the mid-1990s, the publications were few and scattered, but from then on, publication stabilized with 1–2 articles per year, nearly all of which originated from North America (Figure 1). A second break- point occurred in the late 2000s, where the annual publication rate increased to 3–15 articles. This increase coincided with a noticeable rise in the number of articles reporting research from South America, Australia, Europe, and Asia (Figure 1). Figure 1. Number of publications on urban tree inventories per year and their geographical origin.
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