Arboriculture & Urban Forestry 40(2): March 2014 elsewhere, following pioneering work by the U.S. Forest Service on quantifying and modeling urban forest benefits in the Chicago Urban Forest Climate Project (Konijnendijk et al. 2006). It is clear that this work in Chicago stimulated the adoption of urban forestry and urban tree research in Europe and elsewhere, supported by networks such as COST Action E12 on Urban Forests and Trees, which ran from 1997–2002 (Konijnendijk et al. 2005; Schip- perijn et al. 2005; Konijnendijk et al. 2006; Koni- jnendijk et al. 2007), and the re-initiation of an urban forestry unit under the International Union of Forest Research Organizations (IUFRO) in 1986. In 1983, Smiley and Baker rightfully argued that not until the ‘why’ has been clearly defined can specific data be collected at single-tree level. The findings of the present review indicate that the ‘why’ question remains relevant and may be more imperative than ever, not only for city authorities and urban tree managers (Smiley and Baker 1988), but also for the scientific commu- nity. The analysis reveals how data from urban tree inventories at single-tree level have found applications in research with a steadily grow- ing number of different study focus areas. The fact that the 57 articles included in the review are published in 36 journals with differing sci- entific scope underscores the use of urban tree inventory data across disciplinary borders. A notable current trend is the use of data at single-tree level in studies focusing on the ecosys- tem services provided by urban trees (including their economic value), as a supplement to stud- ies linked more directly to daily arboricultural management and planning by city authorities. In fact, all cost-benefit studies and nearly all stud- ies on the regulating environmental aspects of urban trees have been published since 2001, thus coinciding with, and are probably prompted by, the popularization and formal definition of the ecosystem services concept by the United Nations Millennium Ecosystem Assessment 2001–2005 (Millennium Ecosystem Assessment 2000). In a wider perspective, one can therefore argue that the concept of ecosystem services has facilitated an overall change in the scientific use of urban tree inventory data from an applied focus (Sudol and Zach 1987; Smiley and Baker 1988; McBride and Nowak 1989) to the current situation, where 105 it also includes more strategic research (e.g., Dimoudi and Nikolopoulou 2003; Nowak et al. 2006b), as defined by OECD (1994). However, the work in Chicago during the early 1990s pioneered such research and the development of current models, such as STRATUM (now i-Tree Streets) and UFORE (now i-Tree Eco). The very recent increase in studies focusing on technological development and modeling studies also adds to this gradual shift towards more strategic consid- erations. However, most of the studies reviewed here focus on regulating ecosystem services related to the urban heat island effect, CO2 seques- tration, storm water runoff, urban biodiversity, and VOC gases. An interesting perspective for the future would be to explore how tree inventory data at single-tree level can support more strategic research into the cultural ecosystem services pro- vided by urban trees (e.g., non-material benefits related to cultural heritage, social cohesion, rec- reational experiences, and aesthetic experiences). The main contribution of the present study is that it goes beyond the ‘why’ question and establishes a typology of ‘how’ urban inventories are conducted (at least, those applied as the main data sources in current peer-reviewed research in the area) and ‘how’ the different contemporary inventory meth- ods and technical aids affect the parameters that can be collected at single-tree level and their accu- racy of measurement. Four main types of urban tree inventory methods were identified. With 46 out of 57 articles, the ‘classical’ field surveys, where ground staff carried out direct measurements and visual inspections, dominated. One concern in relation to this inventory method is that it is labor-intensive and generally limited to public trees, simply because of the difficulties in obtaining access to trees on private land (Nowak 2008). Satellite- and airplane- supported methods are less labor-demanding and have the potential to provide easy ‘access’ to trees on private land. It is therefore exciting to see the many recent publications using/testing satellite- and airplane-supported methods (e.g., Jutras et al. 2009; Arroyo et al. 2010; Ardila et al. 2012), ground scan- ning (e.g., Park et al. 2010; Rutzinger et al. 2011), and digital photography methods (e.g., Patterson et al. 2011; West et al. 2012) for data collection at single- tree level. The application and testing of these tech- nologies may give an indication of coming advances. ©2014 International Society of Arboriculture
March 2014
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