Arboriculture & Urban Forestry 34(6): November 2008 Arboriculture & Urban Forestry 2008. 34(6):391–392. 391 Assessing Urban Forest Structure: Summary and Conclusions David J. Nowak This special issue has presented data on several topics related to assessing urban forest structure. These topics include means to measure urban forest cover from aerial-based platforms (Walton et al. 2008), national and local ground-based assessments of urban forest structure and functions (Cumming et al. 2008; Nowak et al. 2008a), measurement of street tree environments through both sampling (McBride 2008) and 100% inventories (Jim 2008), evaluating urban forest structure and health in Shenyang, China (Ning et al. 2008), and the effect of plot and sample size on estimate precision (Nowak et al. 2008b). These papers provide insights to aid cities in assessing their urban forest resource. In assessing an urban forest, there are several questions that need to be answered prior to conducting an assessment. These questions include: 1) What is the study area? – is it the entire urban forest (all trees) within a specific area (e.g., city or part of city) or is it just a street or park tree population? 2) Will all trees be measured (complete inventory) or will a sampling procedure be used? 3) Will aerial imagery and/or ground data be collected? and 4) What variables will be measured? STUDY AREA The boundary of the study area is a critical element that defines the scope of the assessment. Often street and/or park trees are measured due to their public ownership, high visibility, and rela- tively easy accessibility. Street and/or park tree assessments pro- vide critical information for public urban forest management, but these public tree studies lack the comprehensive information that an urban forest assessment of an entire city can provide. As private land often dominates a city in terms of area, comprehen- sive assessments that include both public and private trees pro- vide a means to develop more integrated and complete manage- ment plans to sustain the entire urban forest into the future. SAMPLE VERSUS INVENTORY Complete inventories provide the most precise and comprehen- sive data on the urban forest. Inventories also provide essential data for management by providing specific tree data with asso- ciated tree locations that can be mapped. Major drawbacks to complete inventories are the costs associated with the time and effort involved with measuring every tree and costs associated with keeping the inventory current. Unless conducting a street tree assessment or an assessment of a relatively small parcel of land, tree inventories can be impractical to establish and maintain. Typically, sampling provides urban forest data at a lower cost than a complete inventory, but often lacks specific tree location information and provides only an estimate of urban forest data with varying degrees of precision. However, sampling is essen- tial to provide cost-effective urban forest data over large areas. Various sampling designs have been detailed in this special is- sue, but most relate to forms of random sampling of a study area. Random sampling is a common technique for providing accurate estimates of the urban forest population. The number and size of plots used in random sampling can vary based on the specific study purposes, but will affect the precision of the urban forest estimate (Nowak et al. 2008b). The McBride (2008) article was an exception with regard to random sampling. Since random sampling produced plots with no trees, he opted to use an expert decision approach to select street plots that were believed to be characteristic of the city. This approach worked well in provid- ing information on commonly used species, but cannot be used for statistical estimates of the tree population (McBride 2008). AERIAL VERSUS GROUND-BASED MEASUREMENTS Aerial-based information can provide relatively cost-effective data on urban cover types. This cover information can be sampled to provide general cover statistics with known standard errors, or it can be mapped to detail the location of various cover types. Aerial-based cover maps can be used with geographic information systems (GIS) to aid in urban forest management and integration with other city departments. These cover maps have varying degrees of accuracy depending upon resolution, image classification techniques, and quality control procedures used. In using cover map data, the user should be aware of the data accuracy. Urban tree cover is one of the most basic and simple variables in assessing urban forests and is best obtained from aerial im- agery. Aerial imagery can provide spatial information on current and potential locations of tree cover in two or three dimensions (e.g., LIDAR). However, aerial cover assessments cannot easily provide essential structural data needed for urban forest man- agement (e.g., species composition, number of trees, diameter structure, tree health). The current best ways to obtain these data are through ground-based assessments where individual tree at- tributes are measured. An integration of aerial- and ground-based approaches will provide the most comprehensive data to improve urban forest management. DATA VARIABLES Before collecting field data for a ground-based assessment, a key decision is what variables to measure. Each variable will have an associated cost and, therefore, should be collected to fulfill the objectives of the assessment. Among the studies presented in this special issue, there are some consistently measured variables: species, diameter at breast height (dbh), tree height, location information (e.g., address, land use), and tree condition, which was often based on crown and other tree variables (e.g., trunk, roots). Other information commonly collected was crown width, height to base of live crown, tree and ground cover data, and information related to trees’ proximity to buildings. Variables ©2008 International Society of Arboriculture
November 2008
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