Arboriculture & Urban Forestry 34(6): November 2008 (5) Underground constraints: Undertaken by government tree management staff in con- junction with other departments and utility companies when the sites were actually used for tree planting; (6) Remarks: Unusual features not solicited in the previously mentioned attributes were recorded; and (7) Sketch: Where it helped to illustrate special observation, a sketch would be drawn. RESULTS AND DISCUSSION Advantages and Disadvantages of the Data Collection Approach The data collection approach has the advantage of covering all target trees in the study area in a census. It could avoid possible sampling errors and problems of representation of different population strata (Jaenson et al. 1992; Alvarez et al. 2005). The data were either quantitative or could be converted into ordinal ranks to facilitate statistical analysis. Plotting the locations of trees and potential planting sites on digital maps permitted spa- tial analysis of the distribution pattern. Detailed information cov- ered not just individual trees, but also planting sites and the immediate environs. Information on physical constraints to tree growth, including present and future restrictions, and symptoms of main tree defects and disorders could help tree managers design the tree maintenance strategy and program. The information on potential planting sites allowed advanced planning to improve the geographic spread, species diversity, and landscape contribution of roadside trees in different districts. The implementation of the synoptic planting plan is firmly based on in-depth understanding of ground truths at the potential plant- ing sites. The recommended species could take into consider- ation existing species makeup and projection of the landscape, amenity, and environmental and ecologic needs of the area in the future. Different neighborhoods could be allotted a certain sig- nature species to accentuate local landscape identity. The findings could assist tree managers to identify the strengths and weaknesses of the existing urban tree program and design the future management plan according to the identified problems, priorities, and resource envelope. More importantly, the systematic data analysis could yield useful hints to avoid past pitfalls in site preparation, species selection, and tree care; rein- force the good choices and practices; and find new or alternative ways to tighten and enhance the urban tree management pack- age. Overall, the method is comprehensive in scope and depth to serve multiple purposes. The tree survey could be expanded to include a questionnaire survey of citizens’ preferences (Schroeder et al. 2003) and attitudes (Schroeder et al. 2006) so that the urban forestry program could be adjusted to the com- munity’s wishes. Opinions of professional urban foresters could also be explored to identify institutional factors and predictors of better tree performance and management (D’Amato et al. 2002; Lewis and Boulahanis 2008). The approach has its disadvantages. It demanded much labor input and the corollary of a sizeable financial compensation to tree surveyors over an extended period. It required assistants with good knowledge and skills of urban forestry and arboricul- ture and competence in species identification. It necessitated many hours of laborious evaluation of trees and tree sites in the 371 field, filling in a detailed record form and inputting the data into a computer database. Over an extended period, experienced team members would leave and new members had to be recruited and trained. For attributes involving open-ended (nonmultiple choices) an- swers, inconsistency in judgment by different surveyors may lower data quality. The possible sources of errors are registration of wrong records in the field and erroneous judgment, measure- ments, and species identification. Additional errors could also be introduced at the data entry stage. The database could be stored centrally in a network server with access rights assigned to tree managers at the central and district levels. Data collected in this manner have a certain life span. Updating could be coordinated at the management level. Each district’s tree manager will modify the database on a monthly basis to record tree felling, planting, major maintenance operations, changes in tree performance, and alterations in plant- ing site conditions. A continually and diligently updated data- base could maintain its usefulness for many years. It is recom- mended that the tree survey should be repeated once every 10 years to overhaul the information, which will offer chances to introduce new tree survey methods and attributes associated with the latest research findings and practices. The positions of the individual trees were plotted on maps with the help of the AutoCAD program (Autodesk Inc., San Rafael, CA). Because global positioning satellite (GPS) pro- grams are now widely available at a reasonable cost, the tech- nology could be integrated into the field survey methodology to record the locations of trees with reference to a local coordinate system. The spatial data could be transferred to a GIS program to facilitate research and planning for tree management. Once digitized in a GIS setting, the spatial data of trees and potential planting sites could be efficiently analyzed and depicted. The integration of GIS and GPS technologies in urban tree surveys was initiated in the late 1990s (Widdicombe and Carlisle 1999) with recent emergence of some proprietary software products. Photographic records of individual trees or tree groups taken at different times, using the repeat photography method, could yield useful information on changes in urban vegetation. Sequen- tial aerial photographs could trace the changes in urban tree cover in response to urbanization effects (Jim 1989; Nowak 1993). Species Composition and Diversity The census recorded approximately 20,000 trees from 149 dif- ferent species. Considering the size of the study area and the human population, the number of street trees was rather limited. Comparison with other south Chinese cities such as Guangzhou and Taipei indicates the lack of street trees in Hong Kong (Table 3). It signified an improvement in comparison with approxi- mately 10,000 trees found in the 1985 survey. The severe con- straints to roadside trees in the cramped urban environment were reflected by the survey results. The large number of species represented a surprisingly high botanical diversity uncommonly found in other city streets. The 149 species have been classified into five groups with respect to frequencies (Table 4). The roadside trees were dominated by a small cohort of com- mon species with the top eight taking up 50% and the top 14 comprising approximately two-thirds of the total stock. A similar pattern of dominance was found in Guangzhou and Taipei (Table 3). Only the top 17 species had individual frequencies exceeding ©2008 International Society of Arboriculture
November 2008
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