374 Number of trees Jim: Urban Forest Census in Hong Kong Table 4. Roadside tree composition in Hong Kong according to five frequency groups. Frequency group Dominant Abundant Frequent Occasional Rare Frequency range Greater than 500 150 to 500 50 to 149 10 to 49 Less than 10 Total Count 10,674 5,121 2,249 856 254 19,154 understanding of tree values (McPherson et al. 1997; Tyrväinen and Miettinen 2000; Jim and Chen, in press). Benefit–cost analy- sis of individual species could provide hints to selection of spe- cies to maximize their benefits (McPherson 2003). Such findings could provide strong justifications to preserve tree budgets and to compete with other claims for municipal funding. Potential Planting Sites A total of 1,094 potential planting sites with a maximum capac- ity for 12,063 trees were visually identified in the survey. All plantable locations have been marked on 1:1,000 large-scale maps. The sites were unevenly distributed in districts and regions with Kowloon East taking up 49%, Hong Kong East 22.7%, Hong Kong West 14.8%, and Kowloon West 13.5%. For the study area as a whole with at present 509 streets with trees, a further 389 more could be greened. Most potential planting sites were small, each with room for less than 13 trees, including many that could accommodate only a few. Only 138 sites could hold more than 20 trees, and only six were large enough for more than 60. On average, each site could encompass 11 trees and each street 31 trees. Thus, the sites were both small and scattered. By dimensions, most sites were shorter than 100 m (330 ft) and narrower than 5 m (16.5 ft). Some 89% of the sites were free from awnings, and the same proportion was situated on pavements. Most off-pavement sites had already been District Count Count 1,318 Wan Chai Subtotal Kowloon West Yau-Tsim Mong Kok Sham Shui Po Subtotal Kowloon East Kowloon City Wong Tai Sin Kwun Tong Subtotal Total 1,162 1,372 2,534 1,049 161 906 2,116 1,517 1,179 1,441 4,137 10,105 233 350 583 566 288 854 267 53 399 719 503 229 288 1,020 3,176 Broadleaf deciduous Percent 17.6 26.2 22.0 20.7 14.0 17.9 14.1 13.2 22.9 17.8 19.3 13.5 8.5 13.3 16.6 Percent 55.7 26.7 11.7 4.5 1.3 100 Number of species Count 10 14 23 33 69 149 Percent 6.7 9.4 15.4 22.1 46.3 100 enlisted for planting. The spread of sites among different land uses was very uneven. In addition to using field survey, aerial photographs or satellite imageries could offer an additional tool to identify potential planting sites in cities (Wu et al. 2008). However, ground truths with pertinent bearing on tree growth could not be gleaned; hence, at the implementation stage, field evaluation of the planting site would still be necessary. The actual number of plantable trees could be significantly reduced as a result of various above- and belowground con- straints. In practice, at the most, approximately 70% of the sites could actually be used. It was anticipated that these potential sites would be used up soon. Thereafter, plantable sites would have to come from new development and urban renewal areas. The findings demanded a concerted effort to change the town plan if urban roadside greening were to continue in the future. A joint effort between the government and developers, following clearly defined objectives, would be needed to generate addi- tional planting sites in the long term (Jim 2004b). The poor performance of many trees, particularly at roadsides, calls for an overhaul of the tree establishment practice. The survey data provided telltale information on soil characteristics at the existing tree sites and hints at soil qualities at the potential planting sites. It is particularly important to assess soil quality and soil volume available for root growth, because it has been identified as a serious constraint on tree establishment and Table 5. Distribution of roadside trees by growth form in ten urban districts of Hong Kong. Broadleaf evergreen Percent Count Hong Kong West Central—Western Southern Subtotal Hong Kong East Eastern 601 717 45.5 53.7 49.6 42.6 66.8 53.0 55.3 40.0 52.0 52.4 58.3 69.3 42.8 53.9 52.8 ©2008 International Society of Arboriculture 24 34 58 15 31 46 20 6 2 28 56 12 76 144 276 Conifer and related Percent 1.8 2.5 2.2 0.5 1.5 1.0 1.1 1.5 0.1 0.7 2.2 0.7 2.3 1.9 1.4 Count 463 233 696 987 363 1,350 560 183 435 1,178 527 281 1,565 2,373 5,597 Palm and palm-like Percent 35.0 17.5 26.2 36.2 17.7 28.2 29.5 45.4 25.0 29.2 20.2 16.5 46.4 30.9 29.2 Total Count Percent 1,321 100 1,334 100 2,655 100 2,730 100 2,054 100 4,784 100 1,896 100 403 100 1,742 100 4,041 100 2,603 100 1,701 100 3,370 100 7,674 100 19,154 100
November 2008
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