Arboriculture & Urban Forestry 38(2): March 2012 and tree density. Weighted values were used to perform one-way analysis of variance (ANOVA) on the basis of planting area type and parking lot class, followed by pairwise comparisons where significant. This sampling scheme provides a representative sample of (a) publicly accessible planting spaces within the in- teriors of parking lots in Raleigh’s planning jurisdiction, (b) the trees growing within these planting spaces, and (c) the parking lots in which these planting spaces were found. Results were considered to be significant at α = 0.05 for all statistical tests. RESULTS Parking lots were estimated to cover approximately 28.2 km2 (5.97%) of the study area. Most of the city’s parking lot area occurs in three land use zones: industrial (33%), multifamily 53 residential (17.7%), and offices and institutions (17.4%). Ex- tensive lots contained significantly more trees per lot than the other three classes of lots (P < 0.05). There were no signifi- cant differences in total trees per lot among the remaining three lot classes. No significant differences were found in the basal area, canopy area, or height per tree or the per-unit-area num- ber of trees, basal area, or canopy among the four lot classes. Significant differences were found in forest composi- tion between planting spaces that were preserved or designed and on the basis of their size and shape. Preserved planting spaces contained significantly more trees, basal area, and can- opy per area than designed planting spaces (Table 2), and dif- fered in dominant species and diameter distribution (Figure 4). In Raleigh, islands were the most common planting space, followed by rows. Designed rows had significantly greater canopy per tree, basal area per tree, and significant- ly fewer trees per hectare than designed islands (Table 3). Trees in designed chunks and rows were significantly taller than trees in designed slivers. Preserved islands contained greater per-unit area canopy cover and basal area than pre- served chunks and rows (Table 4). Mean tree size var- ied with respect to planting space type and origin (Table 5). Researchers estimated 44,000 ± 24,000 trees (95% con- Figure 4. Estimated density of trees by diameter class and plant- ing space type. fidence interval) were growing within these parking lots, representing at least 60 different tree species. Of these, 18,000 ± 7,600 trees were in designed planting spaces, and 26,000 ± 23,000 trees were in preserved planting spaces. This is a small fraction (~1%) of the 17 million ± 7.4 mil- lion trees estimated to be growing within the entire study area (Hess et al. 2008). Of the 60 tree species identified, 46 species were native to the study area and represent a mix of upland and lowland species, including evergreens and deciduous species. An estimated 5,800 trees, or 31% of trees in designed planting spaces, represent 14 species that are not native to the area. Crapemyrtle (Lagerstro- Table 2. Comparison of forest composition in preserved and designed planting spaces. All ranges are reported with 95% confidence. Preserved planting spaces (n = 41) Top 10 species Sweetgum (39%, n = 87) (Liquidambar styraciflua) Loblolly pine (16%, n = 144) (Pinus taeda) Shortleaf pine (9%, n = 50) (Pinus echinata) Eastern red cedar (7%, n = 10) (Juniperus virginiana) Sourwood (5%, n = 5) (Oxydendrum arboreum) White oak (5%, n = 4) (Quercus alba) Red maple (4%, n = 38) (Acer rubrum) Northern red oak (2%, n = 23) (Quercus rubra) Black cherry (2%, n = 7) (Prunus serotina) Mean basal area per hectare Mean canopy area per planting space area Mean of average tree height per planting space Mean trees per hectare of planting space (TPH) Black oak (2%, n = 9) (Quercus velutina) 47.8 ± 6.7 m2 165 ± 34% 14 ± 1 m 808 ± 126 TPH Designed planting spaces (n = 460) Red maple (27%, n = 362) (Acer rubrum) Willow oak (17%, n = 239) (Quercus phellos) Crapemyrtle (11%, n = 141) (Lagerstroemia indica) Northern red oak (8%, n = 32) (Quercus rubra) Chinese elm (4%, n = 41) (Ulmus parvifolia) Callery pear (3%, n = 27) (Pyrus calleryana) American holly (2%, n = 24) (Ilex opaca) Japanese zelkova (2%, n = 25) (Zelkova serrata) Wax myrtle (2%, n = 30) (Morella cerifera) Green ash (2%, n = 21) (Fraxinus pennsylvanica) 13.3 ± 1.8 m2 82 ± 10% 7.6 ± 0.3 m 334 ± 37 TPH ©2012 International Society of Arboriculture
March 2012
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