MEASUREMENT and PREDICTION of TREE GROWTH REDUCTION from TREE PLANTING SPACE DESIGN in ESTABLISHED PARKING LOTS by Jason Grabosky1 and Edward Gilman2
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154 Grabosky and Gilman: Tree Growth Reduction from Planting Space Design MEASUREMENT AND PREDICTION OF TREE GROWTH REDUCTION FROM TREE PLANTING SPACE DESIGN IN ESTABLISHED PARKING LOTS by Jason Grabosky1 and Edward Gilman2 Abstract. Five species of trees were measured in 12- to 27-year- parking lot environment, and the unfortunate lack of old parking lots in north-central Florida, U.S. Tree species prophetic prowess of professionals for estimating attrition, evaluated were Chinese elm (Ulmus parvifolia Jacq.), live oak car hits, maintenance damage severity, pavement resurfac- (Quercus virginiana Mill.), sycamore (Platanus occidentalis L.), ing needs (accounting for the 15- to 25-year criterion), and shumard oak (Q. shumardii Britton), and laurel oak (Q. laurifolia design changes over time. Michx.). Tree growth in the parking lot planting zones was reduced as the surface area of nonpaved surface was reduced for While field wisdom and experience is useful, quite often all trees except live oak. Regression models yielded significant professionals charged with the design of planting spaces relationships with prediction value between parking lot detail have little or no training or experience beyond any casual space openings and tree size, measured by diameter at breast personal observation. Alternatively, the designer may height or by canopy radius. Normalization of the data within the consult an industry contact who also relies on general site allowed for multiple site analysis. A method for generating observation rather than measurement. As a result, ordi- similar data is introduced for development of regionally species- nance requirements may be met by creating tree planting specific growth reduction multipliers. Those multipliers can be spaces and tree species selection with tree growth expecta- used to better meet long-term canopy expectations or to advocate tions based on references for tree growth in parklike or for larger planting zones. Key Words. Chinese elm; live oak; sycamore; shumard oak; natural settings (Dirr 1990; Gerhold et al. 1993; Bassuk laurel oak; canopy size; trunk dbh; tree height. 1998; Gilman 1997; Porter 2000). Such references do not report reduced growth or basic tree–pavement design limitations; thus, designs meet the design letter of the law Concerns for environmentally sustainable urban develop- in planning for canopy but fail to meet the intent of the law ment and planning have grown with increases in urbaniza- due to lack of growth (McPherson 2001). While there is tion, exemplified by catchphrases such as “smart growth” anecdotal evidence that trees in parking lots are smaller and increased legislation with regard to environmental than those growing in less stressful environments, little data impacts. Outgrowths of these concerns are the development are available to document the effect. The goal of this study of community forest management plans and the adoption of was to determine the relationship between canopy area to municipal shade tree ordinances. Within many ordinances, diameter at breast height (dbh) and attempt a fitting and in recommendation templates for ordinance develop- coefficient for growth expectation based on site restrictions ment, there is language for the assignment of minimum as represented by open soil surface in the planting zone in canopy cover in the design of parking lots and industrial parking lot design detail. The information was used to development areas (Lee County 1990; ISA 2001; City of evaluate the expectation of growth in varied design details Sacramento 2003). for planning parking lot tree planting spaces to better meet Design of parking areas usually provides for some level the provisions of municipal tree ordinances. of tree canopy establishment, in balance with space concerns and capacity parking issues (ISA 2001; Garber MATERIALS AND METHODS 2000a, 2000b, 2000c). Tree ordinance language often In 2002, parking areas located throughout north-central provides formulae for calculating the required number of Florida, U.S., were selected for data collection to test the individual trees. Methods include using a canopy cover hypothesis of decreased canopy expectations for parking lot percentage, meeting a canopy target per unit of paved area, trees. Twelve- to thirty-year-old parking lots at malls, using a set number of trees per unit paved of area, or shopping centers, colleges, and universities were identified. mandating vegetation space per number of parking spaces. Upon securing permission from owners, each site was Often when a percentage of cover is mandated, there is a visited prior to data collection to verify adequate numbers time frame imposed for such a designated level of coverage of trees, the availability of suitable species, and the presence target to be fulfilled, such as 50% shade of the lot within 15 of a variety of tree–pavement design strategies. All sites years (ISA 2001; Barr Engineering 2001; City of Sacra- required a test species in the parking lot as well as the same mento 2003). A limitation of the growth prediction method species planted on the periphery of the lot in nonrestricted is the lack of measured tree growth impacts from the soil areas as part of the site construction project. Sites of Journal of Arboriculture 30(3): May 2004 155 Table 1. Site identification project age and species present on site for protocol for trunk diameter followed canopy analysis. recommendations in the Guide for Plant Age in Live Shumard Laurel Appraisal (CTLA 2000). Soil openings Site years Elms oaks Sycamores oaks oaks without pavement were measured and assigned into a size class rounded up to the UF O’Connell Center 20+ 39 nearest 9.29 m2 (100 ft2). For regression UF Track 20+ 19 analysis of measured tree parameters against Oaks Mall 24 52 24 49 available nonpaved space, several assump- Newberry Square 16 27 41 Santa Fe C.C. ‘A’ 11 10 tions were placed on the delineation of the Santa Fe C.C. ‘B,’ ‘C,’ ‘D’ 20+ 34 nonpaved root zone area. Where trees Santa Fe C.C. ‘E’ 15 7 shared a planting zone, the total zone was Paddock Mall 20 117 divided by the number of trees in the zone. Shands Hospital 20+ 24 Where pavement openings intersected at Orange Park Mall 27 49 points less than 15 cm (6 in.) wide, the Avenues Mall 12 111 68 planting zones were considered separately, The Park 15+ 19 unless visual evidence was present that the Total trees 287 241 78 43 41 roots were in both profiles. Where trees fit into an “unlimited” soil zone, the soil volume for further analysis was arbitrarily set to be 2 times the area within the data collection are shown with replicate counts in Table 1. drip line of the tree where no curbs were within 5 m (16.5 Tree species evaluated were Chinese elm (Ulmus parvifolia ft) of the trunk, and 1.5 times the area within the drip line Jacq.), live oak (Quercus virginiana Mill.), sycamore in cases where pavement was within 5 m of the trunk. To (Platanus occidentalis L.), shumard oak (Q. shumardii prevent undue influence in the regression relationship, the Britton), and laurel oak (Q. laurifolia Michx.). The species mean of the control group calculated areas was used to were chosen because of their prevailing use and availability generate a group endpoint for the regression analysis, rather throughout the region. than regressing on the relationship generated from the Initial treatment codes were assigned to each tree based calculated soil areas based on the size of the control on the size and shape of their respective island, strip, and canopies. open planting area and their proximity to curbs and At older commercial sites, landscape maintenance pavement (Table 2). The design details encountered were companies had changed hands over the years, as had parking lot edge (large rooting zone, portion of canopy over ownership and management. Documentation indicating pavement), closed knuckle divider (zone surrounded by planting dates and installed tree size was difficult to pavement used to direct traffic flow), open knuckle divider acquire, when existent, and site maintenance profiles (surrounded by pavement and directing traffic flow, but (pruning, fertilization, and irrigation) were inconsistent. connected to open soil at one end), and vegetation strips Additionally, site conditions pertaining to construction, [long, interconnected root zone, placed in three classes: < 1 m (3.3 ft) wide, 1 to 3 m (3.3 to 10 ft) wide, and > 3 m Table 2. Description of treatment groupings in park- (10 ft) wide]. Sites used as controls featured open soil, no ing lot design analysis. Number of curbs was assigned pavement below canopy, and a large rooting zone. For for analysis of canopy size by design type. better interpretation, the zones were measured for area of Treatment group Number of curbs nonpaved soil surface. All trees on the site were measured and used except Parking edge distant 0 those known to be replacement trees for earlier losses. Tree Parking edge close 1 height to the nearest 0.31 m (1 ft) was determined using a Vegetation strip Suunto clinometer from a distance of 30.48 m (100 ft). >3 m 2 1–3 m 2 Average canopy radius was calculated from north, east, <1 m 2 south, and west canopy measurements to the nearest 15 cm Dual tree narrow oval 2 (6 in.) from tape measure center from trunk to branch tip. (>3:1 dimension ratio) The area below the drip line was calculated as a circle from the mean canopy radius per tree. Trunk diameter (dbh) was Columnar knuckle, L and T 3 if connected to larger soil zone measured to the nearest 1 mm (0.04 in.) at 1.37 m (4.5 ft) Columnar knuckle, L and T 4 if not connected to larger soil zone Triangulated knuckle 3 if connected to larger soil zone above soil. Trunk diameter on multiple trunks, the elms, Triangulated knuckle 4 if not connected to larger soil zone were calculated by the square root of the summation of the Classic tree pit opening 4 individual trunk diameters squared.