Arboriculture & Urban Forestry 37(4): July 2011 frustum provided highly accurate estimates of crown volume relative to UrbanCrowns (Β1 crown volume (Β1 DISCUSSION On a strictly operational basis, UrbanCrowns software provided highly repeatable estimates of both crown volume and density across a broad range of tree sizes and showed limited sensitivity to photographic distance when photographed on a consistent azi- muth. The majority of crown volume estimates had repeatability errors of less than 5% across photographic distances; errors in the range of 5%–10% were most common for large trees pho- tographed across long distances. Crown density estimates were more robust with the majority of repeated measurement errors less than 1%. These findings suggest that UrbanCrowns has high potential as a practical, reliable technology for assessing crown attributes of urban trees. Previous researchers have document- ed effective methods of estimating tree crown dimensions us- ing photography (Miller and Lightner 1987; Seiler and McBee 1992; Wilkinson 1994; Phattaralerphong and Sinoquet 2005), but their methods typically required specialized equipment/soft- ware or substantial computer post-processing. In contrast, Ur- banCrowns requires only basic photographic, mensuration, and computing equipment and minimal computer processing time. Moreover, the relatively simple field and computational mea- surements make the technology accessible to a broad range of skill levels. The program’s robustness to varying photographic distances also makes the program particularly valuable to chrono- logical studies where landscape changes over time may make it impossible to photograph a given tree at a consistent distance. Although UrbanCrowns has not been subjected to compu- tation validation through destructive sampling of trees (to de- termine true volume of tree structures in the crown), the high correlation of its calculations with those of geometric solids suggests that the program is reasonably accurate. Karlik and Winer (1999) found that estimates of leaf mass based on geo- metric solids (sphere, vertical ellipsoid, and paraboloid) were within ~20% of destructively sampled leaf mass for various ur- ban tree species. Similarly, Wright et al. (2006) found that about 80% of variability in fruit density could be accounted by model- ing apple tree crown volume as an elliptical cone. In the current study, both the sphere and the conical frustum provided highly precise and accurate estimates of sugar maple crown volume relative to UrbanCrowns’ computations, with the sphere being marginally superior to the frustum. These results also suggest that crown volume can be reliably estimated using trunk diam- eter measurement; however, these results were rendered with a carefully selected sample of trees from a single species, which limits applicability of the model. Across multiple species, tree sizes, and crown condition classes, it is suspected that Urban- Crowns would provide more reliable crown dimension estimates than equations based on either geometric solids or trunk diameter. In addition to computation validation, there are other opera- tional limitations of UrbanCrowns that merit consideration. First, there is the analytical assumption that the tree crown is horizon- tally symmetrical (i.e., circular from aerial view). Although the crowns of open-grown trees have a tendency to be symmetrical (Mawson et al. 1976; Mallette 1982), urban tree crowns may be misshapen due to storm damage, directional pruning, or crowd- = 1.4226). Both the sphere and the conical = 1.0033 and 1.0109, respectively). 177 ing. Currently, UrbanCrowns has no built-in feature to adjust its calculations for horizontal crown asymmetry. This limitation might be addressed by simply taking photographs from mul- tiple angles around the tree, computing crown dimensions for each perspective, and then calculating the mean of these values. Peper and McPherson (2003) addressed this challenge in simi- lar fashion when assessing leaf area of sycamores with crowns misshapen by constant directional winds. The researchers took two side-angle photographs of each tree crown from perpendicu- lar perspectives and averaged the leaf area estimates calculated from each photograph. Despite substantial crown asymmetry, this photographic method produced leaf area estimates within 25% of destructively sampled leaf area on average, outperforming so- phisticated digital canopy analyzers in both accuracy and preci- sion. This approach may prove effective for analyzing misshapen crowns using UrbanCrowns and warrants further investigation. Another operational limitation is the effect of elevation on Ur- banCrowns calculations. In the current study, subject trees were chosen to minimize the effect of terrain on the analysis (i.e., trees on relatively flat ground were sampled). On uneven terrain, the ef- fect of the photographer’s elevation could compound the effects of photographic distance due to changes in the profile view of the tree. Finally, UrbanCrowns is limited by the ability of the photogra- pher to frame the entire crown in the image. In the current study, the closest distance practical for all tree size classes was 1.5× tree height; any closer, and it was not possible to capture the entire crown of larger trees. Although this may not be problematic for small trees, finding an unobstructed view of large trees at a distance ≥1.5× tree height could be a challenge in some urban settings. CONCLUSION Measuring tree crown attributes is an important aspect of evalu- ating and managing trees in the urban forest, and objective, re- liable methods for obtaining these measurements are needed. In this study, the robustness of crown dimension calculations made with UrbanCrowns image analysis software was evalu- ated. For open-grown sugar maples across a broad range of sizes, from recently transplanted to full maturity, it was found that software computations of both crown volume and density were highly repeatable across varying photographic distances. For the majority of tree size classes, crown volume and den- sity errors were less than 5% on average over varying photo- graphic distances; however, crown volume errors of 5%–10% were common for larger trees. UrbanCrowns calculations of crown volume showed strong agreement with calculations de- rived from empirical equations for geometric solids and were highly correlated with trunk diameters measured in the field. Although UrbanCrowns has great potential for field applica- tions, practitioners should be aware of several known and po- tential limitations that might exist in urban settings. First, it is critical that the tree photograph be unobstructed (particularly by neighboring trees) in both the foreground and background in order to obtain accurate measurements. In addition, it is cur- rently unknown whether the minor influence of photographic distance on crown measurements documented in this study might be exacerbated by irregular terrain, misshapen crowns, or species-specific crown geometry. Therefore, additional re- search is needed to fully evaluate the software’s robustness to these practical limitations as well as to establish the relationship ©2011 International Society of Arboriculture
July 2011
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