The distance and direction coordinates from the center of each canopy gap to its edge in sixteen compass directions were measured in the secondary forests. Total of 21 gaps of different sizes were measured. The gap size was determined using different calculation methods: octagon method (OM), which was applied with two compass directions (OM_1 and OM_2); sixteen-gon method (SM); and elliptical-sector method (ESM), a new calculation method proposed in this work, including 8-elliptical-sector (8ESM) with two different compass directions (8ESM_1 and 8ESM_2) and 16 elliptical sector (16ESM). The results indicated that within the same gap delimitation, the average estimated gap size varied from 122.0 to 164.4 m2 with different calculation methods. The estimated area of the same gap among all calculation methods and between pairs exhibited significant differences (p < 0.05). Particularly, the estimated area for the same gap was significantly different between OM_1 and OM_2 (p < 0.005), indicating that the starting direction of the OM significantly affected the gap size. However, there were no significant differences among applications of the new method. When SM is used as a standard, the gap size calculated by OM was 10.0% less but, by ESM, was 11.6% larger. Therefore, the new method is proposed for future research involving the gap size calculation. Finally, we concluded that a comparison of forest gap sizes, particularly using OM, should be conducted when discussing the effects of gaps on forest regeneration and succession. © Higher Education Press and Springer-Verlag 2009.
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