A new taper index based on form-factor: application to three bamboo species (Phyllostachys spp.)

The form-factor for the stem surface area is directly proportional to the square root of the form-factor for the stem volume, i.e., the square root law of the form-factor. Although the square root law addresses the stems of coniferous trees, the nature of the proportional coefficient of the law has not been discussed. In this study, we demonstrated that the coefficient indicates the stem taper; therefore, it is named “Taper Index based on Form-Factor (TIFF).” We also examined the relationship between the form-factor for the culm surface area and the form-factor for the apparent culm volume of 570 culms across three bamboo species (Phyllostachys pubescens Mazel ex Houz., P. bambusoides Sieb. et Zucc., and P. nigra var. henonis). The square root law held true for all three bamboo species. The species-specific TIFF was determined to be 0.873 for P. pubescens, 0.897 for P. bambusoides, and 0.901 for P. nigra. This result implied that P. pubescens had a more tapering culm form compared to the other two species, while culm taper was similar between P. bambusoides and P. nigra. Our findings align with empirical observations of the culm taper of the bamboo species. Intra-species variation in TIFF was considerably small, allowing us to evaluate the species-specific culm taper from a small number of samples. Therefore, we conclude that TIFF provides a simple and useful method for quantifying species-specific culm or stem taper, and facilitates the estimation of merchantable or total volume.