Effects of components of the leaf area distribution on drag relations for Cryptomeria japonica and Chamaecyparis obtusa

The objectives of this study were to clarify the effects of components of the leaf area distribution on the drag coefficient of crowns and streamlining (e.g., leaf area index; LAI, outline of the crown shape, and clumpiness) and to contribute to the accumulation of data on drag relations by quantifying data for Chamaecyparis obtusa and Cryptomeria japonica. We conducted drag experiments while simultaneously capturing dynamic crown images for 28 Ch. obtusa crowns and 13 Cr. japonica crowns to analyze the relationships between the leaf area distribution components and drag coefficient or streamlining. The static drag coefficient increased with the LAI for Ch. obtusa and with decreasing clumpiness for Cr. japonica. The reduction rate of the static drag coefficient decreased with increasing clumpiness for Ch. obtusa and with a combination of increasing LAI and decreasing clumpiness for Cr. japonica. The reduction rate of the static drag coefficient had a clear relationship with the decreasing rate of the dynamic crown projected area of obstacles (foliage elements, branches, and stems) calculated from captured video images under windy conditions for Cr. japonica, while Ch. obtusa did not show clear relationship between them. The drag coefficients assuming non-porous crown; Cmax estimated by simple model combining LAI and clumpiness were approximately 1.0 in Ch. obtusa and 0.5 in Cr. japonica and were equivalent to the dynamic drag coefficients from video image under windy condition. The combination of LAI and clumpiness provided simple estimation for drag relations and enable to link crown structure to wind damage easier.