Estimation of Canopy and Woody Components Clumping Indices at Three Mature Picea crassifolia Forest Stands

Many previous studies have shown that the underestimation of the plant area index (PAI) by traditional optical instruments is mainly due to the clumping effects of forest canopies. However, the evaluation of the clumping effects of canopy (leaves, stems, branches, fruits, and flowers) and woody components (stems and branches) within forest canopies is still a challenging task for current indirect instruments and algorithms. In this study, two key issues of the gap size distribution algorithm (CC) were discussed first, such as the stop conditions of the gap-removal procedure and gap-removing methods. Four types of gap-removing schemes were compared and their impacts to the estimation of canopy and woody components clumping indices (Ω_e and Ω_w) were analyzed. Obvious discrepancies were found between the estimates of Ω_e and Ω_w from the four gap-removing schemes at each zenith angle. A new gap-removal scheme was proposed subsequently. After that, a solution was presented to estimate Ω_e and Ω_w at multiple zenithal and azimuthal directions in three mature Picea crassifolia forest stands. These estimates were retrieved based on multispectral canopy imager (MCI) equipment and CC algorithm. The estimates from the MCI equipment were later compared with those from the TRAC instrument. Small differences were found between the Ω_e estimates from the MCI and TRAC instruments, indicating that the MCI instrument was effective for evaluating the clumping effects of forest canopies at multiple zenithal and azimuthal directions. Results showed that both of Ω_e and Ω_w change with the zenithal and azimuthal directions.