Canopy clumping index (CI) indicates the non-random distribution of foliage components in space, and is an important structural parameter for better understanding the radiative transfer process in a canopy. Field measurements of CI have been generally performed with the optical methods. However, actual measurements of CI are difficult to perform due to the complex measurement conditions.
Besides, what we measured using the general optical instruments, such as the LAI-2200, is the apparent CI (ACI) of vegetation canopy. Significant uncertainties exist in these ACIs calculated using different optical instruments due to the different measurement principles and calculation methods, and it is difficult to compare among different ACIs.
Prof. FANG Hongliang Institute of Geographic Sciences and Natural Resources Research (IGSNRR) and his team propose several new ACI metrics and analyze the new metrics using the seasonal data measured at paddy rice fields in northeast China. The new ACI metrics can be applied in the canopy radiative transfer modeling and in the estimation of canopy biophysical parameters from remote sensing data.
The researchers put forward to the necessity of describing different forms of foliage organization in geometric spaces using the directional ACIs at different levels-a particular cell, a concentric ring, an azimuth sector, and over the horizontal landscape.
LAI-2200 is the only instrument that reports the ACI at specified concentric rings until now. They pointed out that the ACI measurements can also be made with digital hemispheric photography (DHP) and AccuPAR.
They carried out a comprehensive comparison of the ACIs using different instruments. DHP provides a far finer directional spatial sampling of canopy gaps than other optical instruments, allowing CI to be derived along multiple zenith and azimuth angles.
"A whole ACI concept was derived from the directional ACIs. The whole ACIs were calculated from the directional ACIs with an angular integration method, a simple averaging method, a non-linear correction method, and a variance-to-mean ratio method. The simple averaging method can well approximate the whole ACI,” said Prof. FANG.
The results indicate that foliage randomness increases with the segment size. This study was published in the ISPRS Journal of Photogrammetry and Remote Sensing, was supported by the National Natural Science Foundation of China and the Hundred Talent Program of the Chinese Academy of Sciences.
Paddy Rice Experiment in the Sanjiang Plain (PRESP 2012): http://www.lreis.ac.cn/kfjl/zlxz/
Data of apparent clumping index (ACI) of paddy rice in the Sanjiang Plain: https://doi.org/10.1016/j.isprsjprs.2018.06.022.
Fang, H., Liu, W., Li, W., and Wei, S., 2018. Estimation of directional and whole apparent clumping index (ACI) from indirect optical measurements. ISPRS Journal of Photogrammetry and Remote Sensing, 144, 1-13.