| Leaf area index is a dimensionless variable and a ratio of leaf area to per unit ground surface area. It is a necessary parameter of vegetation system. It is a particularly significant index of vegetation system, which is widely used for indicating the physicochemical status of crops (e.g., growth, yields, photosynthesis and disease monitoring), and for field management. So far, many studies have been conducted on crop LAI measurement and a variety of different methods have been proposed. Main methods can be divided into several groups, including traditional methods (e.g., weighing method, square grid method, computer-aided scanning method, length-width coefficient method), canopy analysis system (e.g., LAI-2000, Sunscan), hyper-spectral inversion models (e.g., vegetation index, principal component analysis (PCA), partial least squares (PLS)) and digital image methods. For different crops and different stages of crop growth, different methods have their own specific characteristics and applicability. However, the issue that which method is more appropriate for LAI inversion under different situations was rarely studies systematically in the past. Therefore, this study was focused on comparing the suitability of several different methods on LAI inversion.In this study, four commonly used methods (i.e., length-width coefficient method, Sunscan canopy analysis system, hyper-spectral inversion model and digital image method)were used for LAI measurement of crops. Considering that different crops and the different period have different applicability on the LAI measurement, we selected three crops-wheat, rice and cotton in this study. The methods mentioned above were used to collect the data of traditional leaf area index, canopy spectral reflectance, canopy image and Sunscan-LAI of each crop during its whole growing period. We analyzed the collected data and compared the accuracies and applicability of different methods in measuring LAI of different crops at different growth periods. The purpose was to provide some guidance on method selection for leaf area index estimation of different crops.The following conclusions were drawn from this study:(1) The digital image method is more suitable for measuring crop LAI during the early growth period before canopy closure. The reason is that, in this period crops are relatively sparse with fewer leaves in field, and thus the canopy coverage can provide a good inversion of the true leaf area index. However, during the late period of crop growth (after canopy closure), this method is short of applicability due to its low accuracy. (2)The Sunscan method of LAI measurement is fast and convenient. It is more suitable for measuring crop LAI during the late growth period. The reason is that, in this period the crop canopy structure is more uniform that the measurement accuracy would be higher. By comparing the three crops (rice, wheat, cotton), it can be seen that it is more suitable for cotton LAI measurement than rice and wheat, the measurement precise is higher.(3) The spectral model method is more stable in measuring LAI than the digital image method, and has a wider range of measuring time, it can be used not only in the early period, but also in the late period. However, the modeling effectiveness of this method decreases during the ripe period of wheat and rice, because the grain is ripe and turned yellow during this time, to some extend it would have some influence on the spectral data collection. |
