数字半球摄影法获取植被表观聚集指数（ACI）采样尺度效应研究

The Clumping Index (CI) represents the degree of leaf aggregation in a canopy. It is an important vegetation structural parameter that affects radiative transport and is crucial to understanding radiative transport in canopies. The apparent clumping index (ACI) was proposed to deepen the understanding of the clumping effect and expand the field indirect measurement of CI. Studies have shown that ACI is closely related to the angular sampling scale. Even ACI measured by the same instrument changes considerably with the sampling scale, which presents a new challenge to the verification of CI products. The combination of digital hemispherical photography (DHP) and logarithmic average methods is widely used in field ACI estimation, and one of the key steps of this combination is the determination of the angular sampling resolution. At present, the influence of angular sampling resolution on ACI estimation and its variation with different vegetation types need to be further explored. When using the logarithmic average method based on DHP measurement data, the angular sampling resolution is jointly determined by two factors: view zenith angle sampling scale (∆θ) and view azimuth angle sampling scale (∆ϕ). On the basis of DHP images, this study divided view zenith and view azimuth angles by using a series of progressively increasing angular sampling scales. A total of 30 sampling methods and 17 levels of angular sampling resolutions were obtained and applied to forest, shrub, grass, and crops. The variation in ACI under different vegetation types was investigated with the decrease in the resolution of the 17 levels of sampling. At the same time, the relationship among the angular sampling scale effect of ACI, the spatial distribution of vegetation, and the growth stage of crops was discussed. Results showed that ACI varied from 0.5 to 0.8 in the four vegetation types with the decrease in the 17-level angular sampling resolution and exhibited an average increase of 26%, 29%, 14%, and 35%. At the different growth stages of soybean crops, ACI increased (up to 60%) with the decrease in the 17-level angular sampling resolution. In general, ACI was directionally dependent on the change in the angular sampling scale, and this characteristic was affected by the spatial distribution of the observed objects. When the observed objects were randomly distributed (forest, shrub, and grass), ACI generally showed a linear growth trend with the increase in VZA and VAA sampling scales; when the observed object had a regular distribution (soybean crop), ACI was still affected by VZA and VAA sampling scales, and the influence of the VZA sampling scale on ACI was highly significant. The results of this study showed that ACI increased with the increase in the angular sampling scale, that is, with the increase in the angular sampling scale, vegetation aggregation weakened and gradually exhibited a random distribution. This study further explored the directional dependence of ACI on the change in the angular sampling scale for different vegetation types. How to select the optimal angular sampling scale for different vegetation types is expected be the focus of future research. A detailed analysis of ACI field measurement and an in-depth discussion of the possible uncertainties in the measurement process are prerequisites for performing an authenticity inspection of CI products. They are also essential for improving the accuracy of CI estimation and CI product verification.