Canopy clumping index (CI) characterizes the spatial distribution of leaves or needles within a vegetation canopy. CI is critical in determining the canopy radiation transfer, photosynthesis, and hydrological processes. This paper reviews the field measurement and remote sensing estimation methods, characteristics, and applications of CI. CI is generally estimated in the field using direct, indirect optical, and allometric methods. The basis for the indirect optical approach is the gap-fraction based method. Remote sensing of CI is carried out using passive optical and active LiDAR technology. Current CI products are mainly derived from an empirical relationship with the normalized difference hotspot and darkspot (NDHD) index. Further CI product validation studies should be conducted using enhanced field measurements. CI typically shows vertical, scale, directional, and temporal variations. The overall CI is calculated as the integration of the directional CI values. CI is a key parameter in leaf area index (LAI) estimation, and canopy reflectance and land surface modeling studies. Future studies should focus on the adoption of automated and wireless measurement methods, development of new remote sensing estimation methods, improving our understanding of the CI characteristics, and accounting for CI in land surface models.

冠层结块指数（CI）表征植被冠层内的叶或针的空间分布。CI对于确定冠层辐射转移，光合作用和水文过程至关重要。本文综述了CI的现场测量和遥感估算方法，特征及其应用。通常在现场使用直接，间接光学和异速测量方法估算CI。间接光学方法的基础是基于间隙分数的方法。CI的遥感是使用无源光学和有源LiDAR技术进行的。当前的CI产品主要来自与标准化差异热点和暗点（NDHD）指数的经验关系。应使用增强的现场测量进行进一步的CI产品验证研究。CI通常显示垂直，规模，方向和时间变化。将整体CI计算为方向性CI值的积分。CI是叶面积指数（LAI）估计以及冠层反射率和土地表面建模研究中的关键参数。未来的研究应集中在采用自动和无线测量方法，开发新的遥感估算方法，增进我们对CI特征的理解以及对地表模型中CI的解释上。