Modeling and validation of the angular clumping index of forest canopy

Abstract

The clumping index (CI) is an important factor for deriving forest leaf area index (LAI) at both pixel and point scale. Due to the non-uniform distribution of leaves within trees and random layout of trees under natural conditions, the CI varies with view zenith angle (VZA) as well as the gap fraction. The variation tendency of gap fraction and CI with VZA dominates the potential uncertainty of deriving LAI from directional observations. In this research, the continuous change of gap fraction and CI with VZA was simulated under virtual tree scenarios. Meanwhile, the directional CI was calculated using an algebraic model developed based on previous researches on gap fraction. Both results were validated using ground measurements. The CI discrepancy distributes within ±0.06 coming from the random number distribution restriction and the influence from tree trunk. The analysis using the algebraic model shows that the forest CI reaches an asymptotic level after 55° for forest canopy dense enough. This is an interesting and important phenomenon, which reveal a stable feature of CI to be further used to extract forest information.