Canopy Definitions Shape Canopy Space Filling–Productivity Relationships: Evidence From Terrestrial Laser Scanning

Many important processes in forest ecosystems are influenced by the spatial structure of the canopy. The spatial arrangement of trees and their branches within the canopy is crucial for light interception, often resulting in a positive relationship between canopy space filling and stand productivity. To date, there is no universal definition of canopy space. However, different parts of the canopy have distinct characteristics, making it crucial to assess the significance of varying canopy definitions. In this study, we investigated how canopy space filling changes with four different definitions of canopy space by using a canopy space filling index (CSFI) derived from terrestrial laser scanning. Moreover, we assessed the relative importance of these definitions in explaining stand productivity. Using data from a tropical tree diversity experiment, we found that CSFI strongly varied with the specific delineation of the canopy space. Across canopy definitions, stand productivity was significantly and positively affected by CSFI, indicating that this relationship was generally robust to varying definitions of canopy space. However, excluding the uppermost part and including the lowest part of the canopy space resulted in a distinct decline of explained variance in productivity. In addition, CSFI played a more important role in regulating productivity in mixtures compared to monocultures. Our results highlight the importance of different layers of the canopy space and tree species richness for a better understanding and assessment of forest dynamics and ecosystem functions. Further research is needed on the relationship between canopy space filling and stand productivity based on different canopy space definitions for mature forests and from different biomes.