Estimation of forest leaf area index and clumping index from the Global Positioning System (GPS) satellite carrier-to-noise-density ratio (C/N₀)

ABSTRACT

Forest leaf area index (LAI) and clumping index (CI) are critical structural parameters to study the biomass, biogeochemical cycles, ecological simulation, and radiation transfer in a forest system. The ground-based measurements by traditional methods are costly, time-consuming, and are sensitive to variations in solar illumination conditions. Global Positioning System (GPS) satellites transfer L-band signals, and the presence of the forest canopy leads to an increase in noise. The GPS satellite’s signal strength is generally described by the carrier-to-noise-density ratio (C/N₀). This study explores the potential of using the GPS satellite C/N₀ to estimate forest canopy structural parameters. We performed experiments at 16 forest plots in northeastern China. LAI, CI, and canopy closure were estimated by digital hemispherical photography (DHP) using the CAN_EYE software. The GPS satellite C/N₀ was recorded with a modified smartphone application, GPSTest. The relationship between the forest canopy structural parameters and satellite C/N₀ was analyzed for those observations with satellite elevation angle (SEA) > 30°. The results show that both the true LAI and the effective LAI have strong linear and polynomial correlation with C/N₀ and the coefficient of determination (R²) is larger than 0.7. CI is linearly related to C/N₀(R² = 0.56). The linear relationship between the canopy closure and C/N₀ is much lower (R²= 0.28). Accounting for the SEA effect, LAI can be estimated with R² > 0.8, and CI with R² = 0.6. LAI and CI can be well estimated from C/N₀ when the SEA effect is accounted for. In general, the GPS C/N₀ provides an alternative method to estimate forest canopy structural parameters.

Acknowledgement

This study was supported by the National Natural Science Foundation of China (NSFC, 41531174, 41471295) and the National Key Research and Development Program of China (2016YFA0600201) to H.F. The Android application GPSTest is available upon request to the first author.

Declaration of interest statement

The authors declare no conflict of interest.

Additional information

Funding

This study was supported by the National Natural Science Foundation of China [NSFC, 41531174, 41471295] and the National Key Research and Development Program of China [2016YFA0600201].