Estimation of forest leaf area index and clumping index from the Global Positioning System (GPS) satellite carrier-to-noise- density ratio (C/N_0)

摘要

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/N-0). This study explores the potential of using the GPS satellite C/N-0 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/N-0 was recorded with a modified smartphone application, GPSTest. The relationship between the forest canopy structural parameters and satellite C/N-0 was analyzed for those observations with satellite elevation angle (SEA) > 30 degrees. The results show that both the true LAI and the effective LAI have strong linear and polynomial correlation with C/N-0 and the coefficient of determination (R-2) is larger than 0.7. CI is linearly related to C/N-0(R-2 = 0.56). The linear relationship between the canopy closure and C/N-0 is much lower (R-2= 0.28). Accounting for the SEA effect, LAI can be estimated with R-2 > 0.8, and CI with R-2 = 0.6. LAI and CI can be well estimated from C/N-0 when the SEA effect is accounted for. In general, the GPS C/N-0 provides an alternative method to estimate forest canopy structural parameters.