Needle Chlorophyll Content Estimation: A Comparative Study of PROSPECT and LIBERTY

Abstract

Inversion capabilities of leaf-level radiative transfer models, PROSPECT and LIBERTY, were evaluated to estimate needle chlorophyll content from measured needle reflectance and transmittance spectra. Sensitivity studies, using simulation results, were conducted on the models’ biophysical parameters to determine robust inversion approaches. Unlike PROSPECT, which has only one scattering parameter (N), the LIBERTY model is comprised of two highly sensitive scattering parameters (average cell diameter and intercellular air gap), which influence visible and near infrared spectral characteristics and challenge usual iterative minimization techniques. Validation of inversion algorithms were based on Jack Pine (Pinus Banksiana) needles, collected in forested areas near Sudbury, Ontario, Canada, between June and September of 2001 for laboratory spectral and biochemical measurements. Both PROSPECT and LIBERTY were capable of accurately inverting model-simulated reflectance and transmittance spectra for needle pigment content. However, inversions using actual data have proven more challenging. For PROSPECT, with an empirically determined geometric form factor to account for needle shape, inversions using measured spectra yield pigment estimates with an RMSE of 13 ug/cm2 over a pigment range between 15 and 68 ug/cm2. For LIBERTY, current efforts focus on effectively accounting for the highly sensitive model scattering parameters to permit accurate inversion of measured needle spectra for pigment estimation. Comparisons of inversion results using both PROSPECT and LIBERTY, as well as contrasts in the sensitivity of specific model parameters to retrieval accuracy are presented. Such assessments of leaf-model inversion capabilities are necessary prior to coupling it with canopy models to make stand level pigment estimations.