Abstract：The sensitivity of biochemical effects on leaf reflectance is vital for retieving biochemical parameters with remote sensing. In this study, the chlorophyll and water absorption coefficients of the commonly used model LIBERTY (leaf incorporation biochemistry exhibiting reflectance and transmittance yields) were calibrated using field measured needle spectral reflectance curves based on a look up table (LUT) method. A novel spectra reflectance fitting method were presented by involving a new index (named as yellow index, YI), which could obviously improve the fitting accuracy of Pinus yunnanensis reflection spectrum at highly-stressed status. As a global sensitivity analysis method, the EFAST (extended Fourier amplitued sensitivity test) was implemented to quantitatively assess the sensitivity of biochemical parameters on needle reflectance. Results show that: (1) the reflectauce spectrum of healthy needles (R2=0.999，RMSE＜0.01), slightly stressed needles (R2=0.991, RMSE＜0.02) and moderately stressed needles (R2=0.992，RMSE＜0.03) are simulated fairly well by calibrated LIBERTY model which has less potential in fitting the reflectance spectrum of seriously stressed needles (R2=0.803，RMSE＞0.1). (2) the reflectance spectrum of seriously stressed needles can be successfully simulated by our proposed spectrum reflectance fitting method (R2=0.991, RMSE<0.03), because YI can quantitatively describe different degrees of stress, and (3) the sensitivity of leaf reflectance to chlorophyll and water parameters decreases with the degree of stress; while the sensitivity to other biochemical parameters is increasing, which includ baseline absorption, albino absorption, Lignin and Cellulose content, and nitrogen content, increases with the stress degree. Needle reflectance spectrum also have sensitivive bands for these parameters. For example, the albino absorption have a significant effect on needle reflectance in 505～565 and 705～850 nm). In addition, Albino absorption and chlorophyll also have significant effects on needle reflectance in visible region for seriously stressed needles, which indicates that the prior knowledge of the albino absorption level can help obtain the valid inversion result of chlorophyll content.
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