Reflectance features of water stressed Larix gmelinii needles

In recent years, Xing’an larch (Larix gmelinii) has been seriously infected by pests and drought. In order to improve the accuracy of monitoring the damage to larch by remote sensing (RS) and to predict the health of the larch, we studied the reflectance features of larch needles under different water conditions at the needle level by using the LIBERTY (Leaf Incorporating Biochemistry Exhibiting Reflectance and Transmittance Yields) model. Before applying the LIBERTY model, we recalibrated it for the needles of L. gmelinii based on ten field-measured spectral curves. After recalibration, LIBERTY can accurately model the needle reflectance spectra of L. gmelinii. Based on the recalibrated LIBERTY model, we extracted and analyzed the sensitive bands to needle water content by simulating the needle reflectance spectra under different drought conditions. Then, we established mathematical equations between the spectral indices (MSI, NDWI, and GVMI) and needle water content. Results show that the variations of larch needle water content can significantly change the needle spectra at the near-infrared and short-wave infrared bands. The higher the water content is, the higher the absorption peak is. We believe that our study will provide the theoretical basis and an optional method to investigate the forest water stress using multi-spectral or hyper-spectral remote sensing data.