Retrieval Of Forest Background Reflectance From MODIS BRDF Data

Forest, as the largest terrestrial ecosystem, is not only an important component of global carbon cycle, but also the material basis of human society. Leaf area index (LAI) is the most important factor representing the growth and developmental status of terrestrial vegetation. LAI has been widely used in the global terrestrial carbon cycle and climate change research. When retrieving the LAI, many algorithms only consider the information of canopy and the forest understory LAI is not systematically taken into account. The existence of background reflectivity overestimated the values of remotely sensed global LAI products in some forest areas. Therefore, this study uses the four-scale model to retrieve the forest background reflectance of Jiagedaqi District and Wuyishan mountain District based on MODIS BRDF product. The characteristics of temporal properties of bidirectional reflectance (BRF) and background reflectivity in the study areas would be discussed. The main conclusions of this study are as follows:(1) The MODIS BRDF model parameters product can simulate the characteristics of the forest bidirectional reflectance well. In the red band, the bidirectional reflectance values of deciduous forest is both lower than the coniferous forest and mixed forest in the two research regions, and in the near infrared wavelength, the difference of forest bidirectional reflectivity between deciduous, coniferous and mixed forest in Wuyishan mountain District is not obvious, but the bidirectional reflectivity values of deciduous forest is larger than coniferous and mixed forest in Jiagedaqi District. There is no significant difference in the characteristics of bidirectional reflectance between coniferous and mixed forest which is so similar.(2) The forest background reflectances of the two study areas in red band are relatively low, mainly between0-0.1. In the near infrared band, the background reflectance is generally high and has a broader range, most values are higher than0.2.(3) The seasonal variation of the background reflectance in the two study areas has the same trend:the background reflectance in the red band reaches to its lowest when it comes to the summer in July; the background reflectance in the near infrared band reaches to its highest in June; compared to the Jiagedaqi District, the seasonal variation of the background reflectance in Wuyishan mountain District is relatively low, especially in red band, the background reflectance is around0.05, and it doesn’t change much with the variation of the season.(4) Indirect validation of results using measured LAI show that the forest canopy reflectance considering the impact of background information has higher correlations with measured LAI data, which confirmed the impact of forest background reflectance on LAI inversion accuracy and the necessity of improving the forest canopy LAI inversion accuracy of remote sensing.