Seasonal change of bidirectional reflectance distribution function in mature Japanese larch forests and their phenology at the foot of Mt. Yatsugatake, central Japan

We constructed a bi-directional reflectance distribution function (BRDF) observation system on a 25-m-high tower surrounded by mature larch forests at the foot of Mt. Yatsugatake, central Japan. The BRDF feature (strong/weak reflections in a backward/forward scattering areas) is related closely to the 3D structure and biophysical parameters of forests. However, because of the difficulty of observations, observational BRDF data of mature forests are scarce, except for those of Boreal Ecosystem –Atmosphere Study (BOREAS). We made BRDF observations 10 times during 2007 to ascertain BRDF characteristics in each season. For comparison to these BRDF data, a semi-empirical BRDF model (Ross–thick Li–sparse kernel model) was introduced. This model can reproduce the BRDF data fairly well under identical solar conditions in each season. Then, seasonal changes of the vegetation structure and possible relations with BRDF were discussed. They are summarized as follows: (1) The forest BRDF changes distinctly and seasonally depending on the phenology. (2) The forest BRDF is influenced primarily by clarity of the geometrical profile on the forest surface, which is affected by shadow. However, the forest BRDF might be influenced by other factors such as the physiological characteristics of leaves and slight change of vegetation structure such as forest floor conditions and leaf angles during summer. (3) Features of forest BRDF must therefore appear during summer when the crown shape is distinctive. Furthermore, BRDF features appear distinctly as seasons progress, e.g., by autumn leaves and defoliation, when the unevenness of forests increases. These findings can promote monitoring of vegetation using multi-angular satellite observations.