Phenology of leaf optical properties and their effects on canopy reflectance in cool-temperate deciduous broadleaf forest

Abstract:

Canopy reflectance spectrum of deciduous forest shows remarkable seasonal change. Since the canopy reflectance is regulated by the scattering and absorption of radiation by leaves in the canopy, the geometrical arrangement of the leaves and their optical properties (reflectance, transmittance and absorptance spectra) are the major determinants of the canopy reflectance. However, our quantitative understandings on these relationships throughout the canopy phenology are still insufficient. In this study we first examined the leaf optical properties of a pioneer species Betula ermanii and a late-successional species Quercus crispula during their seasonal development for four years and analyzed their consequences with chlorophyll content and mesophyll structure. We then investigated the effects of the leaf optical properties on the phenology of canopy reflectance by a radiative transfer model SAIL. After leaf unfolding in spring, the reflectance in the photosynthetically active radiation region decreased gradually, while the transmittance dropped rapidly. Both reflectance and transmittance increased before leaf fall. In the near-infrared region, reflectance increased, and transmittance decreased during the leaf development period. Value of parameter N (the mesophyll structural parameter estimated by the leaf data and the PROSPECT-5 model) in young leaves was very low but increased rapidly. The PROSPECT-5 simulation with our measured data clearly showed that the development of mesophyll tissue and an increase in chlorophyll content are responsible for the seasonal patterns in the optical properties. The SAIL simulation indicated that not only the increase of canopy leaf area index, but the seasonal change in the leaf optical properties is also responsible for the seasonal patterns of canopy reflectance. Our results indicate that the structural and biochemical developments of leaf are essential biological processes for analyzing seasonal change of canopy reflectance.