State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics LASG,Institute of Atmospheric Physics,Chinese Academy of Sciences
Forest canopy as an active interface between vegetation and environment transmits energy by reflecting, absorbing and transmitting solar radiation by leaves. The radiation above, within and below the forest canopy is very important in the energy balance, water and carbon cycle. Its variation with season and distribution within forest canopies is few studied in Huainan area. Using the data of total radiation measured at Huainan forest observation station from 1 July 2018 to 30 June 2019, the temporal change characteristics of solar radiation above the Sawtooth Oak canopy, and the spatial distribution and transfer of solar radiation through the canopy were analyzed, and also the albedo, transmittance and absorptance of the canopy were given. The results show that: (1) The downward shortwave radiation above the Sawtooth Oak canopy increases from spring to summer, then decreases gradually to winter. But the downward shortwave radiation within and under the canopy demonstrate a different trend with smaller value. It decreases from the early spring and increases from autumn to winter as opposite to that above the canopy. Concerning the upward shortwave radiation, whether above, within or under the canopy, the seasonal variation pattern is the same as the downward one, but the value is much smaller. (2) The downward longwave radiation above, within and under the canopy gradually increases with time from spring to summer, then decreases gradually and reaches the minimum in winter; in terms of spatial change, the radiation value of longwave within and under the canopy is higher than that above the canopy, which enhances longwave radiation and can be as large as 1.3 times under clear skies. (3) The annual average albedo above the canopy in Huainan forest area is 0.14, which is 0.01 lower than that in the temperate monsoon climate area (mainly mixed forest) in northern China (350N), indicating that the forest is denser in Huainan. (4) The shortwave radiation transmittance of the upper part and the whole canopy is mainly affected by the leaves. In summer, the average shortwave transmittance of the whole canopy (τ) is 0.1. But in winter, the leaves wither and fallen, the transmittance increases and tends to a stable fluctuation. The absorptance of shortwave radiation in the canopy was the highest in summer and decreased gradually in autumn, and decreased rapidly in winter as the leaves withered, tending to a stable fluctuation. These results could be useful for validating layered radiative transfer and photosynthesis models and for further study the energy, water and carbon cycle of the forest ecosystem.