Characteristics of Solar Radiation and Radiative Transfer of a Forest Canopy in Huainan, Anhui Province

The forest canopy, as an active interface between vegetation and the environment, transmits energy by reflecting, absorbing, and transmitting solar radiation through its leaves. The radiation levels above, within, and beneath the forest canopy are considerably important factors that affect the energy balance and water and carbon cycles. The variation of radiation with the seasons and the distribution of radiation among the forest canopies of the Huainan area have rarely been studied. Using total radiation data obtained by the Huainan forest observation station from July 1, 2018 to June 30, 2019, we investigated the temporal changes in solar radiation above the Sawtooth Oak canopy, analyzed the spatial distribution and transfer of solar radiation through the canopy, and determined the albedo, transmittance, and absorbance of the canopy. The results show the following. (1) The downward shortwave radiation above the Sawtooth Oak canopy increases from spring to summer and then decreases gradually toward winter. Unlike that above the canopy, the downward shortwave radiation within and under the canopy demonstrate a different trend with smaller values that decrease from early spring and increase from autumn to winter. Concerning the upward shortwave radiation, the seasonal variation pattern is the same as the downward pattern, whether above, within, or under the canopy, but the values are much smaller. (2) The downward longwave radiation above, within, and under the canopy gradually increases from spring to summer, then decreases gradually, and reaches a minimum in winter. In terms of spatial change, the radiation longwave values within and under the canopy are higher than that above the canopy, enhancing the longwave radiation by as much as 1.3 times under clear skies. (3) The annual average albedo above the canopy in the 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 (35°N), which indicates that the forest is denser in Huainan. (4) The shortwave radiation transmittance values of the upper part and the whole canopy are mainly affected by the leaves. In summer, the average shortwave transmittance of the whole canopy is 0.1, whereas in winter, as the leaves fall, the transmittance increases and tends to a stable fluctuation. The absorbance of shortwave radiation in the canopy is highest in summer, decreases gradually in autumn, and then decreases rapidly in winter as the leaves fall, tending to a constant value. These results are useful for validating layered-radiative-transfer and photosynthesis models as well as for further investigations of the energy, water, and carbon cycles of forest ecosystems.