Objective  Solar radiation is the main source of energy and also one of the most important influencing factors affecting the local and regional climate and environment of forest ecosystem. However, long-term changes of radiation data are needed to focus on climate change research. Exploring the long-term characteristics of solar radiation of forest is very important to establish a research foundation for climate change, tropical forest productivity, radiation energy balance, and the establishment and verification of radiation models. In order to explore the long-term changes of radiation characteristics of tropical seasonal rainforest in Xishuangbanna of southwestern China, the observation of ground based long-term radiation had been conducted.

  Method  The characteristics of the ratio of various components to solar radiation had been explored in a tropical seasonal rainforest in Xishuangbanna on different time scales from 2003 to 2016.

  Result  The multi-year mean annual values of total radiation, net radiation, reflected radiation, atmospheric invers-radiation and upward canopy long-wave radiation were: 5 268.8, 3 151.5, 513.8, 12 468.6, 13 299.3 MJ/m², respectively. The inter-annual variation of global radiation showed a fluctuating upward trend, whereas the net radiation fluctuates showed a downward trend, and a significant decrease in the proportion of global radiation. The inter-annual variability of atmospheric invers-radiation was higher due to the different atmospheric conditions were mainly affected, which resulting in a large inter-annual variability of the effective radiation. Whereas the inter-annual variability of upward canopy long-wave radiation was lower, indicating that the canopy long-wave radiation of tropical seasonal rainforest was not changed too much. The diurnal variation curve of atmospheric inverse-radiation was bimodal in the dry season and unimodal in the rainy season. On different time scales, the upward long-wave radiation from the forest canopy was higher than the atmospheric inverse-radiation, indicating that the tropical seasonal rainforest canopy was a heat source for the atmosphere. The ratio of net radiation to Comment annual global radiation was about 60%, and the inter-annual variation showed a significant downward trend. The ratios of reflected radiation and effective radiation to annual global radiation were relatively stable. The ratio of net radiation to global radiation was high during the daytime but low at dawn and dusk, which showed an inverted U-shaped curve. The ratios of reflected radiation and effective radiation to total radiation were lower during the daytime and higher at dawn and dusk, which showed U-shaped curves.

  Conclusion  In addition to being affected by general factors such as solar activity, cloud cover and water vapor, the radiation of Xishuangbanna tropical rainforest is also affected by regional climates, such as seasonal defoliation and seasonal fog weather. The study of long-term changes in forest radiation characteristics have great significance for climate change research, climate model verification and forest productivity research.