Modelling the effects of changes in solar radiation on gross primary production in subtropical evergreen needle-leaf plantations

Aims Solar radiation is the energy source of terrestrial ecosystem carbon and water cycles. Observation shows that solar radiation has experienced noticeable variations in recent decades and significantly impacted on plant photosynthesis. Our objective was to investigate the impacts of changes in solar radiation on gross primary production (GPP) at the Qianyanzhou eddy tower site in subtropical forest in China.
Methods We first established the relationship between diffuse radiation fraction and clearness index based on the observed solar radiation and diffuse radiation data. Then, a two-leaf ecological process model, the Boreal Ecosystem Productivity Simulator (BEPS), was used to simulate the impacts of different changes in solar radiation on shaded GPP, sunlit GPP, and canopy GPP in the typical subtropical evergreen needle-leaf plantations.
Important findings Results showed that the effects of changes in solar radiation on shaded leaves predominantly determined the changes in the canopy photosynthesis as the shaded leaves contributed 67% to the total GPP. The impacts of changes in solar radiation on GPP varied inter-annually due to variations in the intensity and distribution of solar radiation from year to year. The GPP during 2003-2005 reached maximum when the solar radiation changed by -5.44%, -1.83%, and 6.26% in each year, respectively. Increased solar radiation enhanced the GPP during May and June, but reduced the GPP from July through September. The shaded leaves responded to changes in radiation differently in different seasons, and the sunlit and shaded leaves had different responses in the same season. Consequently, there were patterns of apparent offset in the total GPP and reduced sensitivity to changes in solar radiation on an annual basis. The changes in solar radiation had the smallest impact on GPP when the clearness index was at 0.43.