Shifting carbon sink patterns driven by reduced sunlight and increased contribution from shaded leaves

Global warming has led to pronounced differences in photosynthesis and respiration between sunlit and shaded leaves. However, assessments of the resulting disparities in carbon sequestration potential and contributions remain limited, and the underlying mechanisms have yet to be systematically elucidated. This study used three carbon flux indicators—gross primary productivity (GPP), sunlit leaf GPP (GPPsun), and shaded leaf GPP (GPPshade)—to reveal the spatiotemporal dynamics and driving mechanisms of carbon sinks in the terrestrial ecosystems of southern China. The Lindeman-Merenda-Gold model was applied to quantify the relative contributions of climate change to carbon sink variations. The results showed the following: (1) GPP, GPPsun, and GPPshade exhibited increasing but fluctuating trends during the period 2001–2020, with growth rates reaching 10.88, 5.69, and 5.19 g C m ⁻² yr ⁻¹ , respectively. In 44.79% of the study area, GPPshade increased faster than GPPsun. (2) GPPshade/GPP showed an increasing trend (0.03%), with a mean value of 0.54. The contribution of shaded leaves to the carbon sink was 1.79 times higher than that of sunlit leaves. (3) Declining solar radiation (SR) dominated this shift. The contribution rates of SR to GPP, GPPsun, and GPPshade were 28.01%, 24.55%, and 34.52%, respectively. SR was the primary driver in 37.46%, 31.02%, and 50.19% of the entire study area. (4) In areas with decreased SR, GPPsun exhibited slow growth, and GPPshade decreased. In areas with increased SR, GPPshade surged, while GPPsun growth decelerated significantly. Shaded leaf carbon sinks emerged as the dominant contributor to the overall enhancement of vegetation carbon sinks. These findings demonstrate a key mechanism—increased GPPshade potential driven by SR decline, suppression of GPPsun, and a resulting restructuring of carbon sink dynamics. This study provides theoretical support for enhancing terrestrial ecosystem carbon sinks and offers valuable insights for advancing global carbon neutrality objectives.