Solar radiation differences drive karst sun and shade leaf carbon sink contribution shifts

Global warming has led to pronounced differences in photosynthesis and respiration between sun and shade leaves. However, assessments of the resulting disparities in carbon sink potential and contributions remain limited, and the underlying mechanisms have yet to be systematically elucidated. This study used three carbon sink indicators-gross primary productivity (GPP), sun leaf GPP (GPPsun), and shade leaf GPP (GPPshade)-to reveal the spatiotemporal dynamics and driving mechanisms of carbon sink in the terrestrial ecosystems of southern China. The Lindeman-Merenda-Gold (LMG) 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. However, GPPshade increased faster than GPPsun in 44.79% of the study area. (2) GPPshade/GPP showed an increasing trend (0.0003 yr^-1), with a mean value of 0.54. The average contribution of shade leaf to the carbon sink was 1.79 times higher than that of sun leaf. (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. Shade leaf carbon sink emerged as the dominant contributor to the overall enhancement of vegetation carbon sink. 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 a theoretical support for enhancing terrestrial ecosystem carbon sink and offers valuable insights for advancing global carbon neutrality objectives.