Climate change weakens the positive effect of human activities on karst vegetation productivity restoration in southern China

China has conducted large-scale eco-afforestation projects in karst areas for mitigating rocky desertification in recent decades. However, the benefits of karst vegetation productivity restoration and the contributions of influencing factors are unclear. We analyzed the impacts of climate change (CC) and human activity (HA) on vegetation productivity change based on the net primary productivity (NPP) by using the partial derivatives and designing eight different scenarios. Results demonstrated that the average NPP over the entire vegetation-covered karst area exhibited an unremarkable increasing trend (0.92 g C m−2 yr−1) and a major decline in NPP was detected in the areas where NDVI increased from 2000 to 2015 (0.44 million km2, 29.07%). Solar radiation (−0.91 g C m−2 yr−1) was the preponderant climatic factors exhibiting negative contribution to NPP changes. A significant positive contribution was caused by HA (1.53 g C m−2 yr−1) on NPP variations, while a negative contribution was induced by CC (−0.61 g C m−2 yr−1). CC and HA showed the more similar contribution proportion to NPP increasing (51.94% vs 48.06%), but with great difference (68.43% vs 31.57%) for NPP decreasing. 39.83% of the areas involved in NPP decreasing was attributed to the accelerating consumption of autotrophic respiration while the rest (60.17%) was contributed by rapid decrease of gross primary productivity, respectively. In southern karst area, HA showed a positive impact (59.07%) on NPP increasing. However, the negative contribution from CC (70.72%) due to the rapid and constant decline of solar radiation completely counteracted this, leading to a greater NPP decrease. This study stresses the importance of negative effect from CC on karst vegetation productivity change and provides location guidance for further implementation of ecological protection projects in southern China.