Differential specific leaf area due to different stand structures drives alterations in soil organic carbon in artificial forests of karst areas

After setting targets for carbon peaking and carbon neutrality, Guangxi of China has adjusted its goals to establish artificial afforestation, which boasts the highest coverage rate of artificial forests in nationwide karst areas. Recognizing limitations in land resources, the focus has shifted from expanding the planting area to enhancing the quality of these artificial forests. The objective is to maximize forest carbon storage and soil carbon sequestration within the limited available land. As a result, artificial forest managers now require extensive and detailed information regarding growth of these forests and the mechanisms involved in their interaction with soil carbon from the micro-level scale. We selected 22 sites of artificial forests in the Liujiang river in Guangxi, known for its typical karst topography, to conduct our investigation. Additionally, 5 natural grasslands were investigated as control groups. We determined various parameters including vegetation distribution, plant growth, leaf traits, and soil organic carbon content. The results showed that when the understory vegetation coverage in artificial forests exceeds 45%, the leaf biomass accumulation rate was higher than the leaf area expansion rate (significant difference in slope from 1.0, P < 0.05). Furthermore, the soil organic carbon content in artificial forests ranged from 15.65 g·kg−1 to 25.91 g·kg−1, which was significantly different from the highest recorded value of 9.39 g·kg−1 found in natural grasslands (P < 0.05). In Eucalyptus globulus artificial forests, The forest structure with low tree height and understory vegetation coverage, promotes a high specific leaf area. This accelerates litter weight and carbon accumulation, enhancing organic carbon input to soil. For balanced carbon sequestration and preventing rocky desertification, Eucalyptus globulus artificial forests can be established with an understory vegetation coverage of 40–45% and a stand density of at least 15 plants per 100 square meters. In Bauhinia purpurea artificial forests, high stand density leads to Bauhinia purpurea leaves as primary source of litter, but low stand density relies on understory vegetation residue for soil organic carbon. Bauhinia purpurea artificial forests can be established with the same understory vegetation coverage as Eucalyptus globulus but a stand density of 10 plants per 100 square meters. In Pinus massoniana artificial forests, the extent of understory vegetation coverage is important for determining the weight and carbon content of litter. For Pinus massoniana artificial forests, it is best to maintain understory vegetation coverage above 40%.