Fragmentation has long been considered the primary cause for ecosystem degradation and biodiversity loss worldwide. Forest fragmentation affects ecosystem functioning and biodiversity in multiple ways. Here, we ask how forest fragmentation influences aboveground biomass storage (AGB) in sub-tropical forests in China. We established 207 20 m × 20 m plots within 69 forest fragments of varying size. Forest fragmentation process simulation was carried out via repeated quadrat sampling using different sized quadrats in two non-fragmented stands. AGB was estimated and compared across forest fragments and quadrats with different sizes within two non-fragmented stands. Our results indicate that AGB is significantly lower in forest fragments than in quadrats within two non-fragmented forests. In addition, species richness and abundance were lower in fragmented stands, respectively. In fragmented forests, the average diameter at breast height (DBH) increased with decreasing patch size, while declined for non-fragmented plots. Species richness, abundance, and mean DBH have strongly positive effects on AGB. This was the case both in forest fragments and quadrats within two non-fragmented forests. Forest fragmentation leads to lower richness, lower abundance, and higher mean DBH in forest fragments than in the two non-fragmented forests. Our results suggest that forest fragmentation increases edge habitats, which drastically decreases forests aboveground biomass storage. These results show that land degradation not only reduces the area of forests, but also reduces the aboveground biomass carbon density of forests.

The decomposition of deadwood is a crucial process for the accumulation and sequestration of soil organic carbon (SOC) in forest ecosystems. However, the response of SOC to different decay classes of deadwood and the underlying mechanisms remain poorly understood. Here, we investigated the dynamics of SOC, soil properties, extracellular enzyme activities, and phospholipid fatty acid biomarkers across five decay classes (ranging from 1 to 5) of Masson pine (Pinus massoniana Lamb.) downed deadwood in a subtropical–temperate ecotone forest in Central China. Our results revealed a nonlinear response pattern of SOC along the deadwood decomposition gradient, with the maximum value at the decay class 4. Soil available nitrogen content, bacterial biomass, fungal biomass, the ratio of fungal-to-bacterial biomass, cellulase, activity and ligninase activity all increased with the intensification of deadwood decay, while soil pH decreased. The increase in SOC content was associated with a direct positive effect of bacteria and both direct and indirect positive effects of fungi by cellulose activity, but ligninase activity showed no significant relationship with SOC content. These findings suggest that cellulose and microbial biomass are key determinants of soil C formation and sequestration during deadwood decomposition. This study highlights the importance of the nonlinear response of SOC to deadwood decay, providing valuable insights for predicting future carbon-climate feedbacks.

Due to climate change, subtropical forests are increasingly exposed to severe disturbance, which may lead to increased tree mortality. To date, previous research has not sufficiently studied the deadwood biomass within forests over large study plots. To address this research gap, we calculated the deadwood biomass within a 20 hectare (ha) permanent old-growth forest plot in southern China during two censuses and assessed the factors contributing to it. The deadwood biomass was estimated by applying allometric regression equations. There was a total of 11,283 (22.4%) dead individuals in the study plot. Most of these dead trees had very small diameters (1–10 cm). The spatial distribution of the dead individuals differed across subplots and was determined by biotic and abiotic factors. The deadwood biomass storage was 142.5 ton (t) in our study plot. Small (DBH: 0–30 cm) and medium trees (DBH: 30–50 cm) were the largest contributors (54.9% and 30.7%) to deadwood biomass storage. Three dominant tree species contributed 64.8% of the deadwood biomass storage, and the deadwood biomass of 38 tree species was less than 1 t ha⁻¹. Finally, the deadwood biomass was determined by breast diameter and the number of dead individuals, which was influenced by neighboring individuals and environmental factors. This study provides a detailed assessment of the patterns of the deadwood biomass in a subtropical forest and underscores the importance of including community characteristics and abiotic factors (e.g., topography) into research on forest ecosystem carbon. The results of this research provide valuable information that can deepen the understanding of the contribution of subtropical forests to the global carbon cycle and that can be used to improve forest protection and planning strategies.