Functional Effects of the Pioneer Fern Dicranopteris on Soil Fertility and Its Spatial Patterns in Degraded Hilly Landscapes

Vegetation recovery is vital for reversing soil degradation in subtropical red soil regions. This study evaluates the role of the pioneer fern Dicranopteris in enhancing soil fertility and its spatial variability across degraded hills in South China. Soil samples were collected from five provinces (Guangdong, Guangxi, Jiangxi, Fujian, and Hunan) in the red soil region of southern China, including 84 samples from slopes covered by Dicranopteris and 21 from adjacent bare land controls. Samples were taken at two depths (0–10 cm and 10–20 cm) to assess vertical differentiation, and vegetation characteristics including plant height, canopy cover, and root mat thickness were measured at each Dicranopteris site. Results showed that Dicranopteris significantly improved soil physicochemical and biological properties, especially in the surface layer, with notably higher levels of organic matter, total nitrogen, available potassium, microbial biomass carbon, and phosphatase activity ( p < 0.05). These improvements are likely driven by continuous organic matter inputs from the dense litter layer and root exudates, which stimulate microbial activity and accelerate nutrient cycling in the rhizosphere. A soil quality index (SQI) was constructed using principal component analysis, revealing consistently higher SQI values under Dicranopteris across surface and subsurface soil layers. Mapping of SQI values across all sampling points showed substantial spatial heterogeneity, with SQI values ranging from 0.177 to 0.579; higher values (0.396–0.579) were concentrated in the southern and southwestern regions, while lower values (0.177–0.334) were observed in the northeastern and central‐eastern zones. This spatial variation reflects the interplay between the consistent ameliorative effect of the root mat and the localized influence of topographic features on water redistribution and erosion patterns. Multiscale geographically weighted regression (MGWR) identified both biotic (e.g., root mat thickness) and abiotic (e.g., slope, topographic roughness) drivers of soil quality, with the effects of topographic variables exhibiting clear spatial heterogeneity across the landscape. These findings demonstrate the effectiveness of Dicranopteris in ecological recovery and underscore the importance of integrating vegetation traits and spatial heterogeneity into land management strategies in subtropical degraded hills regions.