Interspecific Association and Environmental Interpretation of Dominant Species in Shrub Layer of <i>Pinus massoniana</i> Community on Chinese Islands

Understanding the factors driving species coexistence and competition in the shrub layer of semi-natural forests is crucial for effective forest management and conservation. However, there is limited knowledge about the interspecific associations of the main species in the shrub layer of <i>Pinus massoniana</i> communities in the semi-natural forest of Sandu Gulf, Ningde, Fujian Province, China. Therefore, this study aimed to investigate the influence of the abiotic environment on plant communities within the semi-natural forest of <i>P. massoniana</i> on the islands of Sandu Gulf. By exploring these interspecific associations, we sought to provide a more accurate understanding of the causes and processes of species coexistence and competition. The ultimate goal of this project was to offer a reference basis for optimizing the shrub layer structure in <i>P. massoniana</i> (plantation) forests. We found that (1) <i>Heptapleurum heptaphyllum</i> was the most dominant species in the shrub layer, while <i>Smilax china</i> demonstrated the broadest range of environmental adaptability and correspondingly broader niche than other species. (2) Our analysis revealed a predominance of positive associations among the dominant species in the shrub layer. However, the overall association was not significant, with relatively small positive and negative associations between species pairs. The significant test rate was low, and the <i>NRI</i> exhibited a non-significant aggregation. These findings suggest that the plant community in the shrub layer has not yet reached its most stable stage. (3) We also observed that the distribution of dominant species in the shrub layer was primarily affected by factors such as total potassium, pH, available potassium, and light (canopy density). (4) Soil pH value decreased gradually as sampling points moved inward away from the coastline, and island isolation, temperature, and precipitation indirectly affected the species' importance in the shrub layer. Considering the intense competition among the understory species, it is crucial for conservation efforts to prioritize species pairs with reduced ecological niche overlap or significant positive associations. This approach will effectively reduce competition and contribute to the maintenance of stability in the shrub layer. This study provides a theoretical basis for environmental and vegetation restoration, optimizing the species composition of island plantation forests, rationalizing plant composition, and implementing effective operation and management practices for local <i>P. massoniana</i> forests.