Responses of belowground fine root biomass and morphology in Robinia pseudoacacia L. plantations to aboveground environmental factors

The adaptation of fine roots to a variety of environmental conditions is crucial for promoting the growth and long-term survival of forest trees. To achieve sustainable forest ecology management requires a comprehensive understanding of the intricate relationships between belowground and aboveground factors in the forest ecosystem. Equally important is identifying the factors that directly affect the growth and development of fine roots belowground. This study was carried out in 25-year-old pure plantations of Robinia pseudoacacia L. within the Caijiachuan watershed, Jixian County, Shanxi Province, China. Twenty-seven permanent sample plots were established, with each plot measuring 20 x 20 m. These plots covered nine different stand densities ranging from 775 to 2975 trees/ha, with three replicates per density. Through field surveys and multivariate analysis, along with the use of a simplified Generalized Linear Model, this study revealed the alterations in belowground fine root biomass and morphology resulting from variations in stand density. Additionally, it explored the mutual relationships between belowground fine root biomass and morphology and aboveground environmental characteristics. The results show that, under different stand densities, only fine root biomass, specific fine root length, fine root tissue density, and fine root tip density displayed significant differences (P < 0.05), while specific fine root surface area exhibited a decreasing trend with increasing stand density. Moreover, lower densities (1025 and 1175 trees/ha) promoted the growth of specific fine root length and specific fine root surface area, while higher densities (2100 and 2475 trees/ha) supported the development of fine root biomass and fine root tip density. The combined contributions of semi-decomposition layer litter layer thickness (contribution value: 0.1340), mean tree height (contribution value: 0.2710), and neighborhood comparison (contribution value: 0.1840) significantly explain the variations in fine root biomass (P = 0.014) and specific fine root surface area (P = 0.031). Fine root biomass shows statistically significant correlations only with total phosphorus and available phosphorus (P < 0.05). The influence of soil factors (contribution value: 0.0306) on subterranean fine roots is much less pronounced compared to that of stand structure (contribution value: 0.1152) and other environmental factors (contribution value: 0.1128). These factors together account for the variations in subterranean fine roots, with stand structure and other environmental factors making the most significant contributions (contribution value: 0.5840). These research findings offer valuable insights into the connections between subterranean fine roots and aboveground biotic and abiotic factors in Robinia pseudoacacia L. plantations in the northern Loess Plateau region. Furthermore, they provide vital information for the prudent management and operation of low-efficiency Robinia pseudoacacia L. plantations.