Downward growth dynamics of Pinus thunbergii taproots and their relation to site environment in a coastal forest

Abstract Background and Aims Estimating the time required for tree root systems to reach deep soils (> 1 m) and elucidating the characteristics of soil properties and fungi associated with deep roots are crucial for understanding tree establishment in harsh coastal environments. This study aimed (1) to determine the downward elongation rate of Pinus thunbergii taproots using tree-ring analysis and (2) to explore the relationships among deep roots, deep-soil properties, and fungal associations. Methods Annual growth rings of taproots in a coastal forest in Japan were analyzed using a cross-dating procedure. Soil physicochemical properties were assessed, and root-associated fungi in deep soils were identified by DNA sequencing. Results Taproot growth followed a sigmoid function; trees with a larger stem diameter at breast height (DBH) had greater total taproot depths. Taproot elongation of the tree with the largest DBH, located at a site with the coarsest gravel, did not slow even within deep gravel horizons. Fine soils in deep horizons had C and N concentrations comparable to or higher than surface horizons, and soil C:N ratios correlated positively with taproot elongation rates. DNA sequencing of fine roots revealed the presence of endophytic and ectomycorrhizal fungi with potential saprotrophic functions. Conclusion Taproot downward elongation dynamics appear to depend on aboveground tree characteristics and soil properties. Root detritus generated through friction with gravel likely supplies C and N to deep soils, supporting sparse saprotrophic fungi. These findings underscore the importance of deep root–soil–fungi interactions in supporting tree establishment in nutrient-poor coastal environments.