Root intersection densities in Japanese forests: Insights from a nationwide soil profile survey

Tree roots underpin key ecosystem functions, including carbon, water, and nutrient cycling, yet national-scale information on their spatial distribution remains limited. Here, we present the first nationwide assessment of root intersection density (RID; the number of roots intersecting a unit area of soil profile) across Japanese forests, based on 3236 soil horizons from 829 soil profiles collected in the National Forest Soil Carbon Inventory. RIDs of fine (≤ 2 mm), medium-sized (2–20 mm), and coarse (≥ 20 mm) roots were analyzed across managed and natural forests using ordinal and binary logistic regression models. Fine and medium-sized roots were detected in 73 % and 57 % of soil horizons, respectively, whereas coarse roots were rare (9 %). Based on non-zero observations, maximum rooting depths averaged 64.7 ± 25.0 cm for fine roots, 49.0 ± 27.6 cm for medium-sized roots, and 33.2 ± 27.0 cm for coarse roots, and these depths were further reduced when zero observations were included. Mean annual air temperature showed a consistent positive effect on root occurrence across root-size classes, whereas soil depth primarily constrained root density. Gravel content, slope, precipitation, and forest management produced root-size–specific responses, and the effects of temperature and soil depth on medium-sized roots were weaker in managed forests than in natural forests. These results support revising rooting-depth parameterizations in forest carbon cycle models, particularly for drought assessments, while caution is required when applying RID-derived depths of medium and coarse roots to mechanical models. Overall, this study provides a foundational national-scale RID dataset for improving assessments of root distribution and ecosystem functioning. • We present the first nationwide dataset of root intersection density in Japanese forests. • Maximum rooting depth decreases from fine to coarse roots and with zero observations. • MAT and soil depth act broadly, while management type modifies root responses. • Results inform carbon cycle models but require caution for mechanical applications.