Root neighbourhood level and species-level relative root distribution
Examination of relative root distributions revealed the concentration of fine roots in the 0-10 cm soil zone in our experimental subtropical forest. Both root richness and abundance were largely reduced below 40 cm (Fig. 2). Similarly, individual species’ root abundance declined sharply and became variable in the deeper zones, presumably due to higher edaphic heterogeneity (Fig. S5). Consequently, although we examined root distributions only within the 0-30 cm soil zone, this allowed the examination of c.  70% of fine roots’ placement patterns and interspecific interactions across the 0-100 cm soil profile (Fig. 2b).
Our study partially supported the differentiation of relative root distributions among species. Although a majority of species had the highest root abundance in the 0-10 cm soil zone, some canopy trees predominantly placed their roots in the 10-20 cm soil zone or were homogeneously distributed throughout the 0-30 cm soil zone (Fig. 3b). These findings expand prior studies indicating similar root depths of coexisting temperate trees (Valverde-Barrantes et al. 2015), raising the possibility of interspecific vertical root segregation through differentiation in relative root distributions (Schmid & Kazda 2001; Herben et al. 2018).