Differentiation of relative root distributions among species
At the root neighbourhood level, root species richness per sampling unit (5Ă—6 cm) in the 0-10, 10-20 and 20-30 cm soil zone was 4.15, 2.73 and 2.18, respectively (Figs. 2a, S1a) and decreased to approximately one below 40 cm (Fig. 2a). Root abundance also decreased sharply as increasing soil depths with over 65% of roots concentrating within the 0-30 cm soil zone (Figs. 2b, S1b).
In support of our hypothesis, although most species were found in the 0-10 cm soil zone (Figs. 3a, S2), species differed in their relative root distributions that could broadly be categorized into five groups (Table S3; Fig. 3b). Specifically, for over 50% of species, root abundance was the highest in the 0-10 cm soil zone and had similar root abundances in 10-20 and 20-30 cm soil zone (group A). For 21% of species, root abundance was the highest in the 0-10 cm soil zone and decreased significantly toward the deeper zones (group B). 11% of species were homogeneously distributed throughout the 0-30 cm soil zone (group C; Fig. 3b). An additional 18.8% of species similarly had the highest fraction of roots in the 0-10 cm soil zone with their root abundance exhibiting inconsistent patterns among the deeper two zones (group D). For some species, root abundance in the 20-30 cm zone was significantly higher than the 10-20 cm zone or that root abundance was similar among the uppermost two zones. One species’ roots were concentrated in the 10-20 cm soil zone (Xanthophyllum hainanense , group E).