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).