Rooting plasticity in response to edaphic heterogeneity and root neighbours
Our hypothesis that relative root distribution would express high plasticity in response to edaphic heterogeneity or/and root neighbours was not fully supported. For the 29 most common species, we only found two species’ relative root abundance in the 0-10 cm zone to be influenced by the presence of the remaining 28 species in the 0-30 cm soil zone (Fig. S3). When interspecific root interactions were examined by correlating pairwise species’ relative root abundance of the 29 species in the 0-10, 10-20 and 20-30 cm soil zone, there were 7.6%, 4.9 %, and 7.1% of species pairs that exhibited a significant correlation, respectively (Fig. 5). However, contrary to our prediction, excepting for some negative correlations observed in the 0-10 cm soil zone (Fig. 5a), all the correlations were positive (Fig. 5b, 5c), suggesting that, if such interactions were direct, the increase of one species’ root abundance tends to increase, but not overwhelm other species’ root abundance (Fig. 5). Again, contrary to our hypothesis, species’ relative root abundance in the 0-10 cm zone was neither affected by heterospecific root richness (Fig. S4) nor by root abundance (i.e. total root length) in the 0-30 cm soil zone (Table S5).
When heterospecific root richness and abundance were quantified only for the 0-10 cm soil zone, only two of the six most common species exhibited significant rooting plasticity in response to neigbours with contrasting responses. While Schima superba tended to place fewer roots in the 0-10 cm soil zone with increased heterospecific root richness (Table S6), Altingia chinensis placed more roots in the 0-10 cm soil zone with increased heterospecific root abundance (Table S7), suggesting a relatively minor and inconsistent effect of heterospecific root crowdedness on the focal species relative root abundance. Similarly, there were only four of 29 species whose relative root abundance exhibited significant plasticity in response to edaphic heterogeneity (Fig. S3). For all the four species, relative root abundance in the 0-10 cm soil zone was negatively correlated with soil nutrient availability, suggesting increased root proliferation in nutrient-poor soil patches.