Vertical root segregation in root neighbourhoods
Our hypothesis that individual species in root neighbourhoods would be vertically segregated was generally not supported. Although there were higher proportions of root neighbourhoods where heterospecific roots tended to avoid co-occurring (Fig. 4a) or avoid placing similar root abundance in the same soil zone (Table 1; Fig. 4b-d) than that tended to co-occur, the proportions of root neighbourhoods that either exhibited significant root segregation or aggregation were very low and did not differ substantially (Table 1). Furthermore, the mean of the standardized effect size of the four parameters measuring rooting similarity did not differ significantly from zero (Fig. 4), indicating that the diversity of root-placement patterns in the observed communities did not differ significantly from that of randomly simulated distributions. Moreover, in contrast to our prediction, when only roots that occurred throughout the 0-30 cm soil zone were included in the analysis, we found a higher proportion of root neighbourhoods that exhibited root aggregation (54%-76%) than that exhibited root segregation (24%-47%; Table S4). Such aggregated patterns were significant in the 20-30 cm soil zone when rooting similarity was evaluated based on the variance of co-occurring species’ relative root distribution (Table S4), suggesting that species have similar root abundance in the 20-30 cm soil zone.