Limited evidence of vertical root segregation in root neighbourhoods
Our hypothesis that the differentiation in relative root distribution among species, complementarily combined with substantial rooting plasticity in response to edaphic conditions and neighbours, would ultimately result in vertical root segregation was not supported (Fig. 4). By contrast, the low differentiation of relative root distribution (Fig. 3), the neutral effect of root-to-root interactions on species rooting plasticity (Figs. 5, S3) and the random-to-aggregated root-placement patterns (Fig. 4; Table S4) suggest similar topsoil foraging of heterospecific roots. The generally random root-placement pattern observed here was similar to that reported in grasslands (Franket al. 2010) and greenhouse experiments (Litav 1967; Semchenkoet al . 2007). These findings were consistent with the ‘symmetric root proliferation’ hypothesis in that heterospecific roots can access and proliferate into the same soil patch (Valverde-Barrantes et al. 2015). Potential explanations for the lack of strong root segregation are elaborated below.
First, the differentiation of relative root distribution maybe not high enough to allow species to exhibit distinctive root depths specialized into specific soil zones. For instance, among the 53 species, only one species had roots concentrating within the 10-20 cm soil zone, and no species were concentrated in the 20-30 cm soil zone (Fig. 3b). Although some species may have higher root abundance in the deeper zones, based on neighbourhood- and species-level relative root distributions, we think it unlikely. The accumulation of c .70 % of fine roots in the 0-30 cm soil zone (Fig. 2b) and the co-occurrence of multiple species in the deeper zones (Fig. S5) suggest fairly shallow and homogeneous fine root distributions in the plot. Additionally, as revealed by the Post-Hoc comparison of species relative root abundance in the 10-20 cm soil zone, there were only seven of 2703 possible combinations of species pairs that exhibited significant rooting difference and all were found between X. hainanense  and the other seven species, further suggesting minor interspecific differences in root depth. Moreover, although X. hainanense  had distinctive root depth, most of its individuals were  aggregated into the valley of the plot, rather than being widely distributed that would otherwise increase the diversity of root-placement patterns in root neighbourhoods. By contrast, the two most common species (Pinus massoniana  and Altingia chinensis ) were widely distributed across the plot, sharing similar rooting patterns with multiple co-occurring species (i.e. concentrating in the 0-10 cm soil zone; Fig. 3b), potentially resulting in spatial root aggregation.
Second, the symmetric root competition may also explain the lack of vertical root segregation. It is proposed that a dominant species with large root biomass does not get disproportionate fitness advantages, as an increase of neighbours’ root biomass, even minor, may similarly exert competitive pressure on the dominant species’ fitness (Cahill & Casper 2000; Demalach et al. 2016). The underlying mechanism may be that species competitive abilities cannot be ranked along a hierarchy in which only a single species gains competitive dominance (Gilpin 1975; Buss 1980). Instead, in an intransitive competitive network, when for example species A is superior to species B, and B superior to C, it is more likely that C is superior to A (A>B; B>C; C>A; Buss, 1980). The correlations of species root biomass also imply that both facilitative and inhibitive interactions exist among species (Fig. 5a). Such an intransitive competitive network may prevent the predominance of a single species in root neighbourhoods (Laird & Schamp 2006).
Third, as found in our recent study in the same plot and elsewhere (Weemstra et al. 2016; Luo et al. 2020), the multidimensional nature of roots may also explain the prevalence of symmetric root proliferation. This suggests that species with different trait syndromes may exhibit comparable competitiveness under similar edaphic conditions. For instance, although thin roots with larger specific root surface area appear to be more competitive in nutrient foraging, due to their higher mycorrhizal colonization rates, thick roots may have larger hyphae surface area than thin roots. As a result, for a given amount of carbon for mycorrhizal root construction, thick and thin roots could have comparable absorptive areas and thus foraging capacities (Kong et al. 2014). This also helps explain the widespread co-existence of species with contrasting root morphologies and mycorrhizal types in temperate forests (Chen et al. 2018b; Valverde-Barrantes et al. 2018). Lastly, it is noteworthy that optional mechanisms rather than vertical root segregation also help to promote species co-existence. For instance, despite the similar root abundance partitioned into a specific soil zone, species may differ in the amount of biomass allocated to thin roots that may cause differentiation of root functional traits among species (Mommer et al. 2011). Such alternative strategies may confer a similar competitive advantage in response to edaphic heterogeneity and neighbour root competition.