Plasticity of relative root distribution in response to root
neighbours and edaphic heterogeneity
Our hypothesis that relative root distribution would exhibit high
plasticity in response to edaphic heterogeneity and root neighbours was
not fully supported. For most species, relative root distribution was
neither affected by edaphic heterogeneity nor by the presence of
specific root neighbours (Fig. S3), suggesting that neighbourhood
species composition is not a good predictor of a focal species’ root
depth. Likewise, most species’ relative root distributions were
unaffected by heterospecific root richness or abundance (Tables S5-S7;
Figs. S3, S4), as observed in temperate forests and mountain grasslands
(Valverde-Barrantes et al. 2015; Herben et al. 2018).
Similarly, Cahill (2003) reported that grassland species did not exhibit
substantial changes in root-shoot ratios with increasing below-ground
competition, suggesting that increased heterospecific root mass may not
necessarily be indicative of higher competitive pressure exerted on the
focal species. However, these findings contrasted with previous studies
reporting apparent neighbour-induced rooting plasticity (e.g. Cahillet al. 2010; Belter & Cahill 2015; Zhang et al. 2019). An
explanation for the discrepancy might be that, in contrast to greenhouse
experiments that have usually involved two species (Cahill et al.2010; Belter & Cahill 2015), individual species in natural communities
experience high niche overlap and diffuse competition. Specifically, in
competitive neighbourhoods composed of multiple species, the addition or
removal of a single neighbour may not have a significant effect on the
competition experienced by the target plant (Dyer & Rice 1997). Another
reason might be that in species-rich communities, species exhibit varied
rooting plasticity, both directionally and magnitudinally, in response
to different neighbours, resulting in overall neutral adjustments of
root depth (Litav 1967; Brisson & Reynolds 1994; Caldwell et al.1996; Schenk et al. 1999).
Root proliferation usually decreases as increasing heterospecific root
abundance, presumably due to intensified competitive pressure and
stronger chemical inhibitive effects (Brisson & Reynolds 1994).
However, we found that the increase of one species’ root abundance did
not reduce other species’ root abundance (Fig. 5). Instead, the
pervasive positive associations of species pairs suggest concurrent root
proliferation of co-occurring species into the same soil zones (Fig. 5b,
5c; Belter & Cahill 2015). Nevertheless, based on the positive
association among two species, we could not conclude that such species
can mutually and directly facilitate their root growth, as such
correlations could simply result from the collinearity of species root
abundance with other unaccounted-for factors. For instance, similar
responses of root proliferation to limiting nutrients or soil microbes
may lead to the co-occurrence of the paired species (Rog et al.2020). These uncertainties highlight the need for further manipulative
experiments examining the direct and indirect paths of which
root-to-root interactions play to better understand below-ground
processes in complex forest communities (Belter & Cahill 2015; Zhanget al. 2019).
At the species level, the variation of relative root abundance in the
0-10 cm soil zone was high (CV: 38%-144%), suggesting that, in
addition to the possible effects of edaphic heterogeneity and root
neighbours, other factors also contribute to such variations. For
instance, as root segments were randomly sampled, those sampled on the
margin of the root system could be relatively shallowly placed, giving
that root abundance generally declines with increasing distance from the
parental tree stem (Jones et al. 2011). Such spatial arrangement
of root systems could cause additional rooting variations independent of
soil heterogeneity and root neighbours. However, prior studies also
indicated that root-placement patterns were mainly affected by localized
edaphic conditions, as root systems are modular, and each meristem can
generate a potentially flexible and plastic root branches (Waiselet al. 2002). Nevertheless, the change of whole-plant root system
arrangements still represents adaptive plasticity in response to root
neighbours or soil conditions (Casper et al. 2003). Collectively,
although there were substantial variations in relative root distribution
left unexplained that could be attributable to tree ontogenetic stages
or soil microbial interactions, our study suggests the relatively minor
effect of interspecific interactions on a focal species’ root
distribution.