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.