Introduction
Fine roots (<2 mm) are the most metabolically active part of
root systems responsible for soil nutrient uptake, mycorrhizal symbiosis
and interspecific interactions (Valverde-Barrantes et al. 2015;
Eissenstat et al. 2015). In most ecosystems, fine roots are
concentrated within the upper soil surface (e.g. 0-20 cm) where there
are typically higher nutrient availability and lower physical impedance
(Schenk & Jackson 2002). However, even within such a shallow profile,
species may differ in their vertical relative root distributions, which
is defined as the proportions of roots partitioned among soil zones
(Herben et al. 2018; Zhou et al. 2020). Such difference
may promote species co-existence through a more complete filling of
below-ground space (Ma & Chen 2017) and complementary resource use
among species (Blume‐Werry et al . 2019). Nevertheless, very few
studies have examined species-specific vertical root distributions in
diverse forest communities, primarily due to the difficulty in
identifying root materials to species. The lack of such information has
hindered our understanding of rooting-depth mediated below-ground niche
differentiation, root foraging strategies and species spatial
distributions (Gale & Grigal 1987; Klimešová et al. 2016).
As root systems are highly plastic organs, vertical root distributions
could express high phenotypic plasticity in response to external factors
such as edaphic heterogeneity (Sainju & Good 1993; Case et al.2020) and specific root neighbours (Zhang et al. 2019; Chenet al. 2020). Such adjustments may increase root nutrient uptake
efficiency through optimizing root placement according to nutrient
availability and neighbouring root identities (Mommer et al.2012; Zhang et al. 2019). While effort is increasingly devoted to
elucidating the mechanisms and consequences of root segregation (Holdo
2013; Kulmatiski et al. 2017), most studies are biased to
artificial communities (Mahall & Callaway 1992; Belter & Cahill 2015;
Zhang et al. 2019) or natural communities in arid and semi-arid
regions, typically involving two species (Case et al. 2020; Chenet al. 2020; but see Valverde-Barrantes et al. 2015). As
the direction and extent of rooting plasticity may be highly dependent
on the number and identities of root neighbours, it remains uncertain if
and to what extent relative root distributions would be affected by the
presence of multiple interacting neighbours in complex forests. Such
information is critical for the prediction of how the change of species
composition and edaphic conditions may influence competitive
interactions and root neighbourhood stability (Chen et al. 2018a,
b).
Within soil zones, the degree of interspecific root segregation could be
ultimately determined by their innate difference in root system
architecture (Schenk & Jackson 2005) and rooting plasticity in response
to edaphic heterogeneity or/and neighbour structure (Zhang et al.2019; Case et al. 2020; Zhou et al. 2020). Despite the
proposed mechanisms, the generalization of vertical root segregation in
natural communities have not been fully identified, with both apparent
(Kesanakurti et al. 2011; Herben et al. 2018) and minor
rooting differentiation observed (von Felten & Bernhard 2008; Hoekstraet al. 2015). Whether and how rooting differentiation mediates
species co-existence remains a subject of intensive studies.
Valverde-Barrantes et al. (2015) recently reported that root
depths of temperate trees in the 0-60 cm soil zone were largely
overlapped and exhibited low responsiveness to neighbours. However, root
depths alone may fail to fully capture root niches, as species with
similar root depths may differ considerably in their relative root
distributions (Schmid & Kazda 2001; Bennett et al. 2002; Herbenet al. 2018). It is thus unclear whether relative root
distribution represents a root niche dimension independent of root depth
that may contribute to vertical root segregation.
In this study, we collected > 4000 root segments from 625
soil samples within the 0-30 cm soil zone in a subtropical forest,
China. From these samples, we quantified the relative root distributions
of 53 woody species, modeled rooting plasticity of the 29 most common
species as a function of soil heterogeneity and root neighbours and
finally examined how individual roots of the identified 109 species were
vertically placed. We used these data to test three hypotheses. First,
we predicted high interspecific differentiation in relative root
distributions, with some predominantly placing their roots in specific
soil zones whereas some others being homogeneously distributed. Second,
we predicted high rooting plasticity in response to either edaphic
heterogeneity or/and root neighbours that may optimize root placement
according to nutrient availability and neighbour root competition.
Specifically, due to the potential pre-emptive utilization of soil
resources by some proliferative species (Campbell et al. 1991),
we predicted that the increase of one species’ root abundance would
reduce other species’ root proliferation, resulting in pervasive
negative correlations in relative root abundance among species pairs.
Finally, depending on the differentiation in relative root distributions
among species complementarily combined with high rooting plasticity, we
predicted that heterospecific roots would be vertically segregated
either simply by avoiding co-occurring or by avoiding placing similar
relative root abundance in the same soil zones.