Testing vertical root segregation in root neighbourhoods
Vertical root segregation was tested in two complementary ways using all
the identified root morphotypes (i.e. a specific species composed of
several root segments) belonging to 109 species sampled from 604 root
neighbourhoods. First, we tested whether species were vertically
segregated simply by exhibiting different root-placement patterns (i.e.
root depth) within the 0-30 cm soil zones (Fig. 1a, 1c). Second, we
sequentially tested whether species were vertically segregated by
avoiding placing similar relative root abundance within the 0-10, 10-20
and 20-30 cm soil zone, respectively (Fig. 1b, 1d).
As each root morphotype was assigned a specific rooting type (Fig. 1)
that was treated as a categorical functional trait, we constructed a
trait dendrogram depicting the pairwise functional distance of all the
identified and coded morphotypes based on a Gower Distance and
determined the mean pairwise functional distances (MPD) of root
neighbourhoods (Gower 1971; Laliberté et al. 2014; Swenson 2014).
We then constructed 999 null communities for each root neighbourhood,
from which the mean and the standard deviations of the 999 simulated
MPDs were determined using the picante package (Kembel et al.2010). Null communities were constructed by randomizing the pairwise
functional distances of the coded morphotypes by shuffling the tip names
of the trait dendrogram, keeping species occurrences constant. Finally,
we determined the standardized effect size of the functional MPD
(SESMPD) to quantify the functional similarity of co-occurring species
based on their root-placement patterns following the equation:
SESMPD =\(\frac{(\text{MPD}_{\text{obs}}\ -\ mean\ (\text{MPD}_{\text{null}}))}{sd\ (\text{MPD}_{\text{null}})}\),
where MPDobs represents the observed MPD. The mean
(MPDnull) and sd (MPDnull) represent the
mean and the standard deviations of the 999 simulated MPDs,
respectively. Root neighbourhoods with a positive SESMPD indicated
functional overdispersion (i.e. vertical root segregation) with values
greater than 1.96 indicating significant segregation, whereas root
neighbourhoods with a negative SESMPD indicated functional clustering
(i.e. vertical root aggregation) with values below -1.96 indicating
significant root aggregation (e.g. Swenson 2014). The absolute value of
SESMPD indicated the magnitude of the difference between the observed
and the simulated MPD. Similarly, we quantified root segregation based
on species relative root distributions simply by replacing the
root-placement-pattern-based trait dendrogram with a trait dendrogram
constructed based on species relative root distribution. In addition to
using MPD, we also used the standard deviation and variance of relative
root distributions to evaluate the similarity of root depths among
co-occurring species. For a specific soil zone, we constructed a trait
dendrogram based on species relative root abundance in the soil zone.
Under the prediction of vertical root segregation, either by way of
avoiding co-occurring or placing similar relative root abundance within
the same soil zone, we would expect pervasive functional overdispersion
in root neighbourhoods, which could be supported by the following two
observations. First, there existed a higher proportion of root
neighbourhoods that exhibited functional overdispersion than clustering.
Second, and more importantly, that the difference between the observed
and simulated MPD, measured as the mean of the SESMPD, was significantly
higher than the expectation of zero when using a two-tail Student’st test (Yang et al. 2014).
All statistical analyses were performed in the R 3.6.0 statistical
software (R Development Core Team, 2019).