Testing rooting plasticity in response to edaphic heterogeneity
and root neighbours
The plasticity of relative root distributions in response to edaphic
heterogeneity and root neighbours was evaluated for each species with
separate redundancy analysis (RDA). As it is required in an RDA analysis
the number of observations of the response variable should be greater
than that of explanatory variables to avoid overfitting (Legendreet al. 2013), we focused on the 29 most common species with
occurrences ≥ 30 cubes. This maximized the number of root neighbours
that could be included in the RDA analysis, giving the minimum
observations of 30 in the relative root distribution for the focal
species.
For each species, relative root abundance (%) in the 0-10 cm soil zone
was treated as the response variable. Additionally, two predictor
variable matrices were created, one of which described the
presence-absence of the remaining 28 species within the 0-30 cm soil
zone and the other described edaphic conditions regarding the 13
selected soil variables. We then performed a global analysis to test the
importance of each factor in the matrix separately (ter Braak et
al. 2012).When the global analysis was significant, a step-wise RDA was
performed to select the parsimonious subsets of competitor species and
soil factors as explanatory variables (Blanchet et al. 2008).
Lastly, we performed a variance partitioning to determine the relative
importance of soil variables and root neighbours on rooting plasticity
as described in Peres-Neto et al. (2006).
To examine whether the increase of one species’ root abundance would
reduce neighbours’ root abundance, we performed a pairwise correlation
of relative root abundance between a focal species and each of the
remaining 28 species. Finally, to examine whether species relative root
distributions would be influenced by heterospecific root crowdedness, we
used a correlation analysis to examine the relationship between the
focal species relative root distributions and both heterospecific root
richness and total root length in root neighbourhoods.