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.