In a context of extensive management and disturbance of species composition in all sort of ecosystems, understanding the different roles of biodiversity is crucial. Evidences of its link with ecosystem functions in grassland communities are consolidating, but much is left to do in forests. The first results of a biodiversity experiment with trees are conflicting. While aboveground biomass was generally greater than expected in mixture, belowground biomass responded the opposite direction. This raises many questions: Is there a bias in diversity effects estimations when not accounting for roots? Is the root to shoot ratio affected by diversity? And if so, is it related to a better nutrient acquisition or overyielding? This analysis focus on the first four years of growth at the Montreal site of the International Diversity Experiment Network with Trees (IDENT). The design presents both a species richness (1, 2 and 4 species) and a functional diversity gradient. 24 mixtures and their respective monocultures were analyzed (x4 replicates). The roots were cored in the fall of 2012, and 7 mixtures had their roots identified at the species level.
Results/Conclusions
As expected, a bias was observed when the roots were ignored. Both positive and negative effects were increased on average by 9% towards more extreme values. When expected ratios between below- and aboveground biomass were weighted by specific aboveground biomass, no evidence of a consistent change in ratios relatively to monocultures was found, and these net effects were not correlated to overyielding. A more thorough investigation of the bias in the 7 treatments with identified roots revealed that the cause was otherwise, i.e. an overestimation of the 'selection effects'. This metric, which matches Grime's ratio hypothesis, was influenced by the correlation existing between the dominance of certain species in mixture and their low root to shoot ratios. While selection effects were reduced by roots addition, complementarity effects were elevated proportionally to the mass fraction they represent, strongly suggesting a greater relative importance of these last effects than earlier suspected. The probability of overestimating selection effects is high when roots are ignored. This is because of the likeliness of the correlation between competitive ability and greater aboveground biomass allocation. Another consequence of this correlation is the detection of negative effects on root biomass. In both cases, a more holistic approach is necessary for better estimations of biodiversity effects, and especially for disentangling complementarity and selection effects.