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The relationship between forest productivity and tree species diversity has been described in detail, but the underlying processes have yet to be identified. One important issue is to understand which processes are at the origin of observed aboveground overyielding in some mixed forests. We used a beech-maple plantation exhibiting aboveground overyielding to test whether belowground processes could explain this pattern. Soil cores were collected to determine fine root (FR) biomass and vertical distribution. Correlograms were used to detect spatial arrangement. Near-infrared reflectance spectroscopy was used to identify the tree species proportion in the FR samples and spatial root segregation. An isotopic approach was used to identify water acquisition patterns. The structure and the composition of the ectomycorrhizal fungal community were determined by high-throughput sequencing of DNA in the soil samples. We found no spatial pattern for FR biomass or for its vertical distribution along the gradients. No vertical root segregation was found, as FR density for both species decreased with depth in a similar way. The two species displayed similar vertical water acquisition profiles as well, mainly absorbing water from shallow soil layers; hence, niche differentiation for water acquisition was not highlighted here. Significant alterations in the fungal community compositions were detected in function of the percentage of maple in the vicinity of beech. Our findings do not support the commonly suggested drivers of aboveground overyielding in species-diverse forests and suggest that competition reduction or between-species facilitation of belowground resource acquisition may not explain the observed aboveground overyielding. [ABSTRACT FROM AUTHOR]

Foliar δ15N, %N and %P in the dominant woody and herbaceous species across nutrient gradients in New Zealand restiad (family Restionaceae) raised bogs revealed marked differences in plant δ15N correlations with P. The two heath shrubs, Leptospermum scoparium (Myrtaceae) and Dracophyllum scoparium (Epacridaceae), showed considerable isotopic variation (−2.03 to −15.55‰, and −0.39 to −12.06‰, respectively) across the bogs, with foliar δ15N strongly and positively correlated with P concentrations in foliage and peat, and negatively correlated with foliar N:P ratios. For L. scoparium, the isotopic gradient was not linked to ectomycorrhizal (ECM) fractionation as ECMs occurred only on higher nutrient marginal peats where 15N depletion was least. In strong contrast, restiad species ( Empodisma minus Sporadanthus ferrugineus, S. traversii) showed little isotopic variation across the same nutrient gradients. Empodisma minus and S. traversii had δ15N levels consistently around 0‰ (means of −0.12‰ and +0.15‰ respectively), and S. ferrugineus, which co-habited with E. minus, was more depleted (mean −4.97‰). The isotopic differences between heath shrubs and restiads were similar in floristically dissimilar bogs and may be linked to contrasting nutrient demands, acquisition mechanisms, and root morphology. Leptospermum scoparium shrubs on low nutrient peats were stunted, with low tissue P concentrations, and high N:P ratios, suggesting they were P-limited, which was probably exacerbated by markedly reduced mycorrhizal colonisations. The coupling of δ15N depletion and %P in heath shrubs suggests that N fractionation is promoted by P limitation. In contrast, the constancy in δ15N of the restiad species through the N and P gradients suggests that these are not suffering from P limitation. [ABSTRACT FROM AUTHOR]