In many ecosystems, experiments have shown that more diverse communities have greater productivity. Where nitrogen is the main limiting mineral nutrient, a plausible mechanism for this effect is complementarity in nitrogen use, which would allow diverse plots to take up more nitrogen than monocultures. The additional nitrogen could be allocated along a spectrum from purely increasing biomass production to purely increasing tissue nitrogen. Both strategies face diminishing returns in enabling light capture and conversion. We sought to determine (1) whether overyielding could be explained by complementarity in nitrogen uptake, (2) how leaf biomass production and leaf nitrogen content (LNC) contribute to increases in the plot-level leaf nitrogen pool size, and (3) whether the prevailing light environment influences allocation of nitrogen. We performed this study at Cedar Creek Ecosystem Science Reserve in East Bethel, MN, using the Forests and Biodiversity (FAB) experiment, a tree diversity experiment containing plots ranging from 1 to 12 species. We measured the amount of leaf area and the leaf mass per area of each species in each plot. We also took leaf-level hyperspectral reflectance scans of each species in each plot, from which LNC can be inferred by a fitted calibration equation.
Consistent with prior results on aboveground stem biomass in the same system, we found a significant overyielding-like effect of biodiversity on plot-level leaf nitrogen pools. Partitioning overyielding into selection and complementarity effects by the method of Loreau and Hector (2001) revealed that complementarity effects dominated, while selection effects tended to be idiosyncratic. Both plot-wide mean leaf nitrogen content and total nitrogen pool size were strong predictors of annual woody biomass increment. Most of the variation in the total leaf nitrogen pool size was driven by variation in leaf biomass, although there was several-fold intraspecific variation in LNC across plots. The prevailing light environment faced by a species in a plot – as assessed by size relative to co-planted species – showed no detectable effects on the balance of allocation of the nitrogen increment to LNC or leaf biomass. These results demonstrate that nitrogen complementarity is a plausible mechanism for the biodiversity-productivity relationship in this tree diversity experiment, in large part because it allows trees to intercept more light by producing more leaf biomass.