Hardwood mixture increases stand productivity through increasing the amount of leaf nitrogen and modifying biomass allocation in a conifer plantation

Recent studies have demonstrated positive effects of hardwood mixtures on stand productivity in conifer plantations. However, the extent to which hardwood mixture increases leaf nitrogen levels and improves stand productivity (in terms of nutrient cycling) remains poorly understood. In this study, we estimated leaf nitrogen per stand (leaf N_stand) by multiplying mass-based leaf nitrogen (leaf N_mass) and leaf mass per stand (leaf M_stand) in conifers, hardwoods, and understory plants in a thinning experiment. The experiment, performed in 2018, was conducted under unthinned (Control) with few hardwoods, 33% thinned (Weak) with a small amount of hardwoods, and 67% thinned (Intensive) with abundant hardwoods in a conifer Cryptomeria japonica (C. japonica) plantation. We also investigated the stand volume of conifers and hardwoods during the 12-year period (2008–2020) after the second thinning (2008), in all three groups. Of all plants—conifers (C. japonica), hardwoods, and understory plants—stand volume, leaf M_stand, and leaf N_stand were all highest in Control and lowest in Intensive. The ratio between the values of Intensive and Control was 27.5% for stand volume, 58.9% for leaf M_stand, and 72.8% for leaf N_stand. This is due to the fact that the leaf M_stand of hardwoods—which had approximately 2–3 times the leaf N_mass of C. japonica—increased rapidly with increasing thinning intensity, and the leaf M_stand of the understory plants—which had approximately twice the leaf N_mass of C. japonica—increased monotonically with increasing thinning intensity, although both the leaf M_stand and leaf N_stand decreased with increasing thinning intensity in both current-year and old C. japonica leaves. In conifers, the relative increment in stand volume was positively correlated with the leaf mass ratio (ratio between the leaf M_stand and stand volume of conifer) and leaf N ratio of conifer (ratio between the leaf N_stand and stand volume), suggesting that disproportionate biomass investment in leaves enhances productivity in conifers. The study suggests that hardwood mixture can significantly increase forest productivity, mainly due to a disproportionate increase in leaf nitrogen in hardwoods and an increase in biomass allocation to leaves in conifers in a conifer, C. japonica plantation.