Interspecific competition for inorganic nitrogen between canopy trees and underlayer-planted young trees in subtropical pine plantations

Abstract Although the area of monospecific plantations in China ranks first in the world, these plantations possess poor disease resistance, low quality, and low ecological benefit. Planting broadleaf seedlings into the understory of monospecific coniferous plantations is an effective and common forest management method to improve plantation structures and enhance their resistance to adverse conditions, disease, and pests. Species that are desired and suitable are selected according to the complementarity of their functional traits to ensure optimal resource utilization and species coexistence. Thus, in Pinus massoniana and Pinus elliottii plantations where three broadleaf species were underplanted, any alterations to root functional traits and inorganic nitrogen (N) uptake rates in response to interspecific competition were studied in situ using 15N isotope labelling technology. The results showed that the five species preferred to absorb ammonium (dominant N form in native soils) rather than nitrate from the soil. Coniferous species showed higher plasticity in root biomass, physiology of N uptake rate, morphology, and mycorrhizal colonization than broadleaf species responding to interspecific competition. Coniferous trees reduced the investment in root biomass while enhanced N uptake rates to acclimate to belowground interspecific competition. Broadleaf species M. maudiae and S. superba were more dependent on mycorrhizal symbiosis to acquire resource in response to environmental variations. The amount of inorganic N uptake by P. massoniana and P. elliottii roots were inhibited by the presence of Michelia maudiae and Schima superba roots, respectively. Based on inorganic N uptake amount, M. maudiae and S. superba may be suitable for underplanting in the P. elliottii and P. massoniana plantations, respectively. Liquidambar formosana was suitable for both two pine plantations. The responses of root N uptake rate, mycorrhizal colonization and morphological traits to interspecific competition can provide supplemental data for species selection to improve structures of artificial plantations.