Effects of simulated nitrogen deposition on the ecophysiological responses of Populus beijingensis and P. cathayana under intra- and interspecific competition

Abstract

High N deposition has been recognized as a major factor influencing plant-plant interactions. We aim to discover whether N deposition could affect competitive relationships between poplars to predict threats posed by the introduction of exotic hybrids to native relative species. Intra- and interspecific competition was investigated for an introduced hybrid poplar (Populus beijingensis) and the native paternal species P. cathayana under two N deposition regimes. Under control conditions, P. cathayana grown under either intra- or interspecific competition showed consistently greater above-ground biomass, root-to-shoot ratio (R/S), photosynthetic capacity, higher activities of N-assimilation enzymes in leaves, and preference for N-NO3− than corresponding P. beijingensis. Interspecific competition increased leaf area, specific leaf area (SLA), R/S, and photosynthetic nitrogen use efficiency (PNUE) in P. cathayana, but decreased PNUE in P. beijingensis compared to the values under monoculture. Thus, P. cathayana can gain a competitive advantage over P. beijingensis at low N availability. In contrast, under N deposition, P. beijingensis under mixture showed more positive responses in growth, higher plasticity in biomass allocation and root architecture, and the shift for N-NO3− preference when compared with P. cathayana, which resulted in relatively higher values in leaf area, SLA, and biomass accumulation in P. beijingensis. Nitrogen deposition increased the intrinsic water-use efficiency of conspecific P. beijingensis and heterospecific P. cahtayana. Our results suggest that N deposition could reverse competitive relationships between the poplars, and exotic P. beijingensis has the potential to outcompete native P. cathayana under growing N deposition.