In the era of extreme weather events, plant resistance to excessive water should be considered for plantations in frequently flooded areas. Therefore, we quantified the resistance to excessive irrigation (REI) in seedlings of six conifer species on the basis of their morphological, physiological, and biochemical traits. Two different irrigation regimes, natural precipitation as a control treatment and additional irrigation by sprinklers (30.89 ± 2.80 % above natural precipitation) as excessive irrigation, were applied for three consecutive years. There were three distinct responses in biomass accumulation: Chamaecyparis obtusa and Abies holophylla significantly increased their aboveground and underground biomass and showed a strong REI; Pinus thunbergii and P. densiflora, both resistant to drought, significantly decreased their aboveground and underground biomass and showed a weak REI. Larix kaempferi and Pinus koraiensis showed a medium REI with only aboveground biomass increment in the former and no biomass change in the latter. Plant biomass responses corresponded well with the changes in morphological, physiological, and biochemical traits. Biomass augmentation resulted from an increase in leaf size, leaf mass per area (LMA), maximum photosynthesis rate, and leaf water potential. Flavonoids, LMA, and stomatal conductance are the most important traits to access species REI.

Down-regulation of photosynthesis in elevated CO2 (eCO2), could be attributed to depletions in nitrogen (N) availability after long term exposure to eCO2 (progressive nitrogen limitation, PNL) or N dilution due to excessive carbon accumulation of nonstructural carbohydrates. To investigate this, we examined N availability, photosynthetic characteristics, and leaf N allocation in Pinus densiflora, Fraxinus rhynchophylla, and Sorbus alnifolia, grown under three different CO2 concentrations, ambient CO2 (aCO2), aCO2 × 1.4 ppm (eCO21.4), and aCO2 × 1.8 ppm (eCO21.8), for 9 years. N availability increased under eCO21.8, and its allocation to chlorophyll (NFchl) and photosynthetic N use efficiency also increased under eCO2. The maximum carboxylation rate, leaf N per mass (Nmass), and N allocation to Rubisco (NFRub), however, were all lower under eCO21.8. There were interactions in NFchl between canopy × species × CO2 concentrations. The greatest changes in N allocation under eCO2 were in the lower canopy of S. alnifolia, the most shade-tolerant species and this species have relatively high flexibility in N operations compared to shade-intolerant species. Based on the reduction in Nmass that was diluted by increased nonstructural carbohydrates and increased N availability, down-regulation of photosynthesis was found to be caused by the dilution and change in N allocations, rather than PNL.