The continuously increasing atmospheric carbon dioxide concentration ([CO2]) has substantial effects on plant growth, and on the composition and structure of forests. However, how plants respond to elevated [CO2] (e[CO2]) under intra- and interspecific competition has been largely overlooked. In this study, we employed Abies faxoniana and Picea purpurea seedlings to explore the effects of e[CO2] (700 ppm) and plant-plant competition on plant growth, physiological and morphological traits, and leaf ultrastructure. We found that e[CO2] stimulated plant growth, photosynthesis and nonstructural carbohydrates (NSC), affected morphological traits and leaf ultrastructure, and enhanced water and nitrogen use efficiencies in A. faxoniana and P. purpurea. Under interspecific competition and e[CO2], P. purpurea showed a higher biomass accumulation, photosynthetic capacity and rate of ectomycorrhizal infection, and higher water and nitrogen use efficiencies compared with A. faxoniana. However, under intraspecific competition and e[CO2], the two conifers showed no differences in biomass accumulation, photosynthetic capacity, and water and nitrogen use efficiencies. In addition, under interspecific competition and e[CO2], A. faxoniana exhibited higher NSC levels in leaves as well as more frequent and greater starch granules, which may indicate carbohydrate limitation. Consequently, we concluded that under interspecific competition, P. purpurea possesses a positive growth and adjustment strategy (e.g., a higher photosynthetic capacity and rate of ectomycorrhizal infection, and higher water and nitrogen use efficiencies), while A. faxoniana likely suffers from carbohydrate limitation to cope with rising [CO2]. Our study highlights that plant-plant competition should be taken into consideration when assessing the impact of rising [CO2] on the plant growth and physiological performance.

中文翻译：

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在种内和种间竞争下，升高的CO2引起两种针叶树种的不同生长刺激，水和氮利用效率以及叶片超微结构响应。

大气中二氧化碳浓度（[CO2]）的不断增加对植物的生长以及森林的组成和结构都有重要影响。但是，在种内和种间竞争下植物如何对升高的[CO2]（e [CO2]）作出反应已被大大忽略。在这项研究中，我们采用了冷杉冷杉和紫云杉幼苗，研究了e [CO2]（700 ppm）和植物-植物竞争对植物生长，生理和形态性状以及叶片超微结构的影响。我们发现e [CO2]刺激了A. Faxoniana和P. purpurea的植物生长，光合作用和非结构性碳水化合物（NSC），影响了形态特征和叶片超微结构，并提高了水和氮的利用效率。在种间竞争和e [CO2]作用下，紫杉显示出更高的生物量积累，与fax草相比，光合能力和外生菌根感染率更高，水和氮的利用效率更高。然而，在种内竞争和e [CO2]的作用下，这两个针叶树在生物量积累，光合能力以及水和氮的利用效率上没有差异。此外，在种间竞争和e [CO2]的作用下，花粉线虫在叶片中表现出较高的NSC水平，并且其淀粉颗粒的出现频率更高且更大，这可能表明碳水化合物的局限性。因此，我们得出结论，在种间竞争下，紫杉具有积极的生长和调节策略（例如，较高的光合作用能力和外生菌根感染率，以及较高的水和氮利用效率），而A. 传真虫可能会受到碳水化合物的限制，以应对不断增长的[CO2]。我们的研究强调，在评估上升的[CO2]对植物生长和生理性能的影响时，应考虑植物与植物之间的竞争。