Abstract How tree growth and the underlying photosynthesis of leaves, especially those multi-aged leaves, change after the application of both pruning and understory removal remains unclear. In this study, the tree growth and photosynthetic responses of various-aged needles to pruning, understory removal and their interactions were investigated in a Chinese fir (Cunninghamia lanceolata) plantation. The biochemical and stomatal limitations to photosynthesis was separated by the combined measurements of stable carbon (δ 13C) and oxygen (δ 18O) isotopes in needles. Our results showed that the tree growth rates did not respond to pruning, understory removal, and their interactions. Pruning significantly stimulated the net photosynthetic rates (PA) and stomatal conductance (gs) of remaining foliage (especially the current- and one-year-needles). Although pruning had no effect on needle total nitrogen (N) concentration, the concentrations of water-soluble N (NS), the ratio of water-soluble N to sodium dodecyl sulfate (SDS)-insoluble N (NS/ND), and the photosynthetic N use efficiency (PNUE) were higher in needles of pruned trees compared with the unpruned trees. A significant and positive correlation between PNUE and NS/ND was also detected. The constant δ 13C and declined δ 18O in the current-year-needles from pruned trees suggested that both the photosynthetic capacity and the gs were responsible for the enhancement in PA of the youngest needles. Conversely, in the previous year needles, δ 13C and δ 18O were not significantly different between the control and pruned trees. Consistent with the response of tree growth rate, the foliar photosynthesis also did not exhibit changes following understory removal in both pruned and unpruned stands. We highlighted that pruning caused an up-regulation in PA of remaining foliage, thereby mitigating the negative effects of canopy loss on carbon assimilation.

中文翻译：

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树冠修剪和林下移除并未改变杉木 (Cunninghamia lanceolata) 人工林的树木生长速度

摘要 修剪和去除林下植物后树木的生长和叶子的潜在光合作用，特别是那些多龄的叶子，如何变化尚不清楚。在这项研究中，在杉木 (Cunninghamia lanceolata) 人工林中调查了不同年龄针叶对修剪、林下移除及其相互作用的树木生长和光合响应。光合作用的生化和气孔限制是通过对针中稳定碳 (δ 13C) 和氧 (δ 18O) 同位素的组合测量来区分的。我们的结果表明，树木的生长速度对修剪、林下植被移除及其相互作用没有反应。修剪显着刺激了剩余叶子（尤其是现针和一年针）的净光合速率 (PA) 和气孔导度 (gs)。尽管修剪对针状总氮 (N) 浓度、水溶性 N (NS) 的浓度、水溶性 N 与十二烷基硫酸钠 (SDS) 不溶性 N (NS/ND) 的比率以及与未修剪的树木相比，修剪的树木针叶的光合氮利用效率（PNUE）更高。还检测到 PNUE 和 NS/ND 之间的显着正相关。来自修剪树木的当年针叶中的常数 δ 13C 和下降的 δ 18O 表明光合能力和 gs 都是导致最年轻针叶 PA 增强的原因。相反，在前一年的针叶中，对照和修剪的树木之间的 δ 13C 和 δ 18O 没有显着差异。与树木生长速度的响应一致，在修剪和未修剪的林分中去除林下植物后，叶面光合作用也没有表现出变化。我们强调修剪导致剩余叶子的 PA 上调，从而减轻冠层损失对碳同化的负面影响。