Abstract

Current carbon cycle models attribute rising atmospheric CO₂ as the major driver of the increased terrestrial carbon sink, but with substantial uncertainties. The photosynthetic response of trees to elevated atmospheric CO₂ is a necessary step, but not the only one, for sustaining the terrestrial carbon uptake, but can vary diurnally, seasonally and with duration of CO₂ exposure. Hence, we sought to quantify the photosynthetic response of the canopy-dominant species, Quercus robur, in a mature deciduous forest to elevated CO₂ (eCO₂) (+150 μmol mol⁻¹ CO₂) over the first 3 years of a long-term free air CO₂ enrichment facility at the Birmingham Institute of Forest Research in central England (BIFoR FACE). Over 3000 measurements of leaf gas exchange and related biochemical parameters were conducted in the upper canopy to assess the diurnal and seasonal responses of photosynthesis during the 2nd and 3rd year of eCO₂ exposure. Measurements of photosynthetic capacity via biochemical parameters, derived from CO₂ response curves, (V_cmax and J_max) together with leaf nitrogen concentrations from the pre-treatment year to the 3rd year of eCO₂ exposure, were examined. We hypothesized an initial enhancement in light-saturated net photosynthetic rates (A_sat) with CO₂ enrichment of ≈37% based on theory but also expected photosynthetic capacity would fall over the duration of the study. Over the 3-year period, A_sat of upper-canopy leaves was 33 ± 8% higher (mean and standard error) in trees grown in eCO₂ compared with ambient CO₂ (aCO₂), and photosynthetic enhancement decreased with decreasing light. There were no significant effects of CO₂ treatment on V_cmax or J_max, nor leaf nitrogen. Our results suggest that mature Q. robur may exhibit a sustained, positive response to eCO₂ without photosynthetic downregulation, suggesting that, with adequate nutrients, there will be sustained enhancement in C assimilated by these mature trees. Further research will be required to understand the location and role of the additionally assimilated carbon.

中文翻译：

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在 175 岁的栎树中，通过三年升高的二氧化碳暴露可以维持光合作用的增强吗？

摘要

当前的碳循环模型将大气 CO ₂上升归因于陆地碳汇增加的主要驱动因素，但存在很大的不确定性。树木对升高的大气 CO ₂的光合作用响应是维持陆地碳吸收的必要步骤，但不是唯一的步骤，但会随着日、季节和 CO ₂暴露持续时间而变化。因此，我们试图量化成熟落叶林中以树冠占优势的物种栎木对升高的 CO ₂ (eCO ₂ ) (+150 μmol mol ^-1 CO ₂ )的光合作用响应。长期自由空气 CO ₂位于英格兰中部的伯明翰森林研究所 (BIFor FACE) 的浓缩设施。在上部冠层进行了超过 3000 次叶片气体交换和相关生化参数的测量，以评估在 eCO ₂暴露的第 2 年和第 3 年期间光合作用的昼夜和季节性响应。通过生化参数测量光合能力，从 CO ₂响应曲线 ( V _cmax和J _max ) 以及从预处理年到 eCO ₂暴露第 3 年的叶片氮浓度得到检查。_{我们假设光饱和净光合速​​率（A sat}）与 CO的初始增强根据理论，_{2富集约 37%，但预期光合能力将在研究期间下降。}在 3 年期间，与环境 CO ₂ (aCO _{2 ) 相比，在 eCO 2中生长的树木的上部冠层叶片的 A sat}高 33 ± 8%（平均和标准误差），并且光合增强随着光照的减少而降低。_{CO 2}处理对V _cmax或J _max以及叶片氮没有显着影响。我们的结果表明，成熟的Q. robur可能对 eCO ₂表现出持续的积极反应没有光合作用下调，这表明在充足的营养条件下，这些成熟树木同化的 C 将持续增加。需要进一步的研究来了解额外同化碳的位置和作用。