Leaf and tree responses of young European aspen trees to elevated atmospheric CO2 concentration vary over the season 

Abstract

Elevated atmospheric CO2 concentration (eCO2) commonly stimulates net leaf assimilation, decreases stomatal conductance and has no clear effect on leaf respiration. However, effects of eCO2 on whole-tree functioning and its seasonal dynamics remain far more uncertain. To evaluate temporal and spatial variability in eCO2 effects, one-year-old European aspen trees were grown in two treatment chambers under ambient (aCO2, 400ppm) and elevated (eCO2, 700ppm) CO2 concentrations during an early (spring 2019) and late (autumn 2018) seasonal experiment (ESE and LSE, respectively). Leaf (net carbon assimilation, stomatal conductance and leaf respiration) and whole-tree (stem growth, sap flow and stem CO2 efflux) responses to eCO2 were measured. Under eCO2, carbon assimilation was stimulated during the early (1.63-fold) and late (1.26-fold) seasonal experiments. Stimulation of carbon assimilation changed over time with largest increases observed in spring when stem volumetric growth was highest, followed by late season down-regulation, when stem volumetric growth ceased. The neutral eCO2 effect on stomatal conductance and leaf respiration measured at leaf level paralleled the unresponsive canopy conductance (derived from sap flow measurements) and stem CO2 efflux measured at tree level. Our results highlight that seasonality in carbon demand for tree growth substantially affects the magnitude of the response to eCO2 at both leaf and whole-tree level.

摘要

大气CO2浓度升高（eCO2）通常会刺激叶片净同化，降低气孔导度，对叶片呼吸则没有明显影响。然而，eCO2对整个树木功能及其季节动态的影响仍远不确定。为了评估eCO2效应的时间和空间变异性，在早期（2019年春季）和晚期（2018年秋季）季节性试验（分别为ESE和LSE）期间，在普通（aCO2，400ppm）和高（eCO2，700ppm）CO2浓度下，在两个处理室中种植一年生欧洲白杨树。测定了叶片（净碳同化、气孔导度和叶片呼吸）和整棵树（茎生长、树液流和茎CO2流出）对eCO2的响应。在eCO2下，早期（1.63倍）和晚期（1.26倍）季节性试验期间刺激了碳同化。随着时间的推移，刺激碳同化的因素发生了变化，春季茎体积生长最高时，碳同化的增加幅度最大，茎体积生长停止时的后期则下调。在叶片水平上测得的中性eCO2对气孔导度和叶片呼吸的影响相当与在树木水平上测得的无反应的冠层导度（来自sap流量测量）和茎CO2流出相当。我们的研究结果强调，树木生长的碳需求的季节性实质上影响了叶片和整棵树对eCO2的响应程度。