Salomón Roberto, Valbuena-Carabaña María, Teskey Robert, McGuire Mary Anne, Aubrey Doug, González-Doncel Inés, Gil Luis, Rodríguez-Calcerrada Jesús
Forest Genetics and Ecophysiology Research Group, E.T.S. Forestry Engineering, Technical University of Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain.
Warnell School of Forestry and Natural Resources, University of Georgia, 180 East Green St, Athens, GA 30602-2152, USA.
J Exp Bot. 2016 Apr;67(9):2817-27. doi: 10.1093/jxb/erw121. Epub 2016 Mar 24.
Since a substantial portion of respired CO2 remains within the stem, diel and seasonal trends in stem CO2 concentration ([CO2]) are of major interest in plant respiration and carbon budget research. However, continuous long-term stem [CO2] studies are scarce, and generally absent in Mediterranean climates. In this study, stem [CO2] was monitored every 15min together with stem and air temperature, sap flow, and soil water storage during a growing season in 16 stems of Quercus pyrenaica to elucidate the main drivers of stem [CO2] at different temporal scales. Fluctuations in sap pH were also assessed during two growing seasons to evaluate potential errors in estimates of the concentration of CO2 dissolved in xylem sap ([CO2*]) calculated using Henry's law. Stem temperature was the best predictor of stem [CO2] and explained more than 90% and 50% of the variability in stem [CO2] at diel and seasonal scales, respectively. Under dry conditions, soil water storage was the main driver of stem [CO2]. Likewise, the first rains after summer drought caused intense stem [CO2] pulses, suggesting enhanced stem and root respiration and increased resistance to radial CO2 diffusion. Sap flow played a secondary role in controlling stem [CO2] variations. We observed night-time sap pH acidification and progressive seasonal alkalinization. Thus, if the annual mean value of sap pH (measured at midday) was assumed to be constant, night-time sap [CO2*] was substantially overestimated (40%), and spring and autumn sap [CO2*] were misestimated by 25%. This work highlights that diel and seasonal variations in temperature, tree water availability, and sap pH substantially affect xylem [CO2] and sap [CO2*].
由于呼出的二氧化碳有很大一部分保留在树干中，树干二氧化碳浓度（[CO₂]）的昼夜和季节变化趋势是植物呼吸和碳预算研究的主要关注点。然而，持续的长期树干[CO₂]研究很少，在地中海气候中通常不存在。在本研究中，在一个生长季节里，每隔15分钟对16棵比利牛斯栎树的树干[CO₂]进行监测，同时监测树干和空气温度、液流以及土壤储水量，以阐明不同时间尺度下树干[CO₂]的主要驱动因素。在两个生长季节中还评估了树液pH值的波动，以评估使用亨利定律计算的木质部树液中溶解的二氧化碳浓度（[CO₂*]）估计值中的潜在误差。树干温度是树干[CO₂]的最佳预测指标，分别解释了昼夜和季节尺度下树干[CO₂]变异性的90%以上和50%以上。在干旱条件下，土壤储水量是树干[CO₂]的主要驱动因素。同样，夏季干旱后的第一场雨导致树干[CO₂]出现强烈脉冲，表明树干和根系呼吸增强，对径向二氧化碳扩散的阻力增加。液流在控制树干[CO₂]变化方面起次要作用。我们观察到夜间树液pH值酸化和季节性逐渐碱化。因此，如果假设树液pH值的年平均值（在中午测量）是恒定的，夜间树液[CO₂*]会被大幅高估（40%），春季和秋季树液[CO₂*]会被错误估计25%。这项工作强调，温度、树木水分可利用性和树液pH值的昼夜和季节变化会显著影响木质部[CO₂]和树液[CO₂*]。
