Woody-tissue carbon budget and influencing factors

木质组织碳排放（ECO2）是陆地生态系统碳平衡的重要组成部分，分别占北方针叶林和温带阔叶林地上碳排放总量的25~50%，因而对林业生产和群落碳收支起到至关重要的作用。在现有研究中，木质组织碳通量观测多局限于树干中的某一个点（通常为胸径位置），测定其白天的CO2释放速率，并基于这种非连续的位点测量，对个体或群落水平的ECO2进行估算。但越来越多的研究认为，ECO2本身的时间变化和铅垂变异性对木质组织碳估算带来很大误差。准确量化ECO2的时间变化规律和空间分异特征，成为准确估算木质组织碳排放的前提条件。为深入探究木质组织碳排放规律及其影响因素，本研究选取北京市4个典型树种（国槐、旱柳、华北落叶松和侧柏）作为研究对象，对每个树种不同高度上的木质组织碳排放规律及其气象因子和形态指标做了连续观测。并对以下问题进行了详细的探讨: (1) 木质组织ECO2的时间变化规律和铅锤变异特征，及其影响因素分析: 对于本研究涉及的4个树种，其ECO2排放呈现相似的单峰型季节变化格局，温度和木质组织的径向生长被认为是造成木质组织ECO2季节变化的先决因素。生长季节和非生长季节，ECO2表现为不同的日变化格局：生长季节，木质组织ECO2日变化为双峰型，存在明显的午间抑制现象；在非生长季节，木质组织ECO2日变化为单峰型，与温度的日变化格局基本一致。铅锤梯度上，木质组织ECO2在生长季节，对测量高度的增加而增加，这种铅锤变异被认为是温度和径向生长的垂直分布造成的。 (2) 木质组织ECO2温度敏感性的时空变异特征及其温度依赖性探究： 在很多过程模型中，植物木质组织温度敏感性被假定为固定的数值2.0，但在本研究中，木质组织温度敏感性Q10无论在时间上还是空间尺度上均表现为高度的异质性。在季节尺度上，生长季节的Q10显著低于非生长季节，即Q10随测量温度的升高而降低；在日间尺度上，Q10在生长季节表现为明显的昼夜不对称现象，白天木质组织ECO2对温度的敏感性明显降低，但在非生长季节，日不对称性消失。(3) 基于木质组织空间异质性的径级分区方法和林分碳总量上推： 本研究根据木质组织内生态因子和形态指标，对木质组织进行了功能区的划分，并对不同功能区内木质组织对环境的生理生态响应进行了深入的探究。研究结果显示：（1）木质组织内的生态因子及生态化学计量（温度、组织含水量和N含量）等指标存在明显的日变化，并受季节的影响；（2）在一天中的某些时段，木质组织温度有明显的铅锤异质性，且不同季节模式相异；（3）木质组织内生态因子在铅锤分布上呈现离散分布。根据其异质性形态，本研究对木质组织进行了详细的分区，并对不同区内的碳排放进行了单独的估算，进而整合到个体或群落水平，大大提高了模型估算中存在的不足。 (4) 生长呼吸和维持呼吸的划分方法和C利用效率计算： 本研究对木质组织生长呼吸的维持呼吸的划分提出了切实可行的计算方法，有助于我们更深入地了解木质组织呼吸过程，并对环境变化带来的木质组织碳通量影响有更好的预测意义。 本研究对木质组织的分区对比和季节比较，相对于传统方法，可以很大程度上提高了碳收支总量的估算精度，并对木质组织呼吸有更好的认识，一定程度上解决了非连续的点位测量在尺度放大中存在的问题。CO2 efflux (ECO2) from stems and branches, is important in the regulation of biomass productivity and maintenance of carbon balance in terrestrial ecosystems, as 25–50 % of net photosynthetic production (NPP) has been estimated to be consumed by plant respiration and woody-tissue alone accounts for ca.11–23 %. Generally, ECO2 measurements were conducted during daytime at a given point (usually at breast height), and the estimation of ECO2 at tree- or stand-level was subsequently scaled up from the infrequent chamber-based measurements with the assumption of breast height ECO2 being representative of the entire stem. Actually, high temporal and vertical variations in ECO2 have hitherto been revealed, resulting in over- or under-estimation in the large-scale carbon budget. Quantitative analysis of ECO2 in temporal dynamics and vertical variations, as well as what factors are driving these changes, would help us to improve our knowledge of ECO2 processes and understand how climatic changes affect forest carbon budgets. In view of this, the hourly ECO2 and meteorological conditions at different heights of four typical tree species (Sophora japonica, Salix matsudana, Larix principis-rupprechtii and Platycladus orientalis) were made on two consecutive days per month throughout 2014, in an artificial mixed forest plantation near Beijing. Four issues were focused as bellows: （1）Temporal dynamics and vertical variations in woody-tissue ECO2 and their influencing factors: The results indicated that the seasonal change of ECO2 roughly followed the seasonal variations in woody-tissue temperature (TW) and stem radial diameter increment (Di). Together TW and Di explained the monthly change of ECO2, and the contributions of TW and Di changed with stem positions and growth stages. The diurnal patterns of ECO2 differed greatly between the growing and dormant season, showing a bimodal distribution with obvious midday depression in the former, and a unimodal distribution in the latter. Strong vertical variation in day-time ECO2 of the growing season was mainly caused by the vertical gradients of TW, Di and difference of sapwood volume per unit of stem surface along the trunk. （2）Temporal dynamics and vartical variations in temperature-sensitivity coefficient Q10 of the ECO2 In some process-based models, stem ECO2 is usually modeled as an exponential function of temperature with a constant Q10 of 2.0. The values of Q10 in current study was found highly variable, no matter in temporal or vertical series. At the seasonal scale, the monthly calculated Q10 was significant lower in growing season than in dormant season. Q10 was not constant as assumed in some models, rather showed obvious decline in summer months. Strong vertical variation in Q10 during the growing season was found in this research, showing gradient increment with height along the trunk. During the growth period, obvious diurnal asymmetry in Q10 was found, and the asymmetry disappeared in the dormant season. （3）Diameter zoning and total carbon estimation of woody-tissue based on the spatial heterogeneity: According to the vertical variations in ecological factors and morphological index, woody-tissue was divided into functional areas, and the physiological response of woody tissue to environmental factors within each functional area was investigated. Results show that: (1) the ecological factors and stoichiometry of wood tissues (temperature, tissue water content and N content) and other indicators showed obvious diurnal variation, and was affected by the seasons; (2) some time during the day, woody-tissue temperature showed obvious heterogeneity, and was different in different season; (3) it was discrete distributed in woody tissue temperature. According to the distribution of morphological heterogeneity, this paper carried out a detailed division of woody tissue, and the carbon estimation in different area was estimated separately, and then integrated into the individual or community level, greatly improved the shortcomings of the model estimate. （4）Division of growth and maintenance respiration and calculation of C utilization efficiency: In this study, we put forward a practical method for partition woody tissue respiration into growth and maintenance components, which is helpful for us to know more about the respiration process of woody tissue and predict the carbon flux of woody tissue