光-湿-气孔耦合自驱动活立木材干燥的研究

Wood drying is an efficient way to make wood keep the excellent finishing and utilization properties by controlling the moisture content. Conventional drying technology, however, has a large proportion of energy consumption during the woodworking process. So, combined-drying technology is rising. It is vital and imperious for combined-drying technology to develop a new and energy-efficient, even zero energy consumption, pre-drying technology. Transpiration is theoretically and practically able to dry the living trees, which is the force self driven for the coupling of light, relative humidity and stomata for long-distance water ascent in xylem and water loss in leaves. On the basis of feasibility study on physiological drying for living trees, physiological drying technology will be improved, and the dynamics and paths of water ascent in xylem and water loss in leaves will be explored in this present work to establish the mechanics of living trees drying self-driven by the coupling of light, relative humidity and stomata. Objectives and main research contents of the project are to a) improve physiological drying process by revealing the effects of weather conditions and laying pattern on the rate and final moisture content of physiological drying, b) develop a mechanics of water loss in leaves by discussing the pathway of water diffusion and site of water evaporation in leaves, c) establish the dynamics of water ascent in xylem in the longitudinal direction on the process of physiological drying by probing the relationships between anatomical and macroscopical structures and the formation, development and distribution of water-stress induced cavitation and embolism, and kinetics of microfluidic.

木材干燥可通过控制含水率使木材保持良好加工和使用性能，常规干燥耗能巨大，联合干燥技术成为发展趋势，因此，探索新型低能耗甚至零能耗干燥预处理技术是十分重要和迫切的。蒸腾作用作为植物生理过程中水分长距离传输和散失的主要动力，可用于木材干燥，且无额外能耗，可在光照、湿度和叶片气孔耦合作用下自驱动木材干燥。因此，本项目在前期对活立木生理干燥可行性探讨基础上，结合试验研究和理论研究，从实践出发，研究不同环境及干燥形式下蒸腾作用对干燥效果的影响，并深入机理，研究生理干燥过程中，处于严重水分胁迫状态下树木叶内水分传输路径和蒸发位点、叶孔蒸腾和角质层蒸腾之间的关系等水分散失机理；木质部内空穴的产生、分布、变化和对水分传输的作用，栓塞的分布及其恢复，纹孔或穿孔解剖结构对空穴和栓塞的作用，以及侧枝分布、同主干微观连接对水分传输路径的影响等木质部水分运输机理，最终建立基于光-湿-气孔耦合自驱动活立木材干燥机制。

本研究通过对新疆杨进行边材切断处理，切断其自根部的水分来源，同时对其进行保留树冠、移除树冠、垂直放置、倾斜45°放置和平躺放置等几种处理，测定了不同处理后新疆杨的含水率、树叶蒸腾特性变化情况，水分横向弛豫时间特性、水分存在状态及变化以及水相关孔隙结构及变化情况，分析了树干边材水分散失的主要动力，影响水分散失的主要因素，并通过分析生理干燥过程中水分状态及水相关孔隙结构变化情况，探讨了水分迁移规律，最后提出了基于蒸腾作用的光-湿-气孔耦合自驱动活立木材水分传输和散失的机理。主要研究结论为：杨树活立木生理干燥过程中的含水率在9天内由60.54%下降到41.32%，平均下降速率为2.30% MC/天，但边材含水率的快速下降主要发生在前3~5天；树叶蒸腾作用是水分下降的主要动力和原因；水分切断处理会使新疆杨的树叶蒸腾速率显著下降，但不同放置方式对树叶蒸腾作用的影响不大，水分的快速散失主要发生在水分切断处理后的3~5天；在常规干燥过程中，新疆杨边材中的水分主要存在自由水和结合水两种状态；在综合分析上述研究结果的基础上，提出了光-湿-气孔耦合自驱动生理干燥过程中水分传输和散失机理：叶内水分在叶片与周围环境蒸汽压差的作用下扩散到大气中，在木质部细胞壁上形成负压；叶脉木质部产生的负压在细胞壁孔隙(r2, r3，微毛细管系统)和导管(r6)或木纤维细胞腔(r5)等大毛细管中交替传递，实现水分的长距离传输。该成果对降低干燥能耗具有重要的理论意义和现实意义。