不同温度下时域反射法反演活立木树干含水率的机理及种特异性模型

Stem moisture content is an important factor for reflecting the physiological characteristics of standing tree, which is closely linked with its physical and mechanical properties and the processing performance of corresponding logs and lumbers. Generally, stem moisture content is much higher than fiber saturation point (FSP), which may cause an inaccuracy and large error in measurement, especially at different temperature conditions. The real-time accurate measurement of stem moisture content is a problem for forestry researchers. Recent studies show that Time domain reflectometry (TDR) is one of the most promising methods to solve this problem. When TDR is used to evaluate the stem moisture content, the mechanism is not clear and the species-specific equations are needed, but these are seldom reported. Therefore, ten standing trees species in northeast forest region of China are selected as the study objects in this project, the research content is: (1) to study the effect of measured medium factors such as standing trees internal structure and composition at different temperatures, from macroscopic and microcosmic levels, on the electromagnetic wave transmission in the medium interfaces using TDR; to construct an inversion method of dielectric constant of standing trees and further to quantitatively analyze the relationship between dielectric constant and moisture content, which are used to reveal the inversion mechanism of stem moisture content based on TDR waveform;(2)to construct some species-specific equations and a detection method for determining the moisture content of heartwood and sapwood of standing trees at different environment temperatures through large number of filed samples test and statistical analysis. The research results of the project will be beneficial to make a precise management of forests (especially precious forests) and can provide the basic data for wood processing and utilization.

树干含水率是活立木生理特征的重要表征因子，与活立木物理、力学特性及采伐后原木、板材的加工性能有紧密联系。通常树干含水率远高于纤维饱和点，导致其测量误差大且不稳定（尤其温差大时），它的实时准确测定一直是个难题。最近研究表明，时域反射法检测高含水率木材时颇具潜力，但目前该方法反演树干含水率时仍有机理不清晰、种间模型差异大等问题，对此却鲜见相关报道。因此本项目拟以东北典型树种活立木为研究对象，利用时域反射法，从宏观、微观层面研究不同温度下被测介质因素（活立木内部结构和组分）对电磁波在阻抗不连续界面反射和透射的影响，构建活立木介电常数反演方法，量化分析介电常数与含水率的关系，揭示活立木树干含水率反演机理；通过野外大样本测试与统计分析，构建不同温度下活立木边、心材含水率种特异性反演模型，确定树干含水率检测方法。研究结果将有利于森林（尤其珍贵树林）精准经营，并期望为木材高效加工与利用提供基础数据。

树干含水率是活立木生理特征的重要表征因子，与活立木物理、力学特性及采伐后原木、板材的加工性能有紧密联系。通常树干含水率远高于纤维饱和点，导致其测量误差大且不稳定（尤其温差大时），它的实时准确测定一直是个难题。课题组前期研究表明，时域反射法检测高含水率木材时颇具潜力，但该方法反演树干含水率时仍有机理不清晰、种间模型差异大等问题。本项目以东北典型树种活立木为研究对象，利用时域反射法，研究了不同温度下被测介质内电磁波在阻抗不连续界面的传播规律，建立了活立木介电常数反演方法，量化分析了介电常数与含水率的关系，揭示了树干含水率反演机理；通过野外测试与分析，构建了不同温度下树干含水率种特异性反演模型及通用模型。研究表明，频率和探针长度对电磁波传播影响显著，本研究中10MHz以上频率和探针深度8cm时介电常数有较稳定的变化规律，木材介电常数接近于仅由电子极化引起的介电常数值，适宜用于树干含水率监测；由含水率引起的介电常数变化机理高度依赖于温度、频率及树种结构的差异（给定条件20℃，10MHz下，白桦、椴树、冷杉、水曲柳木材含水率介电模型R²>0.95，RMSE<0.14）。所研究的东北典型树种含水率-电磁波传播时间预测模型种间差异性大，优选的二次多项式种特异性模型拟合优度高（R²>0.94）。所有树种采用完全统一的通用模型可行性不高，但气干密度接近的树种适宜采用同一模型（如椴树-白桦预测模型R²>0.95）。低温条件下，引入环境温度变量对树干含水率反演模型进行修正能够提高检测精度。研究结果可望为森林（尤其珍贵树林）精准经营、古树名木保护、城市行道树健康监测、木材高效加工与利用提供基础数据和技术支持。