树木细根具有高度的结构和功能的异质性。近20年来, 人们逐渐认识到采用某一直径阈值来定义细根并开展研究的方法(即直径法)不能准确地描述相关的生理学和生态学过程。随着越来越多的研究证实根系功能与其分支等级密切相关, 根序划分的方法(即根序法)在根系研究中得到更多的应用。但是, 采用根序法测定相关功能属性需耗费大量的人力和时间。最近, 有研究者在结合直径法和根序法优点的基础上提出了功能划分的方法。该方法将传统的细根区分为吸收根和运输根, 在充分考虑根系结构和功能联系的同时, 又能兼顾研究工作的效率和结果间的可比性, 特别适用于根系生物量与周转的研究。采用功能划分方法来研究根系生物量(包括其他功能属性)是一个较新的观点, 研究结果的规律性和存在的主要问题仍有待总结。该文作者通过查阅近年来的相关研究报告发现吸收根与运输根生物量在树种间存在较大的差异, 目前对吸收根生物量在全球尺度上的变异格局并不清楚, 吸收根与运输根对细根生物量周转的相对贡献有待探究, 运输根在界定上存在着很大的挑战性。该文最后讨论了在根系研究中应用功能划分方法的优势与不足, 并提出了建议。

The morphology of fine root branching of woody plants is highly variable in their forms and functions. In the past two decades, researchers have increasingly recognized that the root-diameter-based method, using an arbitrary size of root diameter, failed to precisely characterize the physiological and ecological processes involved in finest roots. The number of publications using root-order-based approaches has increased regardless the fact that root trait-measurements based on root order are time-consuming and labor-intensive. A new approach—root functional classification method—was proposed and had been applied in the literature. The functional classification of fine roots separates roots of < 2 mm to absorptive and transport pools, making it more feasible for studies on root biomass and turnover. This new concept redefines fine root guild and has great potentials for future studies. Our literature review of the topic indicates that less is known about the inter-specific differences in estimates of biomass of absorptive and/or transport roots, with a large variation of absorptive roots on global scale. In addition, our review emphasizes the importance in: a) precision estimating of the absorptive biomass of fine roots, and b) proper definition of the range of the transport roots within and among forest ecosystems. Finally, after compare the strengths and weaknesses of the functional classification method, we propose several specific suggestions to improve the applications of this approach.