Grégoire T. Freschet, Catherine Roumet
The long and complex evolution of plant roots, and their occurrence in multiple environmental conditions, have shaped a large diversity of forms and functions among plant species. With the growing recognition that roots constitute a critical component of ecosystem functioning, an increasingly large research community has begun to unravel the complex relationships between root morphology, chemistry, physiology and functions. As such, the field of root ecology is rapidly moving away from the traditional representation of fine roots as homogeneous entities responsible for anchorage and nutrient acquisition to embrace the idea of spatially and temporally changing organs performing many contrasting plant and ecosystem functions.
The multiple functions of roots imply some degree of specialization within a root system. Aboveground, the function of light interception relies on plant traits related to the size, architecture and morphology of stems and leaves, but the photosynthesis function mostly relates to leaf parts. Similarly belowground, the soil exploration function relies on characteristics of the entire root system, whereas nutrient uptake is generally performed by thin root segments only. As such, adequately characterizing root functions requires estimating which parts of the root system are most particularly involved in these functions. In addition, the changing nature (and function) of roots while aging, and the puzzling plasticity of roots to the multiple influences they encounter in soil, makes the adequate measurement of root functions particularly challenging.
Our review suggests that all current root sampling categories present both advantages and pitfalls, and that no single method can appropriately tackle the main current challenge of root functional ecology, that is, linking fine-roots to plant and ecosystem functions in a truly comparable way across all plant species. We argue instead that a small set of complementary sampling methods is necessary to fully capture the linkages between root forms and functions.