Back to the roots: standardizing root length density terminology

Abstract The number of studies investigating root length has increased, particularly in the context of root length measurements observed through windows such as minirhizotrons and rhizoboxes. However, there are currently two obstacles constraining their broader utility: (1) the absence of standardized terminology or units for root length data, and (2) the translation from two-dimensional (2D) to three-dimensional (3D) data. Here, we delineate the fundamental disparities between root length measurements obtained from observation windows and via volumetric soil sampling and propose the adoption of more precise terminology to distinguish 2D planar (pRLD) from 3D volumetric (vRLD) root length density measurements. This differentiation should be accompanied with the use of standardized units and should not endeavour to make blanket conversions between dimensions unless this is supported by specific calibration data.