Divergent intra- and interspecific root order variability identifies a two-dimensional root economics spectrum

Background and Aims: The form-function linkages and variation of fine root traits reflect adaptive strategies to cope with complex soil environments. However, their contributions to the root economics spectrum (RES) remain unclear. Methods: We measured thirteen functional traits in the first four root orders of 59 subtropical woody species, including four morphology functional traits, three chemical functional traits, and six anatomical functional traits. Results: A multi-dimensional RES was observed among the different order roots, including two trade-off axes, one represented by root diameter (RD) and specific root length (SRL) and another represented by root tissue density (RTD) and root nitrogen content (RNC). As the root orders increased, the root function transitioned from nutrient uptake (1st-3rd orders) to resource transport and storage (4th order). The hub traits changed accordingly. The intraspecific variation among root orders was along the RD-SRL axis, whereas the interspecific variation among the root orders was along the RTD-RNC axis in the RES. Furthermore, the data pertaining to plant life history strategies (e.g., leaf size and leaf nitrogen) had effects on the multi-dimensional RES variation. Conclusions: Collectively, a multi-dimensional RES reveals intra- and interspecific variation characteristics in the fine root system. These findings provide empirical data underpinning a theoretical basis for understanding fine root form-function linkages.