Two dimensions define the variation of fine root traits across plant communities under the joint influence of ecological succession and annual mowing

1. Quantifying the variation in community-level fine root (<2 mm) traits along ecological gradients or in response to disturbances is essential to unravel the mechanisms of plant community assembly, but available surveys are scarce. Whether fine root traits covary along a one-dimensional economic spectrum, as previously shown for leaves, is highly debated. 2. We measured six fine root traits at the community level along a 69-year succession, with or without annual mowing, offering a unique design of two nested disturbances. We examined whether (1) there is variation and covariation in community-level fine root traits along the succession and in response to mowing and (2) morphological root traits mirrored analogous leaf traits (using previously acquired data). 3. Early-successional communities were herbaceous-dominated (48 +/- 6% in <10-year-old plots) and possessed fine roots with high specific root length (SRL), low root dry matter content (RDMC) and low root carbon concentration (RCC), while later successional communities were dominated by woody species (56 +/- 9% in >40-year-old plots) and possessed opposite trait values. Root nitrogen concentration (RNC) did not vary across communities along the succession. The trait values at community level were not affected by mowing, except for a reduction in root mass density. 4. We found covariation of fine root traits across communities along two dimensions: the first dimension (60% of total variation) represented changes in root foraging capacity (related to SRL) and resource conservation (related to RDMC, RCC, mean root diameter), whereas the second dimension (17%-20% of the variation) represented variations in RNC, potentially related to root respiration and metabolism. 5. Specific root length and specific leaf area were correlated regardless of the mowing regime, but there was no analogous relationship between leaf dry matter content and RDMC in mown communities, showing a decoupling in the investment in tissue density above- and below-ground. 6. Synthesis. Our study demonstrates coordinated variations of community-level fine root traits along a succession gradient and provides evidence that fine root traits covaried along two dimensions, regardless of mowing regime. The relationship between leaf dry matter content and root dry matter content observed in unmown communities was modified by mowing, reflecting an uncoupled response to mowing.