Characterizing plant functional diversity is essential to decipher community assembly rules and ecosystem functioning. Most studies focused on above-ground traits whereas the analysis of root diversity lags far behind. We analyzed the structure of fine root (< 2 mm) diameter distribution at the community level as an indicator of root morphological diversity, and hypothesized that the shape of the distribution provide insights on root types and root exploration strategies. We tested this hypothesis along a successional gradient (6–69 year-old) with yearly mowing to better understand assembly rules regarding to belowground processes, and explored the relations between the parameters describing its modes and the vegetation composition and ecological properties of plant communities. Most communities showed a multimodal distribution, with two main modes corresponding to absorptive roots (thinner root mode) and transport roots (coarser root mode), and a third mode of lower importance corresponding to large transport roots. In early succession, the thinnest root mode was prominent, reflecting the dominance of thin absorptive roots and a low proportion of transport roots, resulting in a low root morphological diversity. As succession proceeds, the relative proportion of the second mode increased, and the proportions of the two main modes were more balanced, resulting in an increased variance and root morphological diversity. Furthermore, the first root mode (absorptive roots) became wider and shifted from very thin to thicker roots, suggesting the coexistence of various root strategies for resource exploration. Yearly mowing did not affect root diameter distribution, which may relate to the relative low mowing pressure that enabled woody species to remain, with stunted stature. Overall, our study demonstrates that the distribution of fine root diameters sheds light on root morphological diversity at the community level, and provide hints on the co-existence of root types and strategies for resource use and exploration.