Abstract
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.
Highlights
Characterizing plant functional diversity is essential to understand community assembly rules (Bruelheide et al 2018) and ecosystem functioning (Zirbel et al 2017)
Most communities showed a multimodal distribution, with two main modes corresponding to absorptive roots and transport roots, and a third mode of lower importance corresponding to large transport roots
The second mode corresponded to larger roots ( M2 = 0.54 ± 0.21 mm), included most of the remaining proportion of root length ( P2 = 0.2 ± 0.2) and was wider ( SD2 = 0.17 ± 0.07 mm)
Summary
Characterizing plant functional diversity is essential to understand community assembly rules (Bruelheide et al 2018) and ecosystem functioning (Zirbel et al 2017). One reason for that lies in the methodological difficulties to identify and separate roots in plant communities, in order to measure root traits at the individual level (Freschet et al 2021a). Fine roots (< 2 mm) diameter play a key role in plant (Ryser and Lambers 1995, Comas and Eissenstat 2004, Gregory 2006) and ecosystem functioning (Wardle et al 1998, Birouste et al 2012) due to their multifunctionality (Weemstra et al 2016, Laliberté 2017, Freschet et al 2021a) They have a hierarchical organization that consists in different root orders with varying diameters and functions. Several studies have attempted to define metrics characterizing root diameter distribution at the individual level, and considered either the whole distribution (Caplan et al 2019) or the various modes of the distribution (Anderson et al 2007) in terms of shape, notably mean (M), standard deviation (SD) and relative proportions (P) of individual modes (Anderson et al 2007; Fig. 1)
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