Rationale: Changes in fine-root morphology are typically associated with transitions from the ancestral arbuscular mycorrhizal (AM) to the alternative ectomycorrhizal (ECM) or non-mycorrhizal (NM) associations. However, the modifications in root morphology may also coincide with new modifications in leaf hydraulics and growth habit during angiosperm diversification. These hypotheses have not been evaluated concurrently, which limits our understanding of the causes of fine-root evolution.

Methods: To explore the evolution of fine-root systems, we assembled a 600+ species database to reconstruct historical changes in seed plants over time. We utilize ancestral reconstruction approaches together with phylogenetically informed comparative analyses to test whether changes in fine-root traits were most strongly associated with mycorrhizal affiliation, leaf hydraulics or growth form.

Key Results: Our findings show significant shifts in root diameter, specific root length and root tissue density as angiosperms diversified, largely independent from leaf changes or mycorrhizal affiliation. Growth form was the only factor associated with fine-root traits in statistical models including mycorrhizal association and leaf venation, suggesting substantial modifications in fine-root morphology during transitions from woody to non-woody habits.

Conclusion: Divergences in fine-root systems were crucial in the evolution of seed plant lineages, with important implications for ecological processes in terrestrial ecosystems.

In the case of leaf vein density, we compiled information from Boyce et al. 2009 (257 species); Brodribb & Feild 2010 (272 species); Walls 2011 (87 species) and Li et al. 2015 (80 species, see Notes 1 for a detailed list). Because “vein density” is defined as the average distance between veins (usually secondary and higher orders) within the mesophyll (Brodribb, Feild & Sack 2010), taxa with single-veined leaves like gymnosperms or parallel venation like most monocots were under-represented in this dataset. In the case of root traits, we used data described in Valverde-Barrantes et al. (2017) for diameter (D, mm), SRL (g/m), and RTD (g/cm³) deposited in the Fine Root Ecological Database (FRED; Iversen et al. 2017, 2018). Habit classification was obtained directly from the FRED database dividing the species between woody shrubs, trees and non-woody herbs. Mycorrhizal state was binned into four mycorrhizal groups based on the classification in Maherali et al. (2016): AM, ECM, transitional (AMNM) and non-mycorrhizal (NM) based on consensus classification from previous studies. Species reported as AM or NM in different studies were binned in the AMNM transitional stage (Maherali et al. 2016).