Independent evolutionary changes in fine-root traits among main clades during the diversification of seed plants

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.