Wood Functional Disparity Lags Behind Taxonomic Diversification in Angiosperms

Early angiosperm fossil record shows a remarkably low wood structural diversity relative to that exhibited by reproductive structures. As wood anatomical traits are related to plant function, paucity of early wood structural differentiation suggests a delay of functional diversification relative to the explosive taxonomic accumulation of the group. We assembled a wood anatomy dataset of over 1000 extant species from Argentina, Brazil, China, Mexico, USA and Suriname to determine the disparification patterns of the cell types associated with the three main wood functional axes: conduction, support and storage. We found that most traits had a temporal shift in their evolutionary patterns, with early conservatism followed by high lability towards the present. Our results indicate that a surge in functional wood traits lability took place after the initial taxonomic diversification in the early Cretaceous and support the observed low wood structural diversity observed in the early angiosperm fossil record. Increased trait lability likely allowed evolutionary flexibility by lifting developmental constraints and boosted the evolution of different cell types involved in multiple wood functions and their associated ecological role. The diversification in wood function coincides with the escalation of angiosperm tree size and leaf hydraulic capacity and likely contributed to the subsequent ecological expansion of the angiosperms.