Late Eocene ring-porous wood signals monsoon seasonality and the rise of deciduousness in East Asia

The Cenozoic development of the monsoon system in Asia drove major biome shifts, however early plant responses to its seasonality remain obscure. Ring-porosity, a functional trait of woody angiosperms associated with deciduousness and seasonal water demand, provides a profound insight into how xylem formation was influenced by monsoon-driven precipitation seasonality. Here, we report the oldest known Cenozoic ring-porous wood in tropical Asia, Parasalicaceoxylon naduongensis Nguyen et Oskolski gen. et sp. nov. (Salicaceae) from the upper Eocene of the Na Duong Basin, northern Vietnam. The fossil wood exhibits distinctive ring porosity, indicating deciduous phenology and seasonal growth in a tropical environment. HadCM3BL paleoclimate simulations reveal strong precipitation seasonality driven by latitudinal migration of the Intertropical Convergence Zone (ITCZ), consistent with an ITCZ monsoon affecting northern Vietnam during the late Eocene. Combined with extant and fossil ring-porous wood records from Asia, this suggests that evolution of deciduousness and ring porosity in East Asia was driven by monsoon seasonality.