LEAF ECONOMIC TRAITS FROM FOSSILS SUPPORT A WEEDY ORIGIN FOR ANGIOSPERMS

Many key aspects of early angiosperms are poorly known, including their ecophysiology and associated habitats. Two competing hypotheses are that the earliest angiosperms grew slowly in dark forest understories or that they grew quickly in sunny riparian zones. Using a power law between petiole width and leaf mass, we estimate the leaf mass per area (LMA) of species from three Albian (110-105 Ma) fossil floras from North America (Winthrop, Patapsco, and Aspen Shale). All LMA’s for angiosperm species are low (<125 g/m²; mean = 76 g/m²) but are high for gymnosperm species (>240 g/m²; mean = 291 g/m²). Based on extant relationships between LMA and other leaf economic traits such as photosynthetic rate and leaf lifespan, we conclude that these Early Cretaceous landscapes were populated with weedy angiosperms with short-lived leaves (<12 months). The unrivalled capacity for fast growth observed today in many angiosperms was in place by no later than the Albian and likely played an important role for their subsequent ecological success.