Invasive annual grasses show decrease in seed size but no change in growth or carbon economy following invasion

The invasion of annual grasses into historically perennial-dominated systems is a global biodiversity threat. While significant, it is not known how evolutionary changes in functional traits during the invasion process may contribute to invader success. We examined how functional traits of a widely distributed invasive annual grass medusahead ( Taeniatherum caput-medusae (L.) Nevski) differed across native-range and invaded-range populations and compared these trait values to those of a common native perennial, squirreltail ( Elymus elymoides (Raf.) Swezey). In greenhouse and growth chamber studies we screened a range of plant, seed and seedling functional traits and determined how variation in these traits contributed to variation in plant biomass. Our results show medusahead maintains a higher relative growth rate (RGR) than squirreltail, but the mechanism differed depending on origin with native-range medusahead achieving a higher RGR through a higher specific leaf area and the invaded-range medusahead achieving a higher RGR though a higher net assimilation rate. Contrary to previous literature, we did not find that medusahead evolved traits allowing larger biomass in its invaded range. Instead, native-range medusahead populations grew bigger than invaded-range populations, primarily due to differences in seed size. While counter to expectations, smaller seed mass may provide invaded-range medusahead populations an advantage in terms of dispersal and higher seed bank density compared to native-range populations. Thus, life history differences, altered disturbance regimes, and a shift towards production of smaller but a greater number of seeds may act, in combination, to facilitate invasion of annual grasses into perennial dominated systems.