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

Authors: MALMBERG, COURTNEY - California Polytechnic State University; Sheley, Roger; JAMES, JEREMY - California Polytechnic State University

Submitted to: Biological Invasions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/4/2023
Publication Date: 1/12/2023
Citation: Malmberg, C., Sheley, R.L., James, J. 2023. Invasive annual grasses show decrease in seed size but no change in growth or carbon economy following invasion. Biological Invasions. 25:1613-1625. https://doi.org/10.1007/s10530-023-02999-4.
DOI: https://doi.org/10.1007/s10530-023-02999-4

Interpretive Summary: Invasion of annual grasses into historically perennial-dominated systems is a global biodiversity threat. Understanding those plant traits that allow annual grasses to dominate rangeland is central to developing strategies to minimize invasion. We found that medsuahead, an invasive annual grass, grows faster than bottlebrush squirreltail, an important native grass. We also found that medusahead from invaded-range produced less biomass than medusahead from its home-range, primarily because home-grown medusahead produced larger seeds. However, medusahead from the invaded-range allocated more carbohydrates to the production of many small seeds than those collected from its native range. We believe the production of many small seeds facilitates more rapid invasion and seed bank domination in newly invaded rangeland.

Technical Abstract: 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.