Nitrogen Fixing Symbionts: Legume Survival and Coexistence in Warming California Grasslands

Session: (In-Person) Oral Panel—Fuel, Fire, Grass and Compost

For the past 200 years, non-native species have been anthropogenically introduced in California grasslands, reducing native species abundance and diversity. Trifolium hirtum is a non-native legume commonly used for cattle fodder and cover cropping because of its ability to fix nitrogen via rhizobial symbionts. T. hirtum currently coexists with California native legume Trifolium willdenovii in established communities, however, T. hirtum has the potential to outcompete T. willdenovii under increased environmental stress, such as that caused by climate change. Considering how symbiotic rhizobia contribute to these Trifolium species’ ability to coexist or compete with other species, as well as how climate change alters these interactions, is essential for understanding potential impacts on native biodiversity and how important forage species react to changing climate conditions.

I tested 1) how rhizobial symbionts contributed to the ability of a native and a non-native legume to coexist with each other and other species in the community. And 2) how drought influences competitive relationships between the two legumes. For both non-native T. hirtum and native T. willdenovii, I measured rhizobia nodule mass, seeded background count, weed species counts, and weighed the aboveground biomass on individuals grown under drought and ambient precipitation treatments. On average, there were no differences in the mass of rhizobia nodules by species, however there was an observed correlation between the biomass and nodule mass of uninoculated background T. willdenovii. Drought positively impacted the biomass of background T. hirtum individuals, however drought treatments negatively affected focal T. hirtum biomass compared to ambient precipitation. Drought had no significant effect on focal T. willdenovii biomass.