Shifts in Weed Diversity in Response to Management Practices and Elevated CO ₂ in Forage Bermudagrass ( <i>Cynodon dactylon</i> )

Abstract High quality bermudagrass ( Cynodon dactylon [L.] Pers.) forage production is a vital component of Southeastern U.S. agriculture. Previous research indicates that environmental stresses like elevated CO 2 may significantly affect crop productivity and weed competitiveness and ecological shifts. Although the effects of elevated CO 2 on C 4 plants like bermudagrass could be marginal, the growth, vigor, and herbicide tolerance of C 3 weeds can be affected profoundly. In the final two years of a seven-year study investigating the effects of management practices and elevated CO 2 levels on bermudagrass forage production, we aimed to evaluate how these treatments impacted weed competition and diversity. From 2018 to 2025, bermudagrass was grown in a coarse-textured soil bin at the USDA-ARS National Soil Dynamics Laboratory in Auburn, AL. Open-top chambers delivered either ambient or elevated CO 2 (+ 200 mg kg -1 ), and plots were either managed annually with fertilizer and herbicide or left unmanaged. In the final two years, elevated CO 2 had no significant effect on total biomass production (bermudagrass + weeds), but in unmanaged plots, elevated CO 2 resulted in a significantly greater proportion of weeds. While some C 4 grasses and sedges were observed in this experiment, most weed species were C 3 . Consequently, C 3 species dominance was generally high, especially in managed plots exposed to elevated CO 2 . Weeds were observed and identified in all plots, but those managed with fertilizer and herbicide had a greater proportion of bermudagrass plants to weeds, as well as lower weed densities. Species diversity indices yielded significantly greater species richness under elevated CO 2 conditions. Moreover, we observed greater weed diversity with elevated CO 2 , which was exacerbated without proper nutrient and weed management. This provides compelling evidence that substantial shifts in weed diversity could occur due to environmental change factors like elevated CO 2 and the lack of a proper crop management program.