Performance of johnsongrass and switchgrass from seeds and rhizome fragments in a mature switchgrass stand

Authors: SCHWINNING, SUSANNE - Texas State University; Fay, Philip; POLLEY, H. WAYNE - Retired ARS Employee

Submitted to: Plant Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/27/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: Johnsongrass (Sorghum halepense) is a widespread invasive grass species that can displace native species and establish as a weed in many crops. The invasiveness of Johnsongrass is increased by nitrogen fertilizers, but few studies have examined how nitrogen may interact with water availability at the crucial stage of initial plant establishment. This study addressed this gap by studying establishment of Johnsongrass in an established stand of switchgrass (Panicum virgatum), a native grass species planted for biomass feedstock production. Seeds and rhizomes of Johnsongrass and switchgrass were planted in switchgrass plots receiving either nitrogen fertilizer, supplemental irrigation, or both. Emergenc time, height growth and persistence of the plants emerging from these propagules was measured during the initial growing season. The study found that Johnsongrass emerged earlier than switchgrass, and that initial height growth and persistence to the end of the growing season of Johnsongrass recruits from rhizomes was increased by nitrogen fertilization. These benefits did not accrue to switchgrass recruits from seed or rhizomes. These findings extend previous greenhouse studies to realistic field conditions to show that early emergence of Johnsongrass, enhanced by nitrogen availability, is a key mechanism allowing this invasive grass to establish in stands of switchgrass.

Technical Abstract: Native plant communities persist by quashing the establishment of new arrivals, but this can change when disturbance and changes in the resource regime tip the competitive balance in favor of the newcomers. Although invasive traits are well characterized, their sway is rarely tested inside mature communities. In this experiment, we tested the ability of one of the most aggressive global invaders, Johnsongrass (Sorghum halepense L.), to establish inside a monoculture of a late-successional perennial bunchgrass stand (switchgrass; Panicum virgatum L.) in central Texas. Specifically, we monitored the growth and persistence of Johnsongrass and switchgrass seeds and rhizome fragments in a factorial experimental design with and without nitrogen fertilizer (NH4NO3) and supplementary irrigation. Johnsongrass emerged two weeks ahead of switchgrass, on average, with seeds and rhizomes of both species emerging at similar times. Emergence was not affected by irrigation because of above-average rainfall. Nitrogen fertilizer input increased the height of recruits across species and propagules, and most strongly in Johnsongrass rhizome plants. Johnsongrass persistence was strongly correlated with early emergence. The persistence of switchgrass recruits was not affected by emergence time but was decreased by fertilizer, suggesting that fertilizer favored mature switchgrass plants more than switchgrass recruits. In the end, no switchgrass and < 2% of Johnsongrass recruits reached the reproductive stage. Species traits, propagule type, resource inputs and characteristics of the resident community interact in complex ways to determine the fate of newcomers to the community and by extension, the community itself.