Invasion promotes invasion: Facilitation of C₃ perennial grass dominance in mixed C₃/C₄ grassland by an invasive C₃ woody sprouter (Prosopis glandulosa)

In the Southern Great Plains (SGP) of the United States, encroachment of the native invasive woody legume, honey mesquite (Prosopis glandulosa Torr.), has caused a decline in C₄ mid-grass abundance. Prosopis glandulosa invasion has also facilitated growth of the C₃ mid-grass species, Texas wintergrass (Nassella leucotricha [Trin & Rupr.] Pohl) initially beneath its canopy but extending to interspaces between P. glandulosa as stand density increases. Little is known about the stability of the Prosopis/Nassella association or C₄ grass recovery following P. glandulosa disturbance.We quantified C₃ and C₄ grass production in interspaces, and basal cover in interspaces and P. glandulosa subcanopy microsites for 9 years following P. glandulosa suppression (top-kill) and compared this to untreated P. glandulosa woodland (woodland).The Prosopis/Nassella association limited the window of C₄ mid-grass recovery to only a few years. Nassella leucotricha dominated grass production during the first 3 years after top-kill. C₄ mid-grass recovery began in year 4, but was interrupted by severe drought in years 5 through 7. Recovery resumed in year 8, due to above-average summer rainfall, but P. glandulosa regrowth was large enough by this time to limit C₄ mid-grass production to a third of its potential. Nassella leucotricha basal cover remained dominant and stable in woodland subcanopy microsites, even during drought, and only briefly declined in top-kill subcanopy microsites before returning to pretreatment levels by year 8 as P. glandulosa regrowth increased and provided shade. Synthesis and applications. A single suppression event had little impact on disrupting the Prosopis/Nassella association and allowing C₄ mid-grass recovery. The coupling of a deciduous, N-fixing C₃ woody species with this C₃ perennial grass may be a vegetative "state" that is resistant to multiple woody suppression disturbances and permanently limits the transition back to C₄ grassland.