Invasion Promotes Invasion: Facilitation Of C-3 Perennial Grass Dominance In Mixed C-3/C-4 Grassland By An Invasive C-3 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 C-4 mid-grass abundance. Prosopis glandulosa invasion has also facilitated growth of the C-3 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 C-4 grass recovery following P. glandulosa disturbance. We quantified C-3 and C-4 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 C-4 mid-grass recovery to only a few years. Nassella leucotricha dominated grass production during the first 3 years after top-kill. C-4 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 C-4 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 C-4 mid-grass recovery. The coupling of a deciduous, N-fixing C-3 woody species with this C-3 perennial grass may be a vegetative "state" that is resistant to multiple woody suppression disturbances and permanently limits the transition back to C-4 grassland.