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

Abstract

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.