Invasion promotes invasion: facilitation of C3 perennial grass dominance

1. In the southern Great Plains (SGP) of the USA, 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. 2. 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).  3. 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.  4. 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 pre-treatment levels by year 8 as P. glandulosa regrowth
increased and provided shade. 5. 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.