Pyric‐based restoration of C4 grasses in woody (Prosopis glandulosa) encroached grassland is best with an alternating seasonal fire regime

Abstract

Prescribed fire is a management strategy for limiting or reducing woody plant invasion on grasslands and savannas. However, in dense mature woody stands several fires may be necessary to overcome woody dominance and facilitate grassland restoration. Here we determined if a concentrated series of fires could stimulate recovery of C4 perennial grasses in Southern Great Plains, U.S.A., grasslands that had become dominated by a fire tolerant woody legume, honey mesquite (Prosopis glandulosa). Our study included six treatments: three winter fires in 5 years (3WF), three alternating season winter‐summer‐winter fires in 4 years (3AF), three alternating season fires in a more compressed (extreme) timeframe (3AFX), two summer fires in 3 years (2SF), two summer fires in consecutive years (2SFX), and a no fire “Control.” Cattle grazing was excluded during the study. Treatments with summer fire were most effective at top‐killing mesquite, but no treatments root‐killed >5%. All top‐killed mesquite resprouted. Only the treatments that alternated fire seasons (3AF and 3AFX) increased C4 mid‐grass cover, most from stoloniferous vine mesquite (Panicum obtusum) and not C4 bunchgrasses. The 2SF treatment increased C4 short‐grass cover but not C4 mid‐grass cover. Because none of the treatments root‐killed mesquite, grass recovery declined as woody regrowth increased. Drought also contributed to the decline. Without additional fire disturbances to suppress mesquite regrowth or anthropogenic inputs to root‐kill mesquite, transition from woodland to savanna or grassland stable states was not possible.