Can prescribed fires restore C4 grasslands invaded by a C3 woody species and a co‐dominant C3 grass species?

Abstract

AbstractPrescribed fire is used to reduce woody plant invasion and restore herbaceous production and diversity in grasslands and savannas worldwide. Here, we determined whether a concentrated series of repeated‐winter, repeated‐summer, or alternate‐season (winter and summer) fires in a short timeframe (“transition fires”) could catalyze the restoration of C4 perennial grasses in Southern Great Plains, USA grasslands that had become dominated by a fire‐tolerant C3 woody N2‐fixer (honey mesquite, Prosopis glandulosa) and a C3 perennial bunchgrass (Texas wintergrass, Nassella leucotricha). We applied transition fires over a 5‐yr span and maintenance fires on a portion of each plot 7 or 8 yr later. We measured herbaceous standing biomass and cover and soil variables (soil organic C, N, δ13C, and δ15N) in unburned, transition‐burned, and maintenance‐burned treatments. Greater δ13C at 10–20 (−17‰) than 0–10 (−20‰) cm depth increment confirmed that vegetation was historically mostly C4 grassland that shifted toward C3 dominance. Transition treatments with summer fire were most effective at top‐killing mesquite, but no treatments root‐killed >3%. Regrowth of top‐killed mesquite was similar in all treatments and reached pre‐fire height by 9–10 yr post‐fire. Herbaceous production and cover responses showed that: (1) Alternate‐season transition fires increased C4 mid‐grass, but did not change Texas wintergrass; (2) repeated‐summer fires reduced Texas wintergrass, but did not change C4 mid‐grass; and (3) repeated‐winter fires did not change C4 mid‐grass or Texas wintergrass compared with the unburned control. All maintenance fires stimulated Texas wintergrass biomass and cover, thus eliminating the reduction of Texas wintergrass caused by repeated‐summer transition fires. There were no long‐term effects of transition fires on soil C, N, δ13C, or δ15N. Results advance our understanding of the expectations and limitations of prescribed fire in shifting a woodland alternate state toward what was historically a fire supported C4 grassland/savanna.