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

Prescribed fire is used to reduce woody plant invasion and restore herbaceous production and diversity in grasslands and savannas worldwide. Here we determined if 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-year span, and maintenance fires on a portion of each plot 7 or 8 years 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 towards 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 to 10 years 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 to 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.Measurements of fire, plant and soil responses in remote field study sit