Restoration of C4 grasses with seasonal fires in a C3/C4 grassland invaded by Prosopis glandulosa, a fire-resistant shrub

Questions: Can prescribed fire restore C-4 perennial grasses in grassland ecosystems that have become dominated by fire-resistant C-3 shrubs (Prosopis glandulosa) and C-3 grasses? Do fires in different seasons alter the direction of change in grass composition? Location: Texas, USA. Methods: We quantified short-and long-term (12 yr post-fire) herbaceous functional group cover and diversity responses to replicated seasonal fire treatments: (1) repeated-winter fires (three in 5 yr), (2) repeated-summer fires (two in 3 yr), and (3) alternate-season fires (two winter and one summer in 4 yr), compared with a no-fire control. Results: Summer fires were more intense than winter fires, but all fire treatments temporarily decreased Prosopis and C-3 annual grass cover. The alternate-season fire treatment caused a long-term increase in C-4 mid-grass cover and functional group diversity. The repeated-summer fire treatment increased C-4 short-grass cover but also caused a long-term increase in bare ground. The repeated winter fire treatment had no long-term effects on perennial grass cover. Mesquite post-fire regrowth had increasingly negative impacts on herbaceous cover in all fire treatments. Conclusions: Summer fire was necessary to shift herbaceous composition toward C-4 mid-grasses. However, the repeated-summer fire treatment may have been too extreme and caused post-fire herbaceous composition to "over-shift" toward less productive C-4 short-grasses rather than C-4 mid-grasses. This study provides some of the first long-term data showing a possible benefit of mixing seasonal fires (i.e., the alternate-season fire treatment) in a prescribed burning management plan to restore C-4 mid-grass cover and enhance overall herbaceous diversity.