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

Abstract

Abstract 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.