Effect of Fuel Load, Fire Season, and Fire Temperature Duration on Honey Mesquite Seedling Mortality

Fire-resistant woody species have encroached into grassland and rangeland ecosystems worldwide. Sapling and mature individuals of many of these species are fire resistant because they resprout following fire; however, seedlings may be vulnerable to fire. We quantified fire temperature and flame length of winter and summer prescribed fires in two grass fuel types (native mixed-grass or tallgrass) and their effects on honey mesquite ( Prosopis glandulosa ) seedling mortality in the Southern Great Plains (SGP), US. Mesquite seeds were planted in replicated plots in both grass fuel types in each of two consecutive years. Plots were burned in the winter when seedlings were 10 or 22 mo old, or in the summer when they were 17 mo old. Fire temperature was measured at 1-second intervals at −1 cm, 0 cm, 10 cm, 30 cm, and 1 m heights. Flame length and fire temperature duration in seconds >60°C (FTD60) at 0 cm (soil surface) were better predictors of seedling mortality than was peak fire temperature. Highest seedling mortality was achieved when FTD60 at 0 cm was >2 minutes. We suggest these results modify the “60°C for 1 minute” hypothesis when applied to a population of seedlings exposed to rapidly moving prescribed headfires. Averaged over both cohorts, seedling mortality was greatest in tallgrass/summer fires (84.0%), intermediate in tallgrass/winter (69.2%) and mixed-grass/summer (68.4%) fires, and lowest in mixed-grass/winter fires (40.8%). Results suggest that summer fires or winter fires with high fuel loads are needed to achieve high seedling mortality. Winter fire effects in mixed-grass were diminished by green tissue in C 3 Texas wintergrass ( Nassella leucotricha ). Treatments with greatest mortality likely mimic fire-season and grass fuel conditions that existed prior to European settlement of the SGP and limited woody plant expansion.