Season of burn effects on mortality, forage production and plant functional group omposition in gulf cordgrass vegetation communities

Gulf cordgrass (Spartina spartinae [Trin.] Merr. ex Hitchc.) is a productive, warm season, perennial bunchgrass. It can provide valuable forage for livestock when young, particularly when other forage may be scarce. However, a problem arises when Gulf cordgrass becomes mature: leaves and stems are very coarse and low in palatability and nutritive value for livestock and wildlife. Removing mature growth encourages production of tender, palatable shoots, and improves overall nutritive value. Fire is the most economical management tool to meet these objectives and is the natural process that kept these Gulf cordgrass communities in prime condition historically. In this study I applied prescribed burning at the pasture scale in grassland communities of coastal prairies and marshes dominated by either Gulf cordgrass or seacoast bluestem (Schizachyrium scoparium var. littorale [Nash] Bickn.). I burned two 200-ha pastures each winter and summer beginning in winter 2016 for a total of 8 burn and 2 control (no burn) pastures. My main objective was to determine the optimal season of burning by comparing winter and summer season burns in Gulf cordgrass communities of the Gulf Prairies and Marshes ecoregion of Texas. There was no difference in fire temperature between summer and winter burns. Gulf cordgrass plant mortality was higher in burn treatments compared to control treatments. However, there was no difference in Gulf cordgrass plant mortality between winter or summer burn treatments. There was a strong positive relationship between plant mortality and peak fire temperature, and between plant mortality and duration of heat over 65°C. Forage standing crop growth models were unique for each season; estimated regression coefficients were positive in each model in Gulf cordgrass vegetative groups, indicating that forage standing crop increased as days after burning progressed. Forage regrowth showed similar patterns following winter and summer burning for Gulf cordgrass, grasses other than Gulf cordgrass, or total forage standing crop. Forage production for approximately 90 days following burning did not differ between the four burns regardless of season, or between winter and summer burning. The removal of litter and excess growth of mature Gulf cordgrass by prescribed burning allowed native warm season forbs and native sub-shrubs to increase in their relative abundance of forage standing crop compared to control patches. A Non-metric multidimensional scaling ordination (NMDS) analyses showed there was more variation in functional group composition of burned Gulf cordgrass communities than non-burned communities when functional group composition was based on forage standing crop than when it was based on plant density. Burning Gulf cordgrass communities, regardless of season, enhanced production of palatable forage for livestock and created changes in functional group composition. This resulted in greater structural and species heterogeneity, additionally, creating more varied winter and summer habitat for a variety of wildlife species