Annual Grassland Response to Fire Retardant and Wildfire

Abstract

Diammonium phosphate (DAP), air-dropped in early autumn 1974 to contain a wildfire on the San Joaquin Experimental Range in California, applied high levels of nitrogen and phosphorus to foothill annual grassland. The DAP drop and fire provided 4 treatments for the study-unburned + DAP, burned + DAP, burned and unburned (control). In the first year both of the DAP treatments, with yields of more than 12,000 kg/ha produced twice that of the unburned (control). First-year forage yields for the unburned and burned plots were not significantly different. The second year the burned plot yielded almost twice that of the unburned. The second year, the unburned + DAP plot produced about 4200 kg/ha, the highest yield of all 4 plots, and significantly higher than the burned + DAP plots. Annual and seasonal weather patterns and soil moisture affected herbage composition more than treatments. Although forbs usually increase in annual grassland after fire, and nitrogen fertilizer favors grasses, grasses nonetheless dominated on all 4 treatments in the first year. Forbs were dominant the second year. The difference in relative percent composition of grasses and forbs was greater between years than between treatments.