Fire Behavior In Southern California Chaparral Before Fire Control: The Mount Wilson Burns at the Turn of the Century

Reconstruction from written accounts of three chaparral burns (1896, 1898, 1900) before fire control on Mount Wilson in the San Gabriel Mountains, California reveals an erratic smoulder-and-run fire behavior pattern. Although these burns persisted for two to three months during summer drought, their ultimate sizes were relatively small. Chaparral in southern California watersheds before 1900 was described as having been fragmented by previous burns. Such patchy stand structure can develop from frequent anthropogenic ignitions because the growth of fires is constrained by previous burns; fire size is inversely related to ignition rates. Season-long burns from relatively few ignitions can also result in complex vegetation fragmentation owing to irregular fire behavior and to development of secondary burns from smouldering or by movement of embers long distances beyond the fire zone. Lightning alone could be responsible for widespread fire occurrence because ignitions can be stored in large fuels until the advent of dry weather and because lightning ignitions are frequent relative to the maturation period of stands. Incipient suppression of small fires modified the presuppression fragmented chaparral in the San Gabriel Mountains. In 1919 and 1924, after two decades of fire suppression, enormous high-intensity conflagrations were driven largely by strong Santa Ana winds, and this pattern has continued to the present.