Fire and Blue Oaks
We see roadside fires burn woodlands every year, but how did past fires affect oaks? Recent research has changed the way ecologists and land managers view fire.
Blue oaks have several adaptations to withstand fire. Best known is an ability of small trees to readily resprout. A larger tree several hundred years old will have survived intact through at least a couple of fires. Until Professor Mitch McClaran, now of the University of Arizona, proved that many trees contained hidden fire scars, we neither knew how frequent past fires were, nor how often blue oaks recovered from moderate burn damage. Blue oaks show evidence of fire as scars in cat-faced trunks, but many more trees quickly heal over, concealing scars.
McClaran documented fires back as early as 1681 on two sites above the Yuba River by cross-dating trees with healed and open scars. Prior to 1850 fires occurred about every 25 years. After 1850 fire frequency increased to every 8 years, still lower than for many conifer-dominated types. Blue oak woodlands apparently do not show the 20th century decrease in fire frequency supported by work in coniferous forests. Because most of the trees in McClaran's sites regenerated within one year of a known fire, a rate much greater than expected by chance, fire played an important role in determining stand structure. Most stems apparently originated as sprouts, not acorns.
Fire undoubtedly plays a role in the complex process of stand renewal through natural regeneration. However, fire also can be thought of an eraser which periodically eliminates information by consuming old stems, rings, and scars. Fire's association with establishment of existing blue oak trees in Yuba County, which will be found at other locations, can be considered as either an expression of fire's positive force in renewal or as a negative force eliminating older stems. How fire assists natural regeneration or establishment, especially from acorns, is still unclear. Even the approaches to artificial regeneration, if the objective is to restore natural stand structure, will be heavily influenced by the fires of the past and perhaps the future.
prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc