Resilience of California Black Oak Experiencing Frequent Fire: Regeneration Following Two Large Wildfires 12 Years Apart

Abstract

Historically, oak woodlands in western North America were maintained by frequent fire that killed competing conifers. Today, these woodlands are often in decline as competition from conifers intensifies. Among oak species affected is the ecologically important California black oak (Quercus kelloggii Newberry). Within its range, large high-severity wildfires have become more common. We examined responses of black oak to two mixed-severity wildfires 12 years apart (2000 Storrie Fire and 2012 Chips Fire reburn). Regeneration was examined in relation to fire severity as measured by the Relative differenced Normalized Burn Ratio (RdNBR). We found that the RdNBR of the Storrie Fire was a useful predictor of the RdNBR of the Chips Fire reburn, suggesting that effects of the Storrie Fire on vegetation influenced fire behavior in the Chips reburn (t12 = 2.892, P = 0.014, n =14, r = 0.641). After experiencing a second top-kill (complete cambium mortality above the root collar) in the Chips reburn, 95 % (99 of 104) of black oak sprout clumps resprouted. Twelve years after the Storrie Fire, we found lower proportions of sprout clumps produced acorns in places where more overstory trees survived the fire (t5 = −3.023, P = 0.029, n = 7, r = 0.804). After both fires, the crown volume of entire sprout clumps was lower in areas of higher live overstory tree basal area (Storrie Fire: t5 = 2.527, P = 0.053, n = 7, r = 0.749; Chips reburn: t5 = −3.597, P = 0.016, n = 7, r = 0.849). Our finding that most black oak survived successive top-kill from repeat fire suggested that repeated high-severity or mixed-severity fire is not an immediate threat to black oak presence on the landscape. Our findings also suggest that, if high-severity fire is followed by historically normal fire return intervals, rapidly regenerating black oak may be favored over conifers and recover from decline.