The effects of native conifer encroachment and importance of high-severity wildfire in fire-excluded California black oak ecosystems of northern California

The absence of fire in many oak woodlands that historically experienced frequent fire has resulted in invasion and subsequent overtopping of oaks by fast-growing conifers. Little is known about the effects of these structural and compositional changes occurring in California black oak (Quercus kelloggii) woodlands. This study addresses two broad questions: 1) How will conifer-encroached California black oaks respond to re-introduced fire in areas that have gone unburned for many decades? And 2) what actions can forest managers take toward California black oak woodland restoration and maintenance over time? Tree competition around individual California black oaks, tree age, and California black oak post-fire responses were investigated in Klamath and Lassen National Forests in northern California. Plots were established around focal oaks where attributes of neighboring encroaching trees, tree regeneration, and fire effects were measured. At both sites woodland overstory was heavily dominated by relatively fire-intolerant conifers, particularly Douglas-fir (Pseudotsuga menziesii) and white fir (Abies concolor). At the Klamath site, trees that pierced California black oak crowns were younger than paired oaks. The probability of California black oak mortality from fire was correlated with neighboring Douglas-fir height, indicating a compromising effect of encroachment on oak survival. At the Lassen site 90% of killed California black oak stems sprouted following fire. Oak recovery was strongest in severely burned areas; linear modeling revealed significant negative relationships between overstory tree survival and both California black oak sprout height and basal area. A conceptual model for the formation and persistence of these specific stand structures is proposed in which fire severity and encroachment pressure affect compositional change over time. Unless specific management actions are taken to protect California black oak woodlands, many encroached stands may be converted to conifer forest.