THE PERSISTENCE OF OAK WOODLANDS IN ALTERED FIRE REGIMES OF THE PACIFIC NORTHWEST
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Pacific Northwest oak woodlands and savannas are fire-resilient communities dependent on frequent, low-severity fire to maintain their structure and understory species diversity, and to prevent encroachment by fire-sensitive competitors. These important ecosystems have been severely reduced in both extent and quality during more than a century of land use change and altered fire regimes. The re-introduction of fire into these transformed ecosystems is viewed as essential to their restoration, yet can be fraught with potential unintended consequences. We examined oak response following re-introduction of fire into two distinct oak ecosystems: formerly suppressed California black oak (Quercus kelloggii Newb.) woodlands subject to repeated wildfire, and Garry oak (Quercus garryana Douglas ex Hook.) woodlands experiencing “first entry” restoration burns. Both the black oak woodlands of Lassen National Forest, California, and the Garry oak woodlands of Joint-Base Lewis-McChord, Washington have experienced shifts in vegetative structure and composition during long fire-free intervals. Black oak canopy dominance and vigor of resprouts were positively correlated with increased fire severity (R2=0.41, 0.49, respectively), but black oaks that had sprouted following top-kill in the first fire were easily top-killed in the second fire, even at low severities, implying that long-term survival of regenerating black oaks in fire-prone regions is uncertain. Top-kill of Garry oak was rare (8%) in three prescribed burns, despite high levels (95%) of crown scorching and irrespective of proportional duff consumption around oak bases, demonstrating the high resilience of Garry oaks to first entry burns when compared to historic fires. Bud kill (as measured by lack of bud burst the spring following burns) in Garry oak crowns was correlated with crown scorching (R2=0.42), but responses were highly variable, especially at high levels of scorching. The results of these studies indicate that fire adaptations may be specific to particular fire regimes, and that vegetative responses in oak woodlands are highly dependent on the adaptive traits of individual species. Restoration efforts in oak woodlands are more likely to be successful when reintroduction of fire is carefully tailored to target species, and modeled on characteristics of past disturbance, taking into account altered conditions of modern landscapes.
- Forestry