Born of fire: Chaparral age structure as a clue to disturbance regimes and vegetation community change in southwest Oregon

Background/Question/Methods

Fuels management on Bureau of Land Management lands in SW Oregon, USA, is motivated by the needs to reduce fire hazard and restore ecosystems thought to be impacted by fire suppression. Oak woodlands and chaparral are two of the most characteristic ecosystems of SW Oregon’s interior valleys, and extensive acreages within these systems are targeted annually for fuels treatment. However, these are also two of the least understood ecosystems in the region. Remarkably little is known about their response to fire suppression, and the assumptions justifying the need for fuels reduction and on which treatment prescriptions are based are extrapolations from other ecosystems. We studied patterns in oak woodland and chaparral age structure for insights into past disturbance history and vegetation dynamics. As the study area represents the northernmost extension of the California chaparral vegetation type, we also explore possible divergence of the Oregon chaparral shrub species, Arctostaphylos viscida and Ceanothus cuneatus, from these species in California chaparral over their 4,000 yr separation. Our 31 low to mid-elevation study sites represented four chaparral types and four times since fire (22 yrs to 114+ yrs). Shrub ages were determined from sections cut from stumps left after fuels treatments.  

Results/Conclusions

Chaparral in California typically burns in stand-replacing fires. However, shrubs at our sites that were older than the most recent fire indicate that lower-severity wildfires also occur here. Further, shrubs far younger than the most recent fire suggest significant inter-fire recruitment at our sites, even though the chaparral shrub species we studied are typically described as recruiting only with fire-stimulated seed germinating 1 to 2 yr post-fire. Temperatures measured in study area surface soils during summer 2007, which were as high as 68^o C on a warm day (air temperature 33 ^o C),  suggest that seed scarification from soil heating alone could cause inter-fire recruitment. Ages of shrub stumps and co-occurring live hardwoods and conifers suggest that some chaparral communities have been stable for over a century, while others are transitioning to woodlands. The layering, resprouting, and fire-free recruitment that we observed have not been previously reported for these species, suggesting possible divergence between Oregon and California chaparral populations. Results advance basic understanding of chaparral and oak woodland ecology, and bolster the information with which land managers can evaluate fuels reduction prescriptions.