COS 7-9
Historical fire regimes of southwestern Oregon and associated forest species composition and structure
In the Mediterranean mixed conifer hardwood forests of southwestern Oregon there is significant need for forest restoration treatments to prepare these forests for a future climate of increasing drought and fire. Our research provides stand reconstructions and dendrochronological fire histories for 8 mixed conifer/hardwood sites in forest types found on approximately 800 thousand ha across the 1.3 million ha Rogue River Basin. All living and dead trees were identified on 0.1 ha plots and tree status and size was reconstructed to the date of last wildfire. At seven of the sites 10 plots were used to characterize forest composition and structure across 50 ha. In the Ashland watershed 50 plots were stratified across a 22,000 ac study area. Fire histories for each site were developed using 10 cross-dated fire scar samples at seven of the sites and 100 fire scar samples in the Ashland watershed. This sampling was designed to allow analysis of fine-scale topographic variability in the Ashland watershed to be scaled up to similar biophysical settings at larger scales.
Results/Conclusions
Contemporary forests exhibited significant species conversion with a greater proportion of shade tolerant forest tree species. On warmer insolation sites Pinus ponderosa and scattered large Pseudotsuga menziesii, Quercus kelloggii, and Arbutus menziesii were historically dominant while in 2011 we found hyper-abundant, small Pseudotsuga menziesii, and Arbutus menziesii. Stands on cooler insolation and higher elevation sites dominated by Pinus ponderosa and Pseudotsuga menziesii had been invaded by Abies concolor. The number of trees per hectare more than tripled from the reference condition and basal area more than doubled. The median fire return interval for sites in the Ashland watershed was 11 years while fire return intervals at the seven sites across the Rogue basin had a median of 9 years. These frequent fire return intervals likely reflect the fire prone Mediterranean climate and large populations of Native Americans known to ignite fires intentionally. Looking to forests before the era of fire exclusion helps inform ongoing efforts to protect the largest and oldest trees and both open and closed habitats by promoting open forests where ecologically appropriate and supports the careful reintroduction of frequent fire’s critical ecological role.