A multi-century history of fire regimes along a transect of mixed-conifer forests in central Oregon, U.S.A

Dry mixed-conifer forests are widespread in the interior Pacific Northwest, but their historical fire regimes are poorly characterized, in particular the relative mix of low- and high-severity fire. We reconstructed a multi-century history of fire from tree rings in dry mixed-conifer forests in central Oregon. These forests are dominated by ponderosa pine (Pinus ponderosa Lawson & C. Lawson), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), and grand fir (Abies grandis (Douglas ex D. Don) Lindl.). Across four, 30-plot grids of ~800 ha covering a mosaic of dry mixed-conifer forest types, we sampled 4065 trees for evidence of both highand low-severity fire. From 1650 to ~1900, all four sites sustained frequent, often extensive, low-severity fires that sometimes included small patches of severe fire (50-150 ha during 18%-28% of fire years). Fire intervals were similar among sites and also among forest types within sites (mean intervals of 14-32 years). To characterize the continuous nature of the variation in fire severity, we computed a plot-based index that captures the relative occurrence of low- and high-severity fire. Our work contributes to the growing understanding of variation in past fire regimes in the complex and dynamic forests of North America’s Interior West.