Topography and fire legacies drive variable post-fire juvenile conifer regeneration in eastern Oregon, USA

Abstract

Increasingly frequent large wildfires in the western US raise questions about the effects of climate and site-level factors on forest ecosystem resilience. This study presents findings from seedling and sapling surveys conducted across 179 sites 15–21 years post-fire in eastern Oregon’s Blue Mountain ecoregion. We found wide variation in conifer seedling and sapling densities across low, medium and high burn severity sites in the eight fires surveyed. One-third of sites had zero seedlings and saplings, while a quarter of sites had densities above 2000 juvenile trees ha−1, in part due to high densities of lodgepole pine saplings. We assessed stocking adequacy by comparing observed juvenile conifer densities to local recommendations for specific plant associations, and found densities did not meet minimum stocking levels in approximately 35% of sites. The most important variables explaining juvenile conifer presence were topographic heat load and distance to live seed source, with the probability of juvenile conifer presence declining below 50% at approximately 100 m from a live seed source. Douglas-fir seedlings were less likely to be found on sites with high heat load than ponderosa pine, and drought conditions in the first three years post-fire reduced Douglas-fir regeneration. Post-fire drought also reduced the probability of achieving minimum stocking levels. Our findings indicate that juvenile conifer densities on warmer slopes within large, high-severity burn areas may be insufficient to meet local silvicultural guidelines without supplementary replanting, especially when moisture availability in the first few years post-fire is low. Some of these marginal sites may transition to shrub or grassland for the foreseeable future, though further research is needed to confirm regional post-fire successional trajectories. The findings from this study can inform post-fire and climate-adapted forest management in the inland Northwest.