Post-fire conifer regeneration in California’s diverse forests

Background/Question/Methods

Fire is an essential element of many forest ecosystems, and fire plays many important ecological roles that affect a forest’s resiliency and ability to regenerate to historic conditions. Many forests in California’s Mediterranean climate historically experienced frequent low-severity fires; however, due to fire suppression policies, timber harvest, and other management practices over the last century, many low- to mid-elevation forests in California have accumulated high fuel loads and dense, multi-layered canopies that are dominated by shade-tolerant and fire-sensitive firs and western incense cedar. These conditions promote stand-replacing high-intensity fires, which may alter forest structure and function. Future composition and structure of California’s forests will be dependent on species response to increased fire severity and post-fire management decisions. We established over 1,400 plots in ten National Forests across a range of elevations, forest types, and fire severities to provide insight into 1) the factors that promote natural tree regeneration after wildfires, 2) the differences in post-fire responses of the most common conifer species, and 3) the differences in post-fire responses between conifers and hardwoods. We measured site characteristics, seedling densities, and woody plant growth.  We developed a zero-inflated hurdle model to understand regeneration dynamics of the most common conifers.

Results/Conclusions

Species-specific regeneration trends were consistent with life history traits only in low to moderate severity fires. Shade-tolerant and fire-sensitive firs and incense cedar were the most abundant in low-light conditions of unburned forests and low severity patches. Shade-intolerant and fire-resistant Pinus species had low success in unburned and low severity patches; their greatest regeneration occurred in moderately burned forest patches. Seedling densities of all conifers were lower in patches that burned with high severity, possibly due to a combination of factors including seed mortality, increasing distance to the nearest living seed tree, competition with shrubs, and more severe microclimatic conditions. We found evidence that Ceanothus shrub cover negatively impacted pine regeneration while creating low-light conditions under which shade-tolerant firs and incense cedar thrive.  Shrub and hardwood competition for light and resources was much higher in moderate and high severity plots, and the effects were stronger on pines. Hardwoods are doing well in postfire landscapes; many hardwoods recruit by both seedlings and resprouting from the rootball.  Moderate severity fires may be the best prescription for “rebalancing” regeneration among shade tolerance groups in unplanted stands. We believe the trends in our data may apply to other mixed-species fire-suppressed forests.