Postfire Regeneration Dynamics in California's National Forests

This study examines postfire regeneration patterns in low to mid-elevation forests of northern and central California where conifers co-exist and compete with hardwoods and shrubs in the postfire environment. 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 conifers. These conditions promote high-intensity fires, which have major effects on forest structure, function, and composition.;We established 1,854 survey plots in nineteen wildfires on ten National Forests across a range of elevations, forest types, and fire severities in central and northern California to provide insight into factors that promote natural tree regeneration after wildfires and the differences in post-fire responses of the most common conifer species and hardwood species. We developed a zero-inflated negative binomial mixed model with random effects to understand the importance of ten environmental variables in predicting conifer regeneration. This model identified as important factors distance to potential seed tree, annual precipitation, presence of regenerating shrubs, litter cover, fire severity and pre-existing forest type.;We documented widespread conifer regeneration failure with nearly 50% of all plots devoid of conifer regeneration. When regeneration did occur, it was dominated by shade-tolerant but fire-sensitive firs, Douglas-fir and incense cedar. Active forest restoration (planting, brush control, reduction of undesirable species, etc.) may be necessary in more severely burned areas farther from seed trees where natural regeneration is insufficient to restore forest composition and structure, increasing forest resiliency in the face of climate change and augmented levels of fire disturbance.;However, conifers do not exist in isolation in these forests and the interaction with other functional woody plant types must be considered. This study adds a unique contribution to understanding postfire regeneration dynamics by comparing the relative success of hardwoods to conifers across a fire severity gradient and in the first decade after mixed-severity fires. By utilizing vegetative sprouting (and to a lesser extent sexual reproduction), hardwood trees and shrubs are able to quickly capitalize on available resources and this ability may confer a competitive advantage to hardwoods. The results of this study indicate that increased fire severity leads to greater relative density of hardwoods via a combined impact of resprouts and seedlings, creating alternative states where hardwoods and shrubs (that suppress the relatively few conifer seedlings that do establish) may dominate for many years after disturbance. To a great extent, the future status of California's forests will depend on tree species' responses to patterns and trends in fire activity and behavior and post-fire management decisions.