Fire is a keystone process in California’s forests and is considered both a disturbance and a regulating force. Plants that evolved in Mediterranean climates have developed mechanisms to persist in a variety of fire regimes, including fire-resistant bark, fire-triggered seed germination, self-pruning of branches, and varying mechanisms of sexual reproduction and/or vegetative sprouting. Many hardwood trees and shrubs quickly capitalize on conditions associated with high severity fire through fire-stimulated resprouting and fire-enhanced seed germination. In contrast, conifers’ persistence in our study sites is dependent on seeds stored in the seed bank or dispersed from surviving trees within or directly outside the fire perimeter. Vegetative resprouting may give hardwood trees and shrubs a disturbance advantage that increases across the fire severity gradient. Shifting fire regimes and a projected increase in size and severity of wildfires in northern California have the potential to shift community composition in some conifer-dominated landscapes towards higher shrub densities and greater hardwood presence. We established 1329 plots on 13 fires that burned in 10 National Forests in California and examined regeneration patterns of 4 functional groups (conifer seedlings, hardwood seedlings, hardwood resprouts, and shrubs) across a fire severity gradient.
Preliminary results suggest 1) high severity fires confer a disturbance advantage to hardwoods, 2) fire-stimulated shrub cover may accentuate the competitive advantage of hardwood resprouts in severely burned areas, and 3) changing fire regimes may lead to increases in hardwood density where conifers currently dominate. Regeneration patterns were consistent with known species-specific differences in shade tolerance, drought tolerance, and adaptation to fire in low to moderate severity plots only; severely burned plots had far fewer conifer and hardwood seedlings regardless of ecological constraints. In contrast, hardwood resprouts had their highest densities in severely burned plots. There were significantly fewer resprouts in unburned controls than all three levels of fire severity. Shrub cover and mean shrub height were positively correlated with increasing fire severity and shrub cover was over three times as great as conifer and hardwood cover in severely burned areas across the thirteen fires. With many of California’s forests in a state of high density and heavily loaded with fuels, severe wildfires will continue to act as agents of change by preferentially promoting hardwoods through fire-stimulated resprouting and the competitive advantages this confers, especially at lower elevations where pre-existing hardwoods are more abundant.