Clustering and growth patterns for natural regeneration in Sierra Nevada mixed-conifer forests with a restored fire regime
Many, dry western conifer forests were historically adapted to high frequency, low-to-moderate severity fires but are increasingly susceptible to large, stand-replacing wildfires due to dramatically altered stand conditions, as well as the effects of climate change. The historic spatial patterns of fire adapted forests: individual trees, clusters of trees, and openings (ICO), have displayed heightened resistance and resilience to fire. How this pattern develops, however, is not well understood and could help inform reforestation practices better designed to increase fire resistance in young stands. We investigated growth and spatial patterns among regenerating tree species in mixed-conifer forests with restored fire regimes in California’s Sierra Nevada. We collected and analyzed spatial patterning and growth rate data at 429 plots in eight different sites in old-growth, mixed-conifer forests with two to five low to moderate severity fires within the last 60 years. We compared average stocking rates of regenerating stems across species, shrub conditions, size classes, and fire histories. We also examined the effects of site fire history and topography on spatial patterning, and the effects of clustering, local density and adjacent shrubs on tree growth. We found very high seedling regeneration rates especially among white fir. The majority (75%) of sampled seedlings were within clusters. Our modeling results indicate that although increased crowding within clusters predictably had a negative impact on growth, growth rates within clusters were significantly higher than growth rates for individual trees. Although high shrub cover was found to negatively impact small seedlings (<137 cm tall), it was associated with increased growth and stocking rates among the oldest (>15 Years) and tallest (> 137 cm) seedlings, respectively. Our results suggest that clustered patterns of natural tree recruitment, as well as proximity of older clustered seedlings to shrub cover in active-fire forests may favor the establishment and growth of regenerating conifer stems. Our findings indicate there may be a benefit to establishment and early growth of seedlings in clusters possibly due to their modification of the surface microclimate and their density keeping shrubs ‘at bay’. While our study focused only on the early stages (<30 years old) of regeneration, our results are at odds with common reforestation strategies favoring regularly, widely spaced plantings and aggressive shrub reduction. Our research suggests we need a better understanding of how heterogeneity in seedling, shrub and gap patterns may enhance the resilience of regenerating stands as they mature.