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As fire regimes shift in western North America, patches of stand-replacing fire are becoming larger. Forest succession in these patches is not well understood. There is concern that competition with rapidly reestablishing shrubs, combined with dispersal limitation, may delay or impede conifer recovery and/or shift tree composition toward shade-tolerant species. However, tree–shrub interactions and shrub neighborhood dynamics have not been closely examined. To investigate the patterns and processes determining forest recovery after severe wildfire, we developed a data-driven simulation model that we use to predict conifer emergence above the shrub canopy. Our model results showed that ponderosa pine (Pinus ponderosa) emerged at a faster rate than white fir (Abies concolor) under whitethorn ceanothus (Ceanothus cordulatus) and deerbrush (Ceanothus integerrimus), and at a similar rate under greenleaf manzanita (Arctostaphylos patula). Across all shrub species, ponderosa pine had a relative advantage over fir in the period between conifer establishment and peak shrub competition, requiring a mean of 16 years for 50% of individuals to emerge compared to 22 years for fir. Fir emergence rates then surpassed those of pine, leading to similar overall emergence by simulation end: 83% ± 7% for pine and 82% ± 7% for fir. These results show that, on balance, shrub neighborhood dynamics do not produce an ecological filter favoring firs for the initial cohort of established seedlings, but emergence patterns are sensitive to shrub species. Further applications of this data-driven simulation framework could improve understanding of other important components of post-fire succession. [ABSTRACT FROM AUTHOR]

Management efforts to promote forest resiliency as climate changes have often used historical forest structure and composition to provide general guidance for fuels reduction and forest restoration treatments. However, it has been difficult to identify what stand conditions might be fire and drought resilient because historical data and reconstruction studies are generally limited to accurate estimates only of large, live tree density and composition. Other stand features such as smaller tree densities, dead wood, understory structure, regeneration, and fuel loads have been difficult to quantify, estimate how they may vary across a landscape, or assess how they would be affected by fire under current climate conditions. We sampled old-growth, mixed-conifer forests with at least two low-intensity fires within the last 65 years in 150 plots at 48 sample sites ranging over 400 km of the Sierra Nevada. Recent fire history had the strongest influence on understory conditions with small tree density decreasing and shrub cover increasing with the increased intensity and frequency of fire associated with upper-slope and ridge-top locations. In contrast, stand structures associated with large, overstory trees such as total basal area, canopy cover, and the abundance of large snags and logs increased in topographic locations associated with more mesic, productive sites regardless of fire history. In forests with restored fire regimes, topography, fire and their interaction influence productivity and burn intensity, creating the structural heterogeneity characteristic of frequent-fire forests. [ABSTRACT FROM AUTHOR]