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• Forests with a restored fire regime had high natural regeneration rates. • Regardless of species, the majority of sampled stems, were found in clumps. • Young trees in clumps had higher diameter and height growth rates than individuals. • There was no significant difference in growth rates among clump sizes. • High shrub cover didn't have a significant negative effect on seedling growth rates. Many dry conifer forests in the western United States were historically adapted to frequent low-to-moderate severity fires, but are increasingly susceptible to large, stand-replacing wildfires due to dramatically altered stand conditions and changing climate. The historic tree spatial patterns of mature stands in fire-adapted forests – individual trees, clumps of trees, and openings (ICO) – are associated with heightened resistance and resilience to fire. How this pattern develops over time, however, is not well understood and could help inform reforestation practices better designed to increase fire resistance in developing stands. We investigated growth rates and spatial patterns among regenerating trees in mixed-conifer forests with restored fire regimes in California's Sierra Nevada. We compared average stocking densities across tree species, size classes, shrub cover, and fire histories. We also examined the effects of microsite topography on spatial patterning of these juvenile trees, and the effects of clump patterning, local stem density and adjacent shrubs on tree growth rates. We found that the majority (75%) of sampled stems were found in clumps. Our mixed-effect models indicated that for trees growing within clumps, increased crowding slowed tree growth, as expected. Surprisingly, however, compared with individual trees growing outside clumps, trees growing within clumps grew significantly faster. Shrub cover in proximity to juvenile trees did not have a consistent impact across our models, but was associated with increased annual height growth. Additionally, plots with high shrub cover had higher stocking rates among the tallest regenerating stems (height > 137 cm). Our findings indicate that clumped spatial patterns of natural tree recruitment may favor the establishment and early growth of regenerating conifers in active-fire forests. While our study focused only on the early stages (<30 years old) of regeneration, our results contrast with common reforestation strategies favoring regular, widely-spaced plantings and aggressive shrub reduction. Our research suggests we need a better understanding of how heterogeneity in the spatial patterns of juvenile trees and shrubs may enhance the resilience of regenerating stands as they mature. [ABSTRACT FROM AUTHOR]

• Patterns of individual trees, tree clumps, & openings varied across topography. • Topographic differences were driven by trends in large clumps & large openings. • Stand-level averages were not representative of trends for smaller clumps. • Management by topography may help reconcile wildlife & fire resilience objectives. • Management by topography can align structure with water availability & fire regime. In Sierra Nevada mixed-conifer forests, heterogeneity in overstory tree spatial patterns is an important ecological characteristic associated with resilience to frequent fires. Regional managers often emphasize this heterogeneity as a key component of forest treatments. There is a lack of information about how the dimensions of fire-resilient tree spatial patterns might vary with topography, which is an important influence on forest structure. We mapped, analyzed, and compared overstory tree spatial patterns across three topographic slope positions (ridges, mid-slopes, and valleys) in unlogged stands with multiple recent lower/mixed-severity burns. Using analysis of I ndividual trees, C lumps of trees, and O penings (ICO), we found significant differences in a number of spatial pattern metrics, including stand-level tree density and maximum clump size (highest in valleys), forest opening size (largest on ridges), and species composition (percent shade-intolerant stems highest on ridges). We found notably non-significant differences in within-clump tree densities at all spatial scales. Additionally, stand-level averages for forest structure across topography did not consistently capture spatial patterns for individual trees or smaller clump sizes, but often matched trends for clumps and openings in the largest size classes. As "megafires" become more common, prescribed fire and thinning treatments will be used more often to increase forest resilience. By quantifying variable tree spatial patterns across topographic positions in frequent-fire forests, we provide general guidelines for marking prescriptions that may increase resilience of treated forests to increasing fire intensity. [ABSTRACT FROM AUTHOR]

• Studied first-entry fires following long-term fire suppression, Sierra Nevada, USA. • Examined forest opening & tree clump structure patterns using airborne lidar data. • Compared burns to frequent-fire reference areas and control areas outside fires. • Moderate-severity fires replicated resilient structures of reference areas. • Proportional loss of taller trees with greater burn severity. A century of fire suppression has left fire-dependent forests of the western United States increasingly vulnerable to wildfire, drought, and insects. Forest managers are trying to improve resilience using treatments such as mechanical thinning and prescribed fire; however, operational and resource constraints limit treatments to a fraction of the needed area each year. An alternative is to let wildfires burn under less-than-extreme fire weather where human lives and infrastructure are not at risk. We examined post-fire forest structure using airborne lidar data to determine whether a single wildland fire following an extended fire-free period could produce forest structures resembling fire-resilient historical conditions. We studied forest structures resulting from these "first-entry" fires in a forest with a history of timber management (2008 American River Complex Fires, Tahoe National Forest) and in a wilderness area (2009 Big Meadow Fire, Yosemite National Park). We compared the results of these first-entry fires with nearby reference areas that had experienced 2+ fires that burned predominately at low- and moderate-severity. We identified visible overstory trees from the lidar data and examined their patterns in terms of individuals, tree clumps, and openings. We found that moderate-severity fire effects in these first-entry fires produced similar patterns to the reference areas with area in openings at approximately 40% and trees predominately in small (2 to 4 trees) and medium (5–9 trees) clumps High-severity fire produced mortality likely to lead to large canopy openings that were historically uncharacteristic in these forests. As burn severity increased, the amount of the residual canopy area represented by taller trees (>16 m and especially >32 m) decreased, which could result from fires preferentially killing taller trees or from locations with taller trees more commonly experiencing lower burn severities. Our study suggests that first-entry fires allowed to burn under less-than-extreme conditions can reproduce spatial patterns resembling historical conditions resilient to fires and drought but possibly at the disproportionate expense of larger trees. [ABSTRACT FROM AUTHOR]