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• Forest thinning substantially reduced drought-related bark beetle mortality. • Variable thinning did not increase mortality relative to a more standard thinning. • Higher tree mortality was associated with greater competition. • Burning increased mortality, especially for trees with more competition. California's high density, fire-excluded forests experienced an extreme drought accompanied by warmer than normal temperatures from 2012 to 2015, resulting in the deaths of millions of trees. We examined tree mortality and growth of mixed-conifer stands that had been experimentally treated between 2011 and 2013 with two different thinning treatments, one with more structural variability (HighV) and one with less structural variability (LowV), applied alone or in combination with prescribed burning. Tree mortality between 2014 and 2018 varied by species ranging from 42% of white fir (Abies concolor) to 18% of sugar pine (Pinus lambertiana), 12% of incense cedar (Calocedrus decurrens) and 10% of yellow pine (P. ponderosa and P. jeffreyi). Lower overall tree mortality rates at this location relative to drier locations in the southern Sierra Nevada suggested that drought effects may have been ameliorated by lower water deficits due to our site's more northerly location and deep, productive soils in combination with reductions in tree competition following thinning and burning. Averaged across burn treatments, thinning reduced the overall mortality rate between 2014 and 2018 from 34% to 11%. A total of 23% of the basal area was lost in the unthinned control treatments during this time period, while basal area was unchanged in the thinned treatments, with growth offsetting mortality. There was no significant difference in mortality or basal area change between LowV and HighV, suggesting that leaving trees at variable spacing may not compromise growth or resilience of the stand during a drought. Overall tree mortality was greater in the prescribed burn treatments, most pronounced in the smaller tree size classes, and varied by species, with burning having a significant effect on incense cedar and all pines, but not white fir. Trees with greater competition (Hegyi index) were more likely to die, particularly when also burned. Burning, however, consumed surface fuels and lowered fire hazard. With predictions of warmer droughts and greater weather variability, reducing forest density (basal area) and keeping surface fuel loads low will be important for building greater resilience to future drought stress and wildfire. [ABSTRACT FROM AUTHOR]

• We examined mixed-conifer forest structure resulting from an active fire regime. • The diversity of forest structures present relied on a range of fire and biophysical variables. • Time since high-severity fire is a primary driver of forest structure. • Repeat fires and interactions with biophysical variables further modify structure. • A process-based approach using managed wildfire could improve adaptive capacity of mixed-conifer systems. Nearly a century of fire suppression in most forested land of the United States has limited researchers' ability to construct and rigorously test conceptual models of forest structural development in mixed-conifer ecosystems. As a result, land managers must rely on conceptual models of forest development that may overemphasize idealized stand structures and developmental pathways, which ultimately hampers management of many forest systems for resilience to future climate change impacts. We sought to determine the relative importance of fire history (frequency, severity, and time since fire) and biophysical variables on forest structural diversity and development. Importantly, we conducted our study in an ecosystem with a contemporary active fire regime where wildland fire has been managed as an ecosystem process for four decades. Using data from unburned (≥80 years since fire), once-burned and twice-burned mixed-conifer forests in the Bob Marshall Wilderness of Northwest Montana, we conducted a hierarchical clustering analysis to identify forest stand structure classes. We then used a Classification and Regression Tree analysis, combined with other post-hoc analyses, to elucidate the biophysical and disturbance history drivers that lead to each structure class. The cluster analysis revealed six forest structure classes. The CART analysis indicated that time since fire plays a large role in determining forest structure, but at intermediate time scales structure is further shaped by repeat fires and interactions with biophysical variables. The CART and post-hoc analyses did not, however, indicate a singular fire history or biophysical pathway to any one structure class. We synthesize our results in a conceptual model of forest structural development under an active fire regime. This model supports existing theory that succession following severe fire plays a large role in shaping forest structure. It also recognizes the role of fire at variable severities and frequencies, the physical environment, and tree community composition in influencing forest structural development. The complexity of forest structure and development generated by an active fire regime points to the need to incorporate a process-based view of wildfire if the goal is to manage for improved resiliency and adaptive capacity to future climate change impacts. [ABSTRACT FROM AUTHOR]

Natural disturbances are critical for supporting biodiversity in many ecosystems, but subsequent management actions can influence the quality of habitat that follow these events. Post-disturbance salvage logging has negative consequences on certain components of forest biodiversity, but populations of some early seral-adapted organisms may be maintained in salvage-logged areas. We investigated the influence of an essential group of pollinators – wild bees – to recent post-wildfire salvage logging within managed mixed-conifer forest in the Pacific Northwest. We compared bee diversity (i.e., bee abundance, species richness, alpha diversity, and beta diversity) and habitat features (i.e., floral resources and available nesting substrates) in salvage-logged areas to unlogged sites, both of which experienced high-severity wildfire. Although we found no evidence for differences in bee abundance between salvage-logged and unlogged sites, interpolated estimates of species richness and both interpolated and extrapolated estimates of alpha diversity were greater in unlogged sites. Additionally, beta diversity was greater in unlogged sites when equal weight was given to rare species. Salvage logging did not select for specific functional groups of bees, as both logged and unlogged sites were dominated by generalist, social species that nest in the ground. However, habitat conditions for bees were influenced by salvage logging: flowering plant density was greater in salvage-logged sites during the latter half of the season, with fewer conifer snags and more woody debris than were available in unlogged sites. Our study indicates that salvage logging can support wild bee abundance, but that unlogged patches of severely burned forest supports slightly greater bee diversity within fire-prone mixed-conifer landscapes. • Assessed wild bee communities in mixed-conifer forest after severe wildfire • Sampled in sites that were salvage-logged or served as unlogged controls • Collected >3500 bees representing 111 species/morphospecies over 2 years • Wild bee abundance was not affected by salvage logging. • Bee communities were less diverse in salvage-logged sites relative to control sites. [ABSTRACT FROM AUTHOR]

As wildfire activity increases in many regions of the world, it is imperative that we understand how key components of fire‐prone ecosystems respond to spatial variation in fire characteristics. Pollinators provide a foundation for ecological communities by assisting in the reproduction of native plants, yet our understanding of how pollinators such as wild bees respond to variation in fire severity is limited, particularly for forest ecosystems. Here, we took advantage of a natural experiment created by a large‐scale, mixed‐severity wildfire to provide the first assessment of how wild bee communities are shaped by fire severity in mixed‐conifer forest. We sampled bees in the Douglas Fire Complex, a 19,000‐ha fire in southern Oregon, USA, to evaluate how bee communities responded to local‐scale fire severity. We found that fire severity served a strong driver of bee diversity: 20 times more individuals and 11 times more species were captured in areas that experienced high fire severity relative to areas with the lowest fire severity. In addition, we found pronounced seasonality in the local bee community, with more individuals and more species captured during late summer, especially in severely burned regions of the landscape. Two critical habitat components for maintaining bee populations—flowering plants and boring insect exit holes used by cavity‐nesting bees—also increased with fire severity. Although we detected shifts in the relative abundance of several bee and plant genera along the fire severity gradient, the two most abundant bee genera (Bombus and Halictus) responded positively to high fire severity despite differences in their typical foraging ranges. Our study demonstrates that within a large wildfire mosaic, severely burned forest contained the most diverse wild bee communities. This finding has particularly important implications for biodiversity in fire‐prone areas given the expected expansion of wildfires in the coming decades. [ABSTRACT FROM AUTHOR]