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Ecological forestry experiments typicaly use large treatment units and silvicultural prescriptions that commonly increase within-unit heterogeneity in structural complexity and species composition in large treatment units. Increased heterogeneity influences processes affecting tree responses (e.g., competition) that operate at the neighborhood-level, posing challenges to analysis and interpretation. To investigate whether examining within-unit heterogeneity offers a more meaningful evaluation of project success than comparing categorical treatment effects, we used 20-year data from the Goosenest Adaptive Management Area (AMA) ecological forestry experiment in northern California, U.S. Designed to evaluate management alternatives for reducing fuels and accelerating development of late-seral forest characteristics in ponderosa pine (Pinus ponderosa)/white fir (Abies concolor) mixed-conifer forests, the Goosenest AMA study consists of (1) an untreated Control, (2) a Big Tree treatment using thinning from below to favor retention of large trees of any species, (3) a Pine Emphasis treatment combining thinning from below with radial thinning to favor percent ponderosa pine while increasing structural complexity, and (4) a Pine Emphasis with Fire treatment with added post-thinning prescribed burns. Our objectives were to evaluate 1) how treatments affect within-unit variation in neighborhood competition, structural complexity, and tree species composition, and 2) whether categorical treatment effects versus within-unit variation in competition, complexity, and composition influence individual-tree basal area increment (BAI), vigor as indicated by live crown ratio (ΔLCR), and tree mortality. To accomplish this, we developed and compared a series of generalized linear mixed models. Our analysis included the first investigation into whether fuel-reduction treatments alter tree species mixture-effects in ponderosa pine/white fir mixed-conifer forests. We found that all treatments similarly reduced neighborhood competition relative to the Control. The two Pine Emphasis treatments promoted greater variation in neighborhood competition and higher Percent pine relative to the Big Tree treatment, consistent with restoration objectives. Reduced neighborhood competition improved both ponderosa pine and white fir BAI. Reduced neighborhood competition helped to offset or reverse crown vigor decline in ponderosa pine and white fir, respectively. Species-mixture effects were negative for both small and large ponderosa pine BAI. For white-fir, trees grew faster in neighborhoods with a higher percentage of pine but the probability of mortality increased. Categorical treatment differences consistently reduced ponderosa pine and white fir mortality, except for the Pine Emphasis with Fire treatment, which increased white fir mortality. Our findings suggest that restoring historical ponderosa-pine forest reference conditions could accelerate the development of fire-resistant ponderosa pine tree sizes and sustain large pine growth. • Fuel reduction treatments foster fine-scale variation in structure and composition. • Local competition influences tree growth and vigor in California mixed-conifers. • Tree size and competition modify species mixture effects on growth. • Tree mortality varies with unit-level treatment and local species composition. • Restoration can benefit disturbance resistance and alter species-mixture effects. [ABSTRACT FROM AUTHOR]

Highlights • Landscape fuel treatments reduced wildfire severity in a natural experiment. • Treatments promoted post-fire seedling regeneration, particularly for firs. • Fir seedling densities were higher in areas with lower neighborhood fire severity. • The strategic fuel treatment network promoted both fire resistance and recovery. Abstract Strategically placed landscape area treatments (SPLATs) are landscape fuel reduction treatments designed to reduce fire severity across an entire landscape with only a fraction of the landscape treated. Though SPLATs have gained attention in scientific and policy arenas, they have rarely been empirically tested. This study takes advantage of a strategically placed landscape fuel treatment network that was implemented and monitored before being burned by a wildfire. We evaluated treatment efficacy in terms of resistance, defined here as the capacity to withstand disturbance, and recovery, defined here as regeneration following disturbance. We found that the treated landscape experienced lower fire severity than an adjacent control landscape: in the untreated control landscape, 26% of land area was burned with >90% basal area mortality, according to the remote-sensing-derived relative differenced Normalized Burn Ratio (RdNBR), while in the treated landscape only 11% burned at the same severity. This difference was despite greater pre-treatment fire risk in the treatment landscape, as indicated by FARSITE fire behavior modeling. At a more local scale, monitoring plots within the treatments themselves saw greater regeneration of conifer seedlings two years following the fire than plots outside the treatments. Mean seedling densities for all conifer species were 7.8 seedlings m−2 in treated plots and only 1.4 seedlings m−2 in control plots. These results indicate that SPLATs achieved their objective of increasing forest resistance and recovery. [ABSTRACT FROM AUTHOR]

Highlights • Maximum tree height is associated with climatic variables across an elevation gradient. • Variables such as topography and soil are also important in a water limited system. • Associations between tree height and environmental variables are scale dependent. • At fine spatial scales, topography and soil variables increase in importance. • The results have implications on how forests are managed in the Western US. Abstract Tall trees and vertical forest structure are associated with increased productivity, biomass and wildlife habitat quality. While climate has been widely hypothesized to control forest structure at broad scales, other variables could be key at fine scales, and are associated with forest management. In this study we identify the environmental conditions (climate, topography, soils) associated with increased tree height across spatial scales using airborne Light Detection and Ranging (LiDAR) data to measure canopy height. The study was conducted over a large elevational gradient from 200 to 3000 m in the Sierra Nevada Mountains (CA, USA) spanning sparse oak woodlands to closed canopy conifer forests. We developed Generalized Boosted Models (GBMs) of forest height, ranking predictor variable importance against Maximum Canopy Height (CH Max) at six spatial scales (25, 50, 100, 250, 500, 1000 m). In our study area, climate variables such as the climatic water deficit and mean annual precipitation were more strongly correlated with CH max (18–52% relative importance) than soil and topographic variables, and models at intermediate (50–500 m) scales explained the most variance in CH Max (R2 0.77–0.83). Certain soil variables such as soil bulk density and pH, as well as topographic variables such as the topographic wetness index, slope curvature and potential solar radiation, showed consistent, strong associations with canopy structure across the gradient, but these relationships were scale dependent. Topography played a greater role in predicting forest structure at fine spatial scales, while climate variables dominated our models, particularly at coarse scales. Our results indicate that multiple abiotic factors are associated with increased maximum tree height; climatic water balance is most strongly associated with this component of forest structure but varies across all spatial scales examined (6.9–54.8% relative importance), while variables related to topography also explain variance in tree height across the elevational gradient, particularly at finer spatial scales (37.15%, 20.26% relative importance at 25, 50 m scales respectively). [ABSTRACT FROM AUTHOR]

Large, high severity fires are becoming more prevalent in Sierra Nevada mixed-conifer forests, largely due to heavy fuel loading and forest densification caused by past and current management practices. In post-fire areas distant from seed trees, conifers are often planted to re-establish a forest and to prevent a potential type-conversion to shrub fields. Typical reforestation efforts promote conifer survival and growth by reducing competing shrub cover, yet the effects of these practices on plant species richness and composition are not well understood. We compared the effects of treatment and time since fire on (1) native and exotic plant species richness, (2) plant community composition, and (3) stand structure. Plots were installed throughout three different aged but proximate fires located in the canyon of the South Fork of the American River in California, 10, 22, and 41 years after fire. All three fires included large patches of stand-replacing fire that had been reforested with conifers as well as unplanted areas. Native plant species richness was significantly higher in planted areas where shrub cover was lower and planted trees successfully established. Native species richness decreased as time since fire increased, but the relationship between shrub control and richness persisted. Exotic species richness was higher on treated sites in the more recent fires, while the opposite was true in the oldest fire. As time since fire increased, understory species composition shifted from a community dominated by annuals and perennials to one dominated by shrubs and shade-tolerant trees. Shrub cover and July soil moisture were the top two factors influencing understory richness levels. Natural regeneration was low in the youngest fire and high in the oldest fire but highly heterogeneous in all three fires. Our study suggests that while retaining some shrub cover for post-fire habitat may be desirable, some level of shrub reduction does favor native plant richness and overall herbaceous cover. [ABSTRACT FROM AUTHOR]

Highlights: [•] Arthropod assemblages showed no significant differences among treatments. [•] Arthropod assemblages showed significant differences among tree species and seasons. [•] Mixed-conifer forest restoration treatments had little effect on canopy arthropods. [•] Restoration treatments should maintain a full range of plant species in these forests. [Copyright &y& Elsevier]

Highlights: [•] Different vegetative factors influenced northern spotted owl foraging between seasons. [•] Wintering owls preferentially foraged in evergreen shrub and hardwood patches. [•] Owls foraged in sclerophyllous shrub patches up to 600m from conifer forests in winter. [•] Greatest use of harvested units included those with 10–18m2/ha of conifer basal area. [•] Nearly 20% of owls in mixed-conifer forests migrated to lower elevations in winter. [Copyright &y& Elsevier]

Abstract: Pinus strobiformis (southwestern white pine, abbr. PIST), a species that comprises a small but significant component of the mixed-conifer forests of the U.S. Southwest, faces an increasing threat from white pine blister rust (Cronartium ribicola, abbr. WPBR). This potentially lethal, non-native fungal pathogen is windborne and, therefore, virtually impossible to control. However, silvicultural strategies, such as thinning, reintroduction of natural fire regimes, and encouraging natural selection for rust-resistant genotypes, show promise for maintaining or restoring infected stands. To expand ecological knowledge of the species and guide management, we surveyed PIST in mixed-conifer stands throughout Arizona and portions of New Mexico. We examined PIST forest structure, regeneration, and competitive interactions. We found that PIST occupied lower canopy positions in Pinus ponderosa-dominated plots, thereby supporting its classification as an intermediate shade-tolerant species. Plots where PIST was a dominant component showed evidence of succession towards more tolerant conifers. PIST regeneration occurred relatively rarely and was negatively associated with increasing understory plant cover and open conditions. Although PIST faced heavy tree competition, with Pseudotsuga menziesii the most important competitor, small suppressed trees showed evidence of persistence. [Copyright &y& Elsevier]

Abstract: Fire suppression and other past management practices in the western USA have led to dense conifer forests with high canopy cover and thick layers of surface fuels, changes likely to alter understory microclimate relative to historical conditions. Silvicultural treatments are used to restore forest resilience, but little is known about their microclimate-mediated effects on fire behavior. We measured fire-related microclimate variables for two years before and after experimental, operational-scale application of fuels-reduction thinning and group selection treatments in a Sierra Nevada mixed-conifer forest. Measurements included air speed, temperature, and relative humidity; soil temperature and moisture; and dead fuel moisture. Wind gust speed increased moderately (average 0.7ms−1 or 31% increase) in thinned forest and sharply (average 2.5ms−1 or 128% increase) in group-selection openings. Surprisingly, treatments did not affect air temperature or humidity. Soil temperatures increased by a mean of 4°C in group openings but did not increase in thinned stands. Duff moisture in group selection openings was 72% of that in the control stands, but there were no effects on moisture in other fuel particle size classes, or in thinned stands. Soil moisture increased in group-selection openings at depths down to 0.7m but did not change in thinned stands. Fire spread simulation modeling with FMAPlus indicated that elevated wind speeds could increase the fire rate of spread, but that increases are moderate and largely linear rather than exponential across the observed range of wind gust speeds. In general our results suggest that group selection openings placed in high canopy cover, Sierran mixed-conifer forests are distinct microclimatic environments that will have slightly different fire behavior than the surrounding matrix due to higher surface temperatures and faster wind speeds. Current fuels-reduction thinning practices in dry western forests, however, will have minimal microclimatic-mediated influence on wildfire behavior, and there is little cause for concern about a faster rate of fire spread or drier fuels in such stands. [Copyright &y& Elsevier]

Abstract: In the American Southwest, mixed-conifer forest experienced altered disturbance regimes with the exclusion of fire since the early 1900s. This research analyzes patch development and tree spatial patterns in the middle versus upper mixed-conifer forests at Grand Canyon National Park in Arizona (USA). The methods used include: (1) size–structure analyses, to compare species patch development; (2) dendrochronological dating of tree establishment and fire history; (3) tree ring master chronology, to determine periods of suppressed growth, compared to a palmer drought severity index; (4) spatial analyses by size and age, with univariate and bivariate analyses of spatial association as well as spatial autocorrelation. Results show that unlike the lower ecotone of the mixed-conifer zone, both the middle elevation and upper ecotone were mixed-conifer forests before Euro-American settlement. At the upper ecotone, two decades (1870s and 1880s) had no successful conifer establishment but instead aspen cohorts, corresponding to the fire history of synchronized fires. Overall, the upper ecotone has shifted in composition in the absence of surface fires from mixed conifer to encroachment of subalpine species, particularly Engelmann spruce. Spatial patterns of tree sizes and tree ages imply development of a size hierarchy in an aging patch. In addition, shifts in species composition from ponderosa pine and white fir overstory to Engelmann spruce and Douglas-fir understory affected within-patch spatial patterns. These results provide quantitative evidence of past and present forest conditions for the development of restoration strategies for Southwestern mixed-conifer forests. [Copyright &y& Elsevier]

Burn severity distributions for areas treated then burned (i.e., " treatment-fire") and untreated burned controls. Density plots showing the frequency distribution of the Relativized Burn Ratio (RBR), a measurement of burn severity, for untreated areas (controls; yellow) and silvicultural or fuel treatment (cases; purple) in areas that were treated before a wildfire. Vertical dashed lines show the thresholds: <2% basal area loss (dark purple), unburned-low and moderate severity (magenta), low-moderate and high severity mixes (orange), and >98% basal area mortality (i.e., stand replacing severity; yellow). Data are from wildfires that occurred between 2001 and 2019 and treatments occurring from 1982 to 2017, within the northeastern Washington State, USA study area. [Display omitted] • Fire weather and species composition influenced burn severity. • Reburns reduced burn severity of subsequent fires. • Most fuel and silvicultural treatments burned with lower severity than untreated controls. • Prescribed fire was the most effective treatment at reducing burn severity. • Post-fire harvest and planting weakly influenced subsequent burn severity. We investigated the relative importance of daily fire weather, landscape position, climate, recent forest and fuels management, and fire history to explaining patterns of remotely-sensed burn severity – as measured by the Relativized Burn Ratio – in 150 fires occurring from 2001 to 2019, which burned conifer forests of northeastern Washington State, USA. Daily fire weather, annual precipitation anomalies, and species' fire resistance traits were important predictors of wildfire burn severity. In areas burned within the past two to three decades, prior fire decreased the severity of subsequent burns, particularly for the first 16 postfire years. In areas managed before a wildfire, thinning and prescribed burning treatments lowered burn severity relative to untreated controls. Prescribed burning was the most effective treatment at lowering subsequent burn severity, and prescribed burned areas were usually unburned or burned at low severity in subsequent wildfires. Patches that were harvested and planted <10 years before a wildfire burned with slightly higher severity. In areas managed within 5 years after an initial fire, postfire harvest and planting reduced prevalence of stand-replacing fire in reburns. However, overall, postfire management actions after a first wildfire only weakly influenced the severity of subsequent fires. The importance of fire-fire interactions to moderating burn severity establishes the importance of stabilizing feedbacks in active fire regimes, and our results demonstrate how silvicultural treatments can be combined with prescribed fire and wildfires to maintain resilient landscapes. [ABSTRACT FROM AUTHOR]

• 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]