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Climate change is considered a major driver of recent avian population declines, particularly in the drought-stricken southwestern United States. Predicting how bird populations will respond requires understanding the climatic drivers influencing population density across the region's diverse habitats. We modelled breeding-season densities of 50 bird species in relation to spring and summer drought and the timing of North American monsoon rainfall over a 12-year period (2007–2018) and across 4 habitats comprising an approximately 1,500 m elevational gradient. We estimated annual breeding-season population density in relation to climate in the previous year by fitting a Bayesian hierarchical N -mixture model to point-count data from each of 6 national parks on the Colorado Plateau. Specifically, we asked whether (1) population trends were stable, increasing, or decreasing in the focal parks; (2) breeding densities were affected by drought or the timing of monsoon rains; and (3) climatic effects differed across habitat types and among species that molt on the breeding grounds, the nonbreeding grounds, or stopover to molt in the monsoon region of northwestern Mexico (molt migrants). Population trends varied with habitat. Species of high-elevation mixed-conifer forest declined over the study period, matching regional Breeding Bird Survey trends, likely in response to climate-related habitat loss and disturbance. By contrast, lower-elevation pinyon-juniper and grassland-shrubland species density generally increased. Effects of drought varied by habitat with elevation: mixed-conifer species responded positively to drought in the previous year, likely due to earlier snowmelt and breeding phenology, whereas pinyon-juniper species were unaffected, and grassland-shrubland species responded negatively, perhaps due to reduced nest survival. Later arrival of monsoon rains, a common prediction of climate models, had a positive effect on grassland bird densities, but a negative effect on molt-migrant densities. Late monsoon rains may result in a phenological mismatch between migration timing and the pulse of resources required to molt. [ABSTRACT FROM AUTHOR]

Studies on the effects of wildfires on soil are common in mixed-conifer forests worldwide; however, such studies are less frequent in the context of high-altitude mixed-conifer forests. In this research, we analyzed the soil physicochemical properties 14 years after a high-intensity wildfire. Soil samples were collected from a depth of 0–10 cm across 100 composite samples, representing both control and burnt areas. Our findings revealed significant changes in soil characteristics in the burnt area compared to the control area. Specifically, the average soil pH, electrical conductivity, and available potassium increased by 3 %, 81 %, and 53 %, respectively, in the burnt area, while available phosphorus decreased by 6 %. Conversely, the average total nitrogen and organic matter showed non-significant decreases of 20 % and 12 % in the burnt area compared to the control area. No significant differences were found in slope aspects. Electrical conductivity and available potassium varied significantly among upper (3300–3500 m), middle (3100–3300 m), and lower (2900–3100 m) altitude ranges. Principal component analysis explained a total of 50.1 % of the variance, with approximately 27.4 % explained by the first axis and 22.7 % by the second axis. The control and burnt areas were closely clustered, indicating similar soil properties. Electrical conductivity, available potassium, and pH were more linked to the burnt forests, while nitrogen and organic matter were more associated with the control forests. Consequently, post-fire restoration efforts should consider potential changes in soil nutrient availability to facilitate successful vegetation recovery. This study enhances the formulation of more precise post-fire land management strategies, improves forest restoration initiatives, and promotes sustainable land-use practices by comparing unburnt natural forests.

Soil nutrient distribution is heterogeneous in space and time, potentially altering nutrient acquisition by trees and microorganisms. Ecologists have distinguished "hot spots" (HSs) as areas with enhanced and sustained rates of nutrient fluxes relative to the surrounding soil matrix. We evaluated the spatial and temporal patterns in nutrient flux HSs in two mixed-conifer forest soils by repeatedly sampling the soil solution at the same spatial locations (horizontally and vertically) over multiple seasons and years using ion exchange resins incubated in situ. The climate of these forests is Mediterranean, with intense fall rains occurring following summers with little precipitation, and highly variable winter snowfall. Hot spots formed most often for NO3− and Na+. Although nutrient HSs often occurred in the same spatial location multiple times, HSs persisted more often for PO43− NH4+, and NO3−, and were more transient for Ca2+, Mg2+, and Na+. Sampling year (annual precipitation ranged from 558 to 1223 mm) impacted the occurrence of HSs for most nutrients, but season was only significant for PO43−, NH4+, NO3−, and Na+, with HSs forming more often after fall rains than after spring snowmelt. The frequency of HSs significantly decreased with soil depth for all nutrients, forming most commonly immediately below the surficial organic horizon. Although HSs accounted for less than 17% of the sampling volume, they were responsible for 56–88% of PO43−, NH4+, and NO3− resin fluxes. Our results suggest that macronutrient HSs have a disproportional contribution to soil biogeochemical structure, with implications for vegetation nutrient acquisition strategies and biogeochemical models. [ABSTRACT FROM AUTHOR]

Climate change is expected to lead to higher temperatures in the Mediterranean region of northern California in the Sierra Nevada. Dendroclimatic studies typically focus on large, old trees, but there relatively limited understanding on how climatic sensitivity can vary with trees of different size classes. We collected tree increment cores and compared radial growth responses of small (20.32 to ≤40.64 cm), medium (40.64 to ≤60.96 cm), and large (>60.96 cm) diameter mixed conifer species in the Sierra Nevada to different climate variables (temperature, precipitation, and climate moisture index (CMI)). The most common tree species encountered were white fir (Abies concolor (Gord. & Glend.) Lindl.), followed by sugar pine (Pinus lambertiana Dougl.), ponderosa pine (Pinus ponderosa Dougl. Ex P. & C. Laws), and incense cedar (Calocedrus decurrens (Torr.) Florin). One of the most coherent responses from all diameter groups and across all species was the positive response to increasing minimum winter temperatures. All diameter groups and species also responded positively to precipitation and CMI at some point in the analysis period, which is the seasonal window of April of the prior year to October of the current year of ring formation. Perhaps the most notable difference when comparing the three diameter groups to climate was the higher occurrence of negative responses to the temperature of the previous year from the largest diameter group, as well as the higher number of negative responses to temperature in general. These results suggest that larger trees may be more sensitive to future climate projections compared with smaller trees and they may carry those effects into the next year. The use of dendroclimatology to assess how mixed conifer species in the Sierra Nevada responded to past climate is a key resource that can be used to infer how trees may respond to a future changing climate. [ABSTRACT FROM AUTHOR]

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]

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]

In temperate forests, elevated frequency of drought related disturbances will likely increase the incidence of interactions between disturbances such as bark beetle epidemics and wildfires. Our understanding of the influence of recent drought and insect‐induced tree mortality on wildfire severity has largely lacked information from forests adapted to frequent fire. A recent unprecedented tree mortality event in California's Sierra Nevada provides an opportunity to examine this disturbance interaction in historically frequent‐fire forests. Using field data collected within areas of recent tree mortality that subsequently burned in wildfire, we examined whether and under what conditions wildfire severity relates to severity of prefire tree mortality in Sierra Nevada mixed‐conifer forests. We collected data on 180 plots within the 2015 Rough Fire and 2016 Cedar Fire footprints (California, USA). Our analyses identified prefire tree mortality as influential on all measures of wildfire severity (basal area killed by fire, RdNBR, and canopy torch) on the Cedar Fire, although it was less influential than fire weather (relative humidity). Prefire tree mortality was influential on two of three fire‐severity measures on the Rough Fire, and was the most important predictor of basal area killed by fire; topographic position was influential on two metrics. On the Cedar Fire, the influence of prefire mortality on basal area killed by fire was greater under milder weather conditions. All measures of fire severity increased as prefire mortality increased up to prefire mortality levels of approximately 30–40%; further increases did not result in greater fire severity. The interacting disturbances shifted a pine‐dominated system (Rough Fire) to a cedar–pine–fir system, while the pre‐disturbance fir–cedar system (Cedar Fire) saw its dominant species unchanged. Managers of historically frequent‐fire forests will benefit from utilizing this information when prioritizing fuels reduction treatments in areas of recent tree mortality, as it is the first empirical study to document a relationship between prefire mortality and subsequent wildfire severity in these systems. This study contributes to a growing body of evidence that the influence of prefire tree mortality on wildfire severity in temperate coniferous forests may depend on other conditions capable of driving extreme wildfire behavior, such as weather. [ABSTRACT FROM AUTHOR]

In mixed stands, species complementarity (e.g. facilitation and competition reduction) may enhance forest tree productivity. Although positive mixture effects have been identified in forests world‐wide, the majority of studies have focused on two‐species interactions in managed systems with high functional diversity. We extended this line of research to examine mixture effects on tree productivity across landscape‐scale compositional and environmental gradients in the low functional diversity, fire‐suppressed, mixed‐conifer forests of the U.S. Interior West.We investigated mixture effects on the productivity of Pinus ponderosa, Pseudotsuga menziesii and Abies concolor. Using region‐wide forest inventory data, we created individual‐tree generalized linear mixed models and examined the growth of these species across community gradients. We compared the relative influences of stand structure, age, competition and environmental stress on mixture effects using multi‐model inference. We analysed growth of neighbouring tree species to infer whether a mixture effect in a single species translated to the stand‐level.We found support for a positive mixture effect in P. menziesii, although our results were equivocal in light of a weaker but still plausible alternative model. Growth of P. menziesii neighbouring species in mixed stands declined or held constant depending on aridity, suggesting that a positive mixture effect in P. menziesii does not necessarily extend to the stand level. We found no evidence for mixture effects in P. ponderosa, A. concolor or their neighbouring species.Complementarity appears to have a limited influence on tree growth in the mixed‐conifer systems of the U.S. Interior West, reflecting limited functional diversity. Historical changes in stand structure following fire exclusion, particularly high stand densities, may limit the potential for positive species mixture effects. The limited species pool of Interior West forests increases the risk that, without careful management, what functional diversity exists could be lost to compositional changes resulting from stand dynamics or disturbance. [ABSTRACT FROM AUTHOR]

Warm summer temperatures and longer fire seasons are promoting larger, and in some cases, more fires that are severe in low- and mid-elevation, dry mixed-conifer forests of the Northern Rocky Mountains (NRM). Long-term historical fire conditions and human influence on past fire activity are not well understood for these topographically and biophysically heterogeneous forests. We developed reconstructions of millennial-scale fire activity, vegetation change, and human presence at Black Lake, a small closed-basin lake on the Flathead Indian Reservation in the Mission Valley, Northwestern Montana, United States. Fossil pollen, charcoal, and biomarkers associated with human presence were used to evaluate the interaction between climate variability, fire activity, vegetation change and human activity for the past 7000 years. Comparisons among multiple proxies suggest climate variability acted as the primary control on fire activity and vegetation change from the early Holocene until the late Holocene when records suggest fire activity and climate variability decoupled. Specific biomarkers (5β-stanols including coprostanol and epi-coprostanol) associated with human presence indicate humans were present within the Black Lake watershed for thousands of years, although the inferred intensity of human presence is highly variable. A strong relationship between climate variability and fire activity during the early and mid-Holocene weakens during the last few thousand years, suggesting possible increased influence of humans in mediating fire activity in recent millennia, and/or a shift in the interaction between the distribution and abundance of woody fuel and fire severity. Human-set fires during the cooler and wetter late Holocene may have been aimed at maintaining important cultural resources associated with the heterogeneous mosaic of mixed conifer forests within the Black Lake watershed. The paleoenvironmental reconstruction at Black Lake corroborates archeological records that show humans were present within the Black Lake watershed for over 7000 years. Further research is needed to evaluate the evidence for this continuous presence and the possible role that people played in shaping fire regimes and vegetation within low- to mid-elevation mixed-conifer ecosystems of the NRM.

Although ecological disturbances can have a strong influence on pollinators through changes in habitat, virtually no studies have quantified how characteristics of wildfire influence the demography of essential pollinators. Nevertheless, evaluating this topic is critical for understanding how wildfire is linked to pollinator population dynamics, particularly given recent changes in wildfire frequency and severity in many regions of the world. In this study, we measured the demographic response of the blue orchard bee (Osmia lignaria) across a natural gradient of wildfire severity to assess how variation in wildfire characteristics influenced reproductive output, offspring sex ratio, and offspring mass. We placed nest blocks with a standardized number and sex ratio of pre-emergent adult bees across the wildfire gradient, finding some evidence for a positive but highly variable relationship between reproductive output and fire severity surrounding the nest site at both local (100 m) and landscape (750 m) scales. In addition, the production of female offspring was > 10% greater at nest sites experiencing the greatest landscape-scale fire severity relative to the lowest-severity areas. The finding that blue orchard bees biased offspring production towards the more expensive offspring sex with increasing fire severity shows a functional response to changes in habitat quality through increased density of flowering plants. Our findings indicate that burned mixed-conifer forest provides forage for the blue orchard bee across a severity gradient, and that the increase in floral resources that follows high-severity fire leads females to shift resource allocation to the more costly sex when nesting. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Field Ornithology is the property of Resilience Alliance and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Fire mosses, including Ceratodon purpureus , Funaria hygrometrica and Bryum argenteum , can achieve high cover within months to years after high-severity fire, but do so heterogeneously across space and time. We conducted a survey of moss cover and erosion-related functions after 10 wildfires in Pinus ponderosa and mixed-conifer forests of the southwestern USA. We sampled 65 plots in high-severity patches, stratifying by elevation and insolation over each fire. Using three landscape-scale predictor variables and one temporal predictor, we explained 37% of the variance in fire moss cover using a random forest model. The predictors in order of importance were: equinox insolation (sunlight/day), pre-fire vegetation type, pre-fire soil organic carbon and time since fire. Within each plot we examined differences between bare and moss-covered soil surface microsites and found moss-covered microsites had a mean increase of 55% water infiltration, 106% shear strength, 162% compressive strength and 195% aggregate stability. We tested a suite of nutrients, finding 35% less manganese in the moss-covered soil. This research demonstrated that post-fire colonisation by moss is predictable and that colonisation improves soil surface erosion resistance and hydrological function, with implications for managing severely burned landscapes. Fire mosses are early successional species that can colonise severely burned landscapes. We conducted a survey of 10 wildfires in the southwestern USA. Fire moss cover increased in shaded north-facing hillslopes in relatively wet locations. When compared with the bare soil surface, fire mosses reduced erosion and enhanced water infiltration. [ABSTRACT FROM AUTHOR]

Aims: Wildfires in dry forest ecosystems in western North America are producing fire effects that are more severe than historical estimates, raising concerns about the resilience of these landscapes to contemporary disturbances. Despite increasing fire activity, relatively little is known about the structure and composition of fire refugia — unburned or low‐severity burned patches where trees survived fire — or the degree to which their understory composition differs from fire‐generated early‐seral forests. Locations: Four recent large fires in dry mixed‐conifer forest in eastern Oregon, USA. Methods: We sampled vegetation and environmental factors in 187 plots (100‐m2) in fire refugia (n = 52) and stand‐replacement patches (SRPs) (n = 135). We used non‐metric multidimensional scaling, indicator species analysis, and randomization tests to compare understory plant communities and diversity in fire refugia and SRPs 12–17 years post‐fire. Results: Understory plant communities in fire refugia and SRPs showed strong compositional affinities, but exhibited shifts in reproductive trait prevalence and differences in landscape‐scale species richness. There were no differences in plot‐scale species richness or the occurrence of two prominent invasive annual grasses (Bromus tectorum, Ventenata dubia). The abundance of common obligate seeding species was similar between plot types, but resprouting and seed‐banking species became substantially more abundant in SRPs. Significantly more plant species occurred only in fire refugia, and significantly fewer in SRPs, than expected by chance. Conclusions: Similarities in understory plant communities between fire refugia and early‐seral forest in SRPs provide evidence for the resilience of historically frequent‐fire forest understory communities to contemporary fire effects. However, fire refugia may serve an ecologically important role as patches with relatively intact forest structure, seed sources that contribute to post‐fire forest recovery, and as reservoirs of species and age structure that may be absent from the higher‐severity burned matrix. [ABSTRACT FROM AUTHOR]

Context: Fire regimes in many dry forests of western North America are substantially different from historical conditions, and there is concern about the ability of these forests to recover following severe wildfire. Fire refugia, unburned or low-severity burned patches where trees survived fire, may serve as essential propagule sources that enable forest regeneration. Objectives: To quantify the influence of fire refugia spatial pattern and other biophysical factors on the process of post-fire tree regeneration; in particular examining both the proximity and density of surrounding refugia to characterize the landscape of refugial seed sources. Methods: We surveyed regeneration at 135 sites in stand-replacement patches across a gradient of fire refugia density in eastern Oregon, USA. We characterized the influence of refugial seed source pattern and other environmental factors on the abundance of regenerating seedlings, and examined the relationship between post-fire climate and the temporal pattern of ponderosa pine seedling establishment. Results: Tree seedlings were present in 83% of plots 12–17 years post-fire, and densities varied substantially (0–67800 stems ha−1, median = 1100). Variation in seedling abundance was driven by the spatial patterns of refugial seed sources. Despite widespread post-fire shrub cover, high-severity burned forests have not undergone a persistent type conversion to shrublands. Ponderosa pine seedling establishment peaked 5–11 years after fire and was not closely associated with post-fire climate. Conclusions: Fire refugia and the seed sources they contain fostered tree regeneration in severely burned patches. Management practices that reduce refugia within post-fire landscapes may negatively influence essential forest recovery processes. [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]

Many western North American forest types have experienced considerable changes in ecosystem structure, composition, and function as a result of both fire exclusion and timber harvesting. These two influences co-occurred over a large portion of dry forests, making it difficult to know the strength of either one on its own or the potential for an interaction between the two. In this study, we used contemporary remeasurements of a systematic historical forest inventory to investigate forest change in the Sierra Nevada. The historical data opportunistically spanned a significant land management agency boundary, which protected part of the inventory area from timber harvesting. This allowed for a robust comparison of forest change between logged and unlogged areas. In addition, we assessed the effects of recent management activities aimed at forest restoration relative to the same areas historically, and to other areas without recent management. Based on analyses of 22,007 trees (historical, 9,573; contemporary, 12,434), live basal area and tree density significantly increased from 1911 to the early 2000s in both logged and unlogged areas. Both shrub cover and the proportion of live basal area occupied by pine species declined from 1911 to the early 2000s in both areas, but statistical significance was inconsistent. The most notable difference between logged and unlogged areas was in the density of large trees, which declined significantly in logged areas, but was unchanged in unlogged areas. Recent management activities had a varied impact on the forest structure and composition variables analyzed. In general, areas with no recent management activities experienced the greatest change from 1911 to the early 2000s. If approximating historical forest conditions is a land management goal the documented changes in forest structure and composition from 1911 to the early 2000s indicate that active restoration, including fire use and mechanical thinning, is needed in many areas. [ABSTRACT FROM AUTHOR]

Wildfire refugia are forest patches that are minimally-impacted by fire and provide critical habitats for fire-sensitive species and seed sources for post-fire forest regeneration. Wildfire refugia are relatively understudied, particularly concerning the impacts of subsequent fires on existing refugia. We opportunistically re-visited 122 sites classified in 1994 for a prior fire refugia study, which were burned by two wildfires in 2012 in the Cascade mountains of central Washington, USA. We evaluated the fire effects for historically persistent fire refugia and compared them to the surrounding non-refugial forest matrix. Of 122 total refugial (43 plots) and non-refugial (79 plots) sites sampled following the 2012 wildfires, one refugial and five non-refugial plots did not burn in 2012. Refugial sites burned more severely and experienced higher tree mortality than non-refugial plots, potentially due to the greater amount of time since the last fire, producing higher fuel accumulation. Although most sites maintained the pre-fire development stage, 19 percent of sites transitioned to Early development and 31 percent of sites converted from Closed to Open canopy. These structural transitions may contribute to forest restoration in fire-adapted forests where fire has been excluded for over a century, but this requires further analysis. [ABSTRACT FROM AUTHOR]

Fire is returning to many conifer-dominated forests where species composition and structure have been altered by fire exclusion. Ecological effects of these fires are influenced strongly by the degree of forest change during the fire-free period. Response of fire-adapted species assemblages to extended fire-free intervals is highly variable, even in communities with similar historical fire regimes. This variability in plant community response to fire exclusion is not well understood; however, ecological mechanisms such as individual species' adaptations to disturbance or competition and underlying site characteristics that facilitate or impede establishment and growth have been proposed as potential drivers of assemblage response. We used spatially explicit dendrochronological reconstruction of tree population dynamics and fire regimes to examine the influence of historical disturbance frequency (a proxy for adaptation to disturbance or competition), and potential site productivity (a proxy for underlying site characteristics) on the stability of forest composition and structure along a continuous ecological gradient of pine, dry mixed-conifer, mesic mixed-conifer, and spruce-fir forests following fire exclusion. While average structural density increased in all forests, species composition was relatively stable in the lowest productivity pine-dominated and highest productivity spruce-fir-dominated sites immediately following fire exclusion and for the next 100 years, suggesting site productivity as a primary control on species composition and structure in forests with very different historical fire regimes. Species composition was least stable on intermediate productivity sites dominated by mixed-conifer forests, shifting from primarily fire-adapted species to competition-adapted, fire-sensitive species within 20 years of fire exclusion. Rapid changes to species composition and stand densities have been interpreted by some as evidence of high-severity fire. We demonstrate that the very different ecological process of fire exclusion can produce similar changes by shifting selective pressures from disturbance-mediated to productivity-mediated controls. Restoring disturbance-adapted species composition and structure to intermediate productivity forests may help to buffer them against projected increasing temperatures, lengthening fire seasons, and more frequent and prolonged moisture stress. Fewer management options are available to promote adaptation in forest assemblages historically constrained by underlying site productivity. [ABSTRACT FROM AUTHOR]

Copyright of Fire Ecology is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

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]

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Mixed-severity fire regimes may be the most extensive yet poorly understood fire regimes of western North America. Understanding their long-term spatiotemporal dynamics is central to debates regarding altered fire regimes and the need for restoration in the context of changing climate and nearly a century of active fire suppression. However, the complexity of fire patterns and forest stand and landscape structures characteristic of mixed-severity regimes poses a substantial challenge to understanding their long-term dynamics. In this study, we develop analysis methods aimed at understanding the fire-driven forest dynamics of mixed-severity systems and apply them in the San Juan Mountains of southwestern Colorado. We sampled fire scars, stand structure, and >4300 tree ages across two 1340-ha landscapes (Williams Creek and Squaretop Mountain) that span the environmental gradient of montane mixed-conifer and aspen forests. New approaches were applied to identify pulses of tree recruitment, evaluate climate and fire as potential drivers of synchronous recruitment pulses, and combine fire scar and recruitment data to reconstruct fires. The reconstructions provided detailed fire history for each stand, which in turn was used to develop a fire-severity metric, compare fire frequency and severity by forest type, and develop a simulation procedure to evaluate the degree to which tree regeneration depended on fire by species within each forest type. Twenty fires were reconstructed since 1685 at Williams Creek and 13 fires since 1748 at Squaretop Mountain. Patterns of fire severity varied within each fire and over successive events, including high-severity patches of hundreds of hectares in both study areas. Dry mixed-conifer forests experienced relatively short fire intervals (mean 21 years) and low fire severity, and regeneration of the main conifer species was largely dependent on open conditions sustained over successive fires. Moist mixed-conifer forests experienced longer fire intervals (mean 32 years) and a broader range of severities, and fire-caused canopy openings were important for initiating pulses of tree recruitment. Most (83%) aspen stands included two or more post-fire cohorts. The methods presented here can be adapted to other mixed-severity systems to better understand their long-term spatial and temporal dynamics and develop restoration priorities. [ABSTRACT FROM AUTHOR]

Interspecific variation in stomatal conductance ( GS) and transpiration ( EL) has been documented in stands of co-occurring species, and this variation has been observed to differ with tree size and canopy height increase. In this study, we present data that examine fluctuations in canopy gas exchange across co-occurring species and varying canopy heights for three montane forest chronosequences located in an inland Pacific Northwest mixed-conifer forest. With the exception of Douglas-fir ( Pseudotsuga menziesii var. glauca (Beissn.) Franco), we observed consistent declines in canopy EL and GS with increasing height for the majority of species examined in our 2-year study. Along with declines in canopy GS, we observed decreases in leaf-specific hydraulic conductance ( KL) across species as canopy height increased. Seasonally, we observed declines in canopy GS during warmer and dryer summer months of both years. These decreases in GS were significant (up to 50%) and suggest that carbon assimilation in trees was limited during dryer months due to a combination of high evaporative demand and reduced soil H2O availability. Such reductions in GS during periods of increased plant water stress suggest that forest productivity in the inland Pacific Northwest may be impacted negatively if future climate predictions of increasing growing-season water stress are realized. [ABSTRACT FROM AUTHOR]

We analyzed historical timber inventory data collected systematically across a large mixed-conifer-dominated landscape to gain insight into the interaction between disturbances and vegetation structure and composition prior to 20th century land management practices. Using records from over 20 000 trees, we quantified historical vegetation structure and composition for nine distinct vegetation groups. Our findings highlight some key aspects of forest structure under an intact disturbance regime: (1) forests were low density, with mean live basal area and tree density ranging from 8-30 m2 /ha and 25- 79 trees/ha, respectively; (2) understory and overstory structure and composition varied considerably across the landscape; and (3) elevational gradients largely explained variability in forest structure over the landscape. Furthermore, the presence of large trees across most of the surveyed area suggests that extensive stand-replacing disturbances were rare in these forests. The vegetation structure and composition characteristics we quantified, along with evidence of largely elevational control on these characteristics, can provide guidance for restoration efforts in similar forests. [ABSTRACT FROM AUTHOR]

The physical characteristics of landscapes place fundamental constraints on vegetation growth and ecosystem function. In actively eroding landscapes, many of these characteristics are controlled by long-term erosion rates: increased erosion rates generate steeper topography and reduce the depth and extent of weathering, limiting moisture storage capacity and impacting nutrient availability. Despite the potentially important bottom-up control that erosion rates place on substrate characteristics, the relationship between the two is largely unexplored. We investigate spatial variations in aboveground biomass (AGB) across a structurally diverse mixed coniferous/deciduous forest with an order of magnitude erosionrate gradient in the Northern Californian Sierra Nevada, USA, using high resolution LiDAR data and field plots. Mean basin slope, a proxy for erosion rate, accounts for 32% of variance in AGB within our field area (P, 0.001), considerably outweighing the effects of mean annual precipitation, temperature, and bedrock lithology. This highlights erosion rate as a potentially important, but hitherto unappreciated, control on AGB and forest structure. [ABSTRACT FROM AUTHOR]

Fuel treatment implementation in dry forest types throughout the western United States is likely to increase in pace and scale in response to increasing incidence of large wildfires. While it is clear that properly implemented fuel treatments are effective at reducing hazardous fire potential, there are ancillary ecological effects that can impact forest resilience either positively or negatively depending on the specific elements examined, as well as treatment type, timing, and intensity. in this study, we use overstory tree growth responses, measured seven years after the most common fuel treatments, to estimate forest health. Across the five species analyzed, observed mortality and future vulnerability were consistently low in the mechanical-only treatment. Fire-only was similar to the control for all species except Douglas-fir, while mechanical-plus-fire had high observed mortality and future vulnerability for white fir and sugar pine. Given that overstory trees largely dictate the function of forests and services they provide (e.g., wildlife habitat, carbon sequestration, soil stability) these results have implications for understanding longer-term impacts of common fuel treatments on forest resilience. [ABSTRACT FROM AUTHOR]

Twentieth-century land management has altered the structure and composition of mixed-conifer forests and decreased their resilience to fire, drought, and insects in many parts of the Interior West. These forests occur across a wide range of environmental settings and historical disturbance regimes, so their response to land management is likely to vary across landscapes and among ecoregions. However, this variation has not been well characterized and hampers the development of appropriate management and restoration plans. We identified mixed-conifer types in central Oregon based on historical structure and composition, and successional trajectories following recent changes in land use, and evaluated how these types were distributed across environmental gradients. We used field data from 171 sites sampled across a range of environmental settings in two subregions: the eastern Cascades and the Ochoco Mountains. We identified four forest types in the eastern Cascades and four analogous types with lower densities in the Ochoco Mountains. All types historically contained ponderosa pine, but differed in the historical and modern proportions of shade-tolerant vs. shade-intolerant tree species. The Persistent Ponderosa Pine and Recent Douglas-fir types occupied relatively hot- dry environments compared to Recent Grand Fir and Persistent Shade Tolerant sites, which occupied warm-moist and cold-wet environments, respectively. Twentieth-century selective harvesting halved the density of large trees, with some variation among forest types. In contrast, the density of small trees doubled or tripled early in the 20th century, probably due to land-use change and a relatively cool, wet climate. Contrary to the common perception that dry ponderosa pine forests are the most highly departed from historical conditions, we found a greater departure in the modern composition of small trees in warm-moist environments than in either hot-dry or cold-wet environments. Furthermore, shade-tolerant trees began infilling earlier in cold-wet than in hot-dry environments and also in topographically shaded sites in the Ochoco Mountains. Our new classification could be used to prioritize management that seeks to restore structure and composition or create resilience in mixed-conifer forests of the region. [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]

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]

1. Although it is recognized that plant species vary in their flammability, we currently lack a mechanistic understanding of how plant traits influence fire and how litter mixtures behave in a fire. As modified fire regimes and climate change shift the species composition of communities, a mechanistic perspective is especially important to understand and predict fire in potentially novel plant communities. This work addresses three questions: (i) How do eight species common in Sierra Nevada mixed-conifer forest differ in their litter flammability? (ii) What leaf traits are associated with various flammability components? and (iii) Do individual species measurements predict multi-species combinations, or are there non-additive effects? 2. Leaf litter was collected in Sequoia and Kings Canyon National Parks, California, from eight dominant tree species in mixed-conifer forest. Controlled flammability tests were performed on reconstructed monospecific litter beds and on mixed litter beds, using litter from three species. We tested for non-additive effects in multi-species mixtures using the weighted mean of single-species measures for each flammability component as a null expectation for each mixture; departures from this null indicated non-additive effects. 3. Most flammability components fell within two major axes of variation, one relating to total heat release and another to fire intensity. The eight species differed significantly in all flammability components, with large-leaved species creating litter that burned with higher intensity. 4. Non-additive species mixture effects are common in this system. Flammability tends to be driven by the most flammable component of the mixture. 5. Synthesis. We have demonstrated positive non-additive effects in mixtures of leaf litter. The most flammable constituent species of a mixture has disproportionate effects on the fire environment faced by the entire community. This could potentially influence community assembly and alter the selective environment faced by co-occurring species. [ABSTRACT FROM AUTHOR]

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]

The article presents a study which looks into the impact of the fire suppression, grazing and timber harvesting of the dry, ponderosa pine forests in the west part of the U.S. These forests, which have severely been experiencing a buildup of fuels as a result of the said activities, are suggested to be provided with an important ecological backdrop, such as the Black-backed Woodpecker, in order to prevent the same things from happening. On account of this, researchers of the study highlights several essential details gathered upon the research execution.

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]

• Understanding spatial and temporal patterns present in ectomycorrhizal fungal community structure is critical to understanding both the scale and duration of the potential impact these fungi have on the plant community. While recent studies consider the spatial structure of ectomycorrhizal communities, few studies consider how this changes over time.• Ectomycorrhizal root biomass and the similarity of community composition were measured at scales up to 20 cm replicated in nine plots and over 3 yr. Soil cores were additionally stratified into three depths.• Annual occurrence of the dominant ectomycorrhizal species was constant at larger spatial scales but varied more across years at a fine spatial scale. Turnover of ectomycorrhizal species between years was observed frequently at scales< 20 cm. The ectomycorrhizal community within a plot was more similar across years than it was to other plots sampled in the same year.• Our results demonstrate the dynamic nature of the ectomycorrhizal community even in the absence of large-scale disturbances. The potential role of root turnover and drought stress is discussed.New Phytologist(2005)doi: 10.1111/j.1469-8137.2005.01354.x© New Phytologist(2005) [ABSTRACT FROM AUTHOR]

The article focuses on a study related to modeling the effect of alternative fire management for the forests in the U.S. and Sierra Nevada, California. Topics discussed include use of forest dynamics regarding investigating forest response to changes in the fire regime, deterioration of fire-dependent ecosystems, and use of forest gap model to integrates climate, fire and forest dynamics.

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]

The development of U.S. national park vegetation management policies is briefly traced from 1872, when Yellowstone National Park was created, to the present. Ambiguities in legislative policies are described and partially resolved. Alternative vegetation management objectives, consistent with those policies, are evaluated using quantitative data from a giant sequoia-mixed-conifer forest community in Kings Canyon National Park. It is concluded from an analysis of this data that structural maintenance objectives are biologically infeasible in this forest community because the community does not have a fully regulated or steady-state distribution of aggregation types. Since such a steady-state distribution of aggregation types is probably not present in most forest communities, the Park Service is generally restricted to pursuing process maintenance objectives for vegetation within national parks and monuments. It is also pointed out that implementation of some process maintenance objectives is complicated by the need to return the forest community to its pre-fire exclusion state before reintroducing fire to the ecosystem. To do otherwise would perpetuate the changes in species composition and structure that have resulted from more than three-quarters of a century of fire exclusion. A new alternative for managing vegetation in national parks that is based on a high resolution description of the presettlement forest community, called the reconstruction-simulation approach, is also presented. [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]