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Fuel and restoration treatments seeking to mitigate the likelihood of uncharacteristic high-severity wildfires in forests with historically frequent, low-severity fire regimes are increasingly common, but long-term treatment effects on fuels, aboveground carbon, plant community structure, ecosystem resilience, and other ecosystem attributes are understudied. We present 20-year responses to thinning and prescribed burning treatments commonly used in dry, lowelevation forests of the western United States from a long-term study site in the Northern Rockies that is part of the National Fire and Fire Surrogate Study. We provide a comprehensive synthesis of short-term (<4 years) and mid-term (<14 years) results from previous findings. We then place these results in the context of a mountain pine beetle (MPB; Dendroctonus ponderosae) outbreak that impacted the site 5-10 years post-treatment and describe 20-year responses to assess the longevity of restoration and fuel reduction treatments in light of the MPB outbreak. Thinning treatments had persistently lower forest density and higher tree growth, but effects were more pronounced when thinning was combined with prescribed fire. The thinning +prescribed fire treatment had the additional benefit of maintaining the highest proportion of ponderosa pine (Pinus ponderosa) for overstory and regeneration. No differences in understory native plant cover and richness or exotic species cover remained after 20 years, but exotic species richness, while low relative to native species, was still higher in the thinning+prescribed fire treatment than the control. Aboveground live carbon stocks in thinning treatments recovered to near control and prescribed fire treatment levels by 20 years. The prescribed fire treatment and control had higher fuel loads than thinning treatments due to interactions with the MPB outbreak. The MPBinduced changes to forest structure and fuels increased the fire hazard 20 years post-treatment in the control and prescribed fire treatment. Should a wildfire occur now, the thinning+prescribed fire treatment would likely have the lowest intensity fire and highest tree survival and stable carbon stocks. Our findings show broad support that thinning and prescribed fire increase ponderosa pine forest resilience to both wildfire and bark beetles for up to 20 years, but efficacy is waning and additional fuel treatments are needed to maintain resilience. [ABSTRACT FROM AUTHOR]

A key uncertainty of empirical models of post‐fire tree mortality is understanding the drivers of elevated post‐fire mortality several years following fire, known as delayed mortality. Delayed mortality can represent a substantial fraction of mortality, particularly for large trees that are a conservation focus in western US coniferous forests. Current post‐fire tree mortality models have undergone limited evaluation of how injury level and time since fire interact to influence model accuracy and predictor variable importance. Less severe injuries potentially serve as an indicator for vulnerability to additional stressors such as bark beetle attack or moisture stress. We used a collection of 164,293 individual tree records to examine post‐fire tree mortality in eight western USA conifers: Abies concolor, Abies grandis, Calocedrus decurrens, Larix occidentalis, Pinus contorta, Pinus lambertiana, Pinus ponderosa, and Pseudotsuga menziesii. We evaluated the importance of fire injury predictors on discriminating between surviving trees versus immediate and delayed post‐fire mortality. We fit balanced random forest models for each species using cumulative tree mortality from 1 to 5‐years post‐fire. We compared these results to multi‐class random forest models using first‐year mortality, 2–5‐year mortality, and survival 5‐years post‐fire as a response variable. Crown volume scorched, diameter at breast height, and relative bark char height, were used as predictor variables. The cumulative mortality models all predicted trees that died within 1‐year of fire with high accuracy but failed to predict 2–5‐year mortality. The multi‐class models were an improvement but had lower accuracy for predicting 2–5‐year mortality. Multi‐class model accuracies ranged from 85% to 95% across all species for predicting 1‐year post‐fire mortality, 42%–71% for predicting 2–5‐year mortality, and 64%–85% for predicting trees that lived past 5‐years. Our study highlights the differences in tree species tolerance to fire injury and suggests that including second‐order predictors such as beetle attack or climatic water stress before and after fire will be critical to improve accuracy and better understand the mechanisms and patterns of fire‐caused tree death. Random forest models have potential for management applications such as post‐fire harvesting and simulating future stand dynamics. [ABSTRACT FROM AUTHOR]

Forest restoration treatments primarily aimed at reducing fuel load and preventing high-severity wildfires can also influence resilience to other disturbances. Many pine forests in temperate regions are subject to tree-killing bark beetle outbreaks (e.g., Dendroctonus , Ips), whose frequency and intensity are expected to increase with future climatic changes. Restoration treatments have the potential to increase resistance to bark beetle attacks, yet the underlying mechanisms of this response are still unclear. While the effect of forest restoration treatments on tree growth has been studied, less is known about their impact on resin-based defenses. We measured axial resin ducts in the earlywood and latewood of ponderosa pines (Pinus ponderosa) in western Montana, USA, 20 years before and after the implementation of restoration treatments, with the aim to elucidate changes in the yearly and interannual investment in resin duct defenses following treatments and their sensitivity to climate. Two experiments were established in 1992: a moderate thinning and a retention shelterwood, with 35 % and 57 % basal area reduction, respectively. Each experiment comprised four treatments with three replicates per treatment: cutting only, cutting followed by prescribed burning in either spring or fall or under wet or dry duff moisture conditions, and an untreated control. Cutting treatments stimulated a long-term, sustained increase in resin duct production, more pronounced in the earlywood, which we attribute to a higher availability of resources due to reduced tree density. Prescribed burning following cutting induced a short-term increase in resin ducts, likely aiding in the compartmentalization of fire-killed cambium and enhancing the resistance of fire-injured trees to bark beetle attack. However, the fire-induced spike in duct production was not related to the degree of crown scorch. Treatments had little effect on climate-defense relationships, as ducts remained positively correlated to winter precipitation and, though less significantly, negatively correlated to spring maximum temperature. Our findings show that by reducing stand density, forest restoration treatments induce the synthesis of resin ducts, which are key in mitigating vulnerability of ponderosa pine to mountain pine beetle (D. ponderosae) attacks, thus promoting forest resilience to multiple disturbances. • Forest restoration treatments stimulate resin-based defenses. • Stand density reduction induces a long-term, sustained increase in resin ducts. • Prescribed burning following cutting induces a short-term increase in resin ducts. • Earlywood resin ducts are more sensitive to treatments than latewood ducts. • Treatments have a minor impact on climate-defense relationships. [ABSTRACT FROM AUTHOR]

Turkish pine (Pinus brutia) is a widely distributed species in fire-prone regions of the eastern Mediterranean, where it is of economic and ecological importance. After wildfires, damaged trees are usually salvage logged out of concern that fire-injured individuals will eventually die. While fire is a major cause of Turkish pine mortality, the species has traits, such as thick bark, that should confer some resistance to low-intensity fire. The aim of this study is to develop post-fire tree mortality models for Turkish pine to support post-fire management decisions. We selected 510 Turkish pine trees from four wildfires that occurred in the Mediterranean region of Türkiye in 2018 and 2019. Only fire-damaged trees that had initially survived the fires were selected. We measured tree diameter, height, and bark thickness and fire-caused injuries (crown volume scorched, bark char height, bark char index, cambium kill rating, and ground char index) within two months after fire. Stem-related variables were measured at three different heights (0, 50, and 130 cm). We then quantified delayed mortality for two years after each fire and developed mixed effects logistic regression models to predict probability of mortality. Most Turkish pines (84 %) survived, with the majority of mortality occurring during the first year post-fire. Crown volume scorched and stem damage variables were the most important predictors of mortality. Models were selected based on their explanatory power and practical use in the field. Among these, the most explanatory model includes crown volume scorched and bark char index measured at 0 cm stem height. The suggested model for use in forestry includes crown volume scorched and diameter at breast height. Some trees were able to survive and flush new foliage during the first season after fire despite 100 % crown scorch due to low bud kill. Our results show that Turkish pine can survive fire when stem injury is low. The mortality models can assist managers in making more evidence-based decisions for post-fire management and developing more ecologically sustainable post-fire management operations. • Turkish pine can survive high levels of crown scorch if cambium damage is low. • Turkish pine post-fire mortality is largely apparent at the end of the first year. • Crown scorch and DBH are significant predictors of Turkish pine mortality. • Prescribed burning could be used as a management tool in Turkish pine forests. [ABSTRACT FROM AUTHOR]

As the climate warms, drought will increasingly occur under elevated temperatures, placing forest ecosystems at growing risk of extensive dieback and mortality. In some cases, increases in tree density following early 20th‐century fire suppression may exacerbate this risk. Treatments designed to restore historical stand structure and enhance resistance to high‐severity fire might also alleviate drought stress by reducing competition, but the duration of these effects and the underlying mechanisms remain poorly understood. To elucidate these mechanisms, we evaluate tree growth, mortality, and tree‐ring stable‐carbon isotope responses to stand‐density reduction treatments with and without prescribed fire in a ponderosa pine forest of western Montana. Moderate and heavier cutting experiments (basal area reductions of 35% and 56%, respectively) were initiated in 1992, followed by prescribed burning in a subset of the thinned units. All treatments led to a growth release that persisted to the time of resampling. The treatments had little effect on climate–growth relationships, but they markedly altered seasonal carbon isotope signals and their relationship to climate. In burned and unburned treatments, carbon isotope discrimination (Δ13C) increased in the earlywood (EW) and decreased in the latewood (LW) relative to the control. The sensitivity of LW Δ13C to late‐summer climate also increased in all treatments, but not in the control. Such increased sensitivity indicates that the reduction in competition enabled trees to continue to fix carbon for new stem growth, even when the climate became sufficiently stressful to stop new assimilation in slower‐growing trees in untreated units. These findings would have been masked had we not separated EW and LW. The importance of faster growth and enhanced carbon assimilation under late‐summer climatic stress became evident in the second decade post‐treatment, when mountain pine beetle activity increased locally, and tree mortality rates in the controls of both experiments increased to more than twice those in their respective treatments. These findings highlight that, when thinning is used to restore historical forest structure or increase resistance to high‐severity fire, there will likely be additional benefits of enhanced growth and physiological activity under climatic stress, and the effects may persist for more than two decades. [ABSTRACT FROM AUTHOR]

In the western United States, restoration of forests with historically frequent, low‐severity fire regimes often includes fuel reduction that reestablish open, early‐seral conditions while reducing fuel continuity and loading. Between 2001 and 2016, fuel reduction (e.g., thinning, prescribed burning, etc.) was implemented on over 26 million hectares of federal lands alone in the United States, reflecting the urgency to mitigate risk from high‐severity wildfire. However, between 2001 and 2012, nearly 20 million hectares in the United States were impacted by mountain pine beetle (MPB; Dendroctonus ponderosae), compounding restoration effects in wildfire‐hazard‐treated stands. Knowledge of the effects of treatments followed by natural disturbance on long‐term forest structure and communities is needed, especially considering that fuel treatments are increasingly being implemented and warming climate is predicted to exacerbate disturbance frequency and severity. We tested the interacting effects of treatments designed to reduce high‐severity wildfire hazard in stands subsequently challenged by MPB outbreak on vegetation dynamics using a factorial experimental design (control, thin only, burn only, thin + burn) in a ponderosa pine (Pinus ponderosa)‐dominated forest. Stands were treated by 2002, then impacted by MPB outbreak from 2005 to 2012. We assessed change in overstory and understory forest community structure, composition, and diversity over time. There were distinct thinning, burning, and year effects. Thinning immediately reduced overstory density; pine density then declined 4.5 times more in unthinned than thinned treatments due to MPB. Burning immediately reduced graminoid, shrub, and total understory cover by as much as 52%, resulting in greater species evenness than unburned treatments, but differences disappeared by 2016 due to growth and MPB outbreak. Similarly, multivariate analyses indicated forest communities were starkly different after treatment but became more similar over time, though key understory and overstory attributes still distinguish control and thin + burn. This study shows the value of long‐term silvicultural experiments to evaluate treatment longevity and the compounded effects of treatment and natural disturbance. We demonstrate the homogenizing effects of treatment‐induced growth coupled with MPB‐caused tree mortality on management strategies that just treat the overstory (thinning) or understory (burning), showing that only combined treatments can provide the unique structural and compositional outcomes expected of restoration. [ABSTRACT FROM AUTHOR]

Structurally diverse forests provide resilience to an array of disturbances and are a mainstay of multiple-resource management. Silviculture based on natural disturbance can increase structural heterogeneity while providing other ecological and economic benefits. One useful silvicultural tool for promoting structural heterogeneity is retention harvesting, whereby a portion of forest stands are left unlogged, transitioning even-aged stands to multi-aged. We report stand and tree dynamics 11 years after retention harvest in a central Montana Rocky Mountain lodgepole pine forest with evidence for a mixed-severity fire regime. Treatments were implemented on 16 experimental units with prescriptions for two 50% overstory basal area retention patterns (Aggregated and Dispersed) crossed with two levels of prescribed fire use (Burned and Unburned). The aim of this study was to identify (1) how retention harvest spatial pattern affects stand dynamics, (2) if stand dynamics after retention treatments are more simply attributable to tree size and competition, or if retention pattern affects dynamics beyond those measures, and (3) how stem and basal area heterogeneity varied over the 11-year measurement period. Retention pattern affected overstory density, growth, mortality, and regeneration density and stocking. After controlling for the fine-scale factors of tree size and competition, overstory mortality, regeneration stocking, and regeneration height growth did not vary by treatment-scale factors. Fine-scaled factors explained significant variation in overstory basal area growth, but at the scale of experimental units, growth was also greater in Dispersed treatments. Prescribed burning interacted with retention pattern to influence overstory tree growth, increased overstory mortality, and increased regeneration height growth. Overstory heterogeneity (e.g., in basal area) degraded more rapidly in treatments with the Dispersed spatial pattern than Aggregated. This study evaluates novel silvicultural treatments in a lodgepole pine forest and highlights the tradeoffs between retention patterns combined with broadcast burning on forest change. Our results are useful for planning silvicultural treatments in multiple-use forests designed to promote structural complexity and resilience to disturbances. [ABSTRACT FROM AUTHOR]

Fuel treatments in ponderosa pine forests of the northern Rocky Mountains are commonly used to modify fire behavior, but it is unclear how different fuel treatments impact the subsequent production and distribution of aboveground biomass, especially in the long term. This research evaluated aboveground biomass responses 23 years after treatment in two silvicultural installations with different cutting and underburning prescriptions in western Montana. The thinning installation included control (no treatment), thin/no burn, thin/spring burn, and thin/fall burn treatments. The shelterwood installation included control, cut/no burn, cut/wet burn, and cut/dry burn treatments. Across all fuel treatments in both the thinning and shelterwood installations, tree biomass had recovered to pre-harvest levels by 2015, or 23 years post-treatment. In the thinning, total aboveground and live-tree biomass were greatest in the control, but did not differ among the three thinned fuel treatments. Forest floor biomass was lower in the two burned treatments relative to the two unburned treatments. Seedling, vegetation, stump, and snag biomass did not differ among the four treatments. In the shelterwood, total aboveground and live-tree biomass were both greater in the unburned treatments relative to the burned treatments. Forest floor and snag biomass also tended to be lower in the burned treatments. Seedling, vegetation, and stump biomass were similar across all treatments. This research shows that tree biomass in ponderosa pine stands subjected to common fuels treatments can recover to pre-harvest levels in less than 23 years, while still exhibiting reduced stand densities that promote forest restoration objectives. Burgeoning biomass at the seedling layer suggests that additional understory treatments are necessary in order to abate ladder fuel development and sustain resistance to high-severity wildfire. [ABSTRACT FROM AUTHOR]

Fire is a driving force in the North American landscape and predicting post-fire tree mortality is vital to land management. Post-fire tree mortality can have substantial economic and social impacts, and natural resource managers need reliable predictive methods to anticipate potential mortality following fire events. Current fire mortality models are limited to a few species and regions, notably Pinus ponderosa and Pseudotsuga menziesii in the western United States. The efficacy of existing mortality models to predict fire-induced tree mortality is central to effective forest management. This study validated 54 logistic regression mortality models from seven published articles and two sets of mortality guidelines from two sources. Survival and a suite of fire injury metrics were monitored for 3654 trees representing 14 species that burned in fires between 2002 and 2009 in the Pacific Northwest, USA. Tree species included Abies amabilis , A . concolor , A . grandis , A . lasiocarpa , Calocedrus decurrens , Chamaecyparis lawsoniana , C. nootkatensis , Thuja plicata , Pinus contorta , P . lambertiana , P . monticola , Picea engelmannii , Larix occidentalis , and Tsuga heterophylla . Existing logistic models adequately described post-fire mortality of A . concolor , A . lasiocarpa , C . decurrens , C . lawsoniana , L . occidentalis , P . engelmannii , P . contorta , and P . lambertiana . We also evaluated predictive accuracy of two published mortality guidelines that apply to species in the Pacific Northwest. In addition to validating existing models, we also developed new logistic regression models and simplified mortality guidelines, or thresholds. We created new logistic regression models for species with adequate sample size and which had no existing species-specific model ( A . amabilis , A . grandis , P . monticola , and T . heterophylla ). Most recommended models contained a crown scorch term and either a cambium injury term or a bark beetle infestation term. New post-fire mortality thresholds were developed for A . amabilis , A . concolor , A . grandis , P . contorta , P . lambertiana , P . monticola , P . engelmannii , L . occidentalis , and T . heterophylla . We were not able to validate or develop acceptable logistic mortality models or thresholds for C . nootkatensis or T . plicata . Injury to cambium and crown were both significant predictors in all but one set of new thresholds. The validation of existing models and guidelines allows managers to determine which models will likely perform best and identifies knowledge gaps where no adequate models exist to predict post-fire tree mortality. The new logistic regression models and threshold guidelines provide improved accuracy, with simpler application for fire and forest management. [ABSTRACT FROM AUTHOR]

Fire frequency in low-elevation coniferous forests in western North America has greatly declined since the late 1800s. In many areas, this has increased tree density and the proportion of shade-tolerant species, reduced resource availability, and increased forest susceptibility to forest insect pests and high-severity wildfire. In response, treatments are often implemented with the goal of increasing ecosystem resilience by increasing resistance to disturbance. We capitalized on an existing replicated study of fire and stand density treatments in a ponderosa pine ( Pinus ponderosa)-Douglas-fir ( Pseudotsuga menziesii) forest in western Montana, USA, that experienced a naturally occurring mountain pine beetle ( MPB; Dendroctonus ponderosae) outbreak 5 yr after implementation of fuels treatments. We explored whether treatment effects on tree-level defense and stand structure affected resistance to MPB. Mortality from MPB was highest in the denser, untreated control and burn-only treatments, with approximately 50% and 39%, respectively, of ponderosa pine killed during the outbreak, compared to almost no mortality in the thin-only and thin-burn treatments. Thinning treatments, with or without fire, dramatically increased tree growth and resin ducts relative to control and burn-only treatments. Prescribed burning did not increase resin ducts but did cause changes in resin chemistry that may have affected MPB communication and lowered attack success. While ponderosa pine remained dominant in the thin and thin-burn treatments after the outbreak, the high pine mortality in the control and burn-only treatment caused a shift in species dominance to Douglas-fir. The high Douglas-fir component in the control and burn-only treatments due to 20th century fire exclusion, coupled with high pine mortality from MPB, has likely reduced resilience of this forest beyond the ability to return to a ponderosa pine-dominated system in the absence of further fire or mechanical treatment. Our results show treatments designed to increase resistance to high-severity fire in ponderosa pine-dominated forests in the Northern Rockies can also increase resistance to MPB, even during an outbreak. This study suggests that fuel and restoration treatments in fire-dependent ponderosa pine forests that reduce tree density increase ecosystem resilience in the short term, while the reintroduction of fire is important for long-term resilience. [ABSTRACT FROM AUTHOR]

• Radial thinning aims to reduce focal tree mortality by reducing local competition. • 16 years post-treatment, large sugar pine mortality did not differ among treatments. • Radial thinning 12.5 m around surviving focal trees increased tree diameter growth. • Radial thinning 12.5 m around surviving focal trees increased sugar pine seedlings. • No clear, consistent benefits resulted from the radial thinning treatments tested. White pines (Pinus , subgenus Strobus) in North America are highly vulnerable to the native mountain pine beetle (Dendroctonus ponderosae), invasive white pine blister rust (Cronartium ribicola), and consequences of altered disturbance regimes, with sugar pine (Pinus lambertiana) being among the most in peril. Radial thinning treatments reflect an effort to balance the needs of late seral, high forest cover for some wildlife species with the desire to also promote habitat for species requiring higher light and lower competition, as well as to create heterogenous forest structure that can be more resilient to wildfire and drought. We examined 16-year effects of three radial thinning treatments that cleared approximately 0.017 ha to 0.05 ha around focal large sugar pines on reducing tree mortality and increasing sugar pine recruitment relative to untreated, control trees. Thinning treatments by order of increasing intensity included a compressed radius treatment which removed all trees and shrubs to 3 m from the crown edge, an extended radius with large tree retention treatment which removed trees <64 cm DBH and shrubs to 7.6 m from the crown edge, and an extended radius treatment which removed all trees and shrubs to 7.6 m outside of the crown edge. None of the three radial thinning treatments tested reduced mortality relative to the control, and mountain pine beetle caused the most mortality across the four treatments. However, mortality trended lower in the extended radius with retention treatment (19% compared to 30–37%) and was the only treatment where no mortality from wind or insects other than mountain pine beetle occurred. For trees that survived through the 16-year monitoring period, only trees in the extended radius treatment experienced increased growth above pre-treatment values and higher presence of sugar pine seedlings. Our results show no clear benefit was derived from the radial thinning treatments tested. While radial thinning may be a useful tool when used in conjunction with other silvicultural prescriptions that create heterogeneous stands and landscapes with a variety of forest structures, it seems unlikely that employing only radial thinning will meet restoration efforts that aim to decrease large tree mortality. This study highlights the value of long-term monitoring in the continual effort to develop new treatments that are effective at increasing forest resilience. [ABSTRACT FROM AUTHOR]

• Giant sequoias are a long-lived, fire-adapted species of limited distribution. • The extent of high severity fire in giant sequoia forests is rapidly increasing. • Large sequoias had 84% mortality in remote sensing-defined high severity areas. • Delayed mortality was predicted by percent crown damage and presence of a fire scar. • Forest treatments that reduce fire severity could help protect this unique species. Ancient giant sequoias Sequoiadendron giganteum (Lindl.) J. Buchholz are highly valued trees with limited distribution. They are the most massive trees on earth, and they have exceptional longevity (>3,000 years). Given their extraordinary resilience to wildfire, insects, and disease, most managers hoped giant sequoias would be resistant to anthropogenic change. However, the warming climate and fire exclusion-caused fuel build-up has dramatically increased the amount of high-intensity wildfire in giant sequoia groves since 2015. To better understand recent wildfire effects on large, legacy giant sequoia trees, we surveyed high- and moderate-severity fire areas in groves that burned in three wildfires between 2015 and 2017. Within areas classified as high severity via remote sensing, ∼84% of the 162 large sequoias (>1.2 m diameter at breast height) were killed; in moderate severity, ∼28% of the 239 large sequoias were killed. These rates varied between fires but overall were at the lower end of the mortality range that has been documented in Sierra Nevada mixed-conifer forests. Across all three fire areas, we found large sequoia mortality was linked with presence of significant prior fire damage to the lower tree bole (i.e., fire scars or "catfaces"), lower crown ratios, and lower elevations. At Black Mountain Grove, we tracked trees through time and found total crown damage and the presence of fire scars were the best predictors of mortality by year three, where large sequoias sustained up to ∼ 85% crown damage before succumbing to injury. There, 52 of 204 trees were dead one-year postfire, with 71 dead by year three. Overall, these fires caused significant and concerning mortality levels of these ancient trees, and yet the impact on sequoia groves between 2015 and 2017 was dwarfed in comparison to the more recent 2020 Castle Fire, 2021 Windy Fire and 2021 KNP Complex Fire. With the push to increase forest and fuel treatments to reduce catastrophic fire risk across the Western U.S., more immediate triage to protect irreplaceable resources is needed as fire activity rapidly increases. Our study suggests that prioritizing management action that creates fire resilience in unburned groves, and maintains the fuel reduction benefits accrued by past treatments and lower severity wildfires, will be critical to avoid continued losses of these exceptional, irreplaceable old growth forests. [ABSTRACT FROM AUTHOR]

Abstract: Fire injury was characterized and survival monitored for 5677 trees >25cm DBH from five wildfires in California that occurred between 2000 and 2004. Logistic regression models for predicting the probability of mortality 5-years after fire were developed for incense cedar (Calocedrus decurrens (Torr.) Florin), white fir (Abies concolor (Gord. & Glend.) Lindl. ex Hildebr.), sugar pine (Pinus lambertiana Douglas), Jeffrey pine (P. jeffreyi Balf.), and ponderosa pine (P. ponderosa C. Lawson). Differences in crown injury variables were also compared for Jeffrey and ponderosa pine. Most mortality (70–88% depending on species) occurred within 2 years post-wildfire and had stabilized by year 3. Crown length and crown volume injury variables predicted tree mortality equally well; however, the variables were not interchangeable. Crown injury and cambium kill rating was significant in predicting mortality in all models. DBH was only a significant predictor of mortality for white fir and the combined ponderosa and Jeffrey pine models developed from the McNally Fire; these models all predicted increasing mortality with increasing tree size. Red turpentine beetle (Dendroctonus valens) was a significant predictor variable for sugar pine, ponderosa pine, and Jeffrey pine; ambrosia beetle (Trypodendron and Gnathotrichus spp.) was a significant predictor variable for white fir. The mortality models and post-fire tree survival characteristics provide improved prediction of 5-year post-wildfire tree mortality for several California conifers. The models confirm the overall importance of crown injury in predicting post-fire mortality compared to other injury variables for all species. Additional variables such as cambium kill, bark beetles, and tree size improved model accuracies, but likely not enough to justify the added expense of data collection. [Copyright &y& Elsevier]

• P. sylvestris is less resistant to bark beetle attack than P. nigra. • In P. sylvestris bark beetle attack increases with tree size. • In P. nigra bark beetle attack is limited to smaller trees. • Interspecific variations in bark thickness and duct area explain size preferences. • Intraspecific variation in duct metrics and grow regulate tree resistance to attack. After fire, bark beetles pose a significant threat to trees. Resin duct characteristics in trees can increase resistance to bark beetles. However, little is known about how intra- and interspecific variations in resin ducts due to tree characteristics, fire-caused tree injury and life history traits contribute to resistance. In fall 2013, a mixed-stand of Pinus sylvestris and P. nigra was underburned, with some trees subsequently attacked and killed by bark beetles (Ips sexdentatus) during the following two years. We investigated whether inter- or intraspecific differences in constitutive defenses (i.e. bark thickness, tree size and growth, resin ducts) or fire-caused tree injury could explain tree resistance to bark beetles. Beetles preferentially attacked P. sylvestris over the more fire-resistant P. nigra subsp. salzmannii. In P. nigra, attacks were limited to smaller trees, whereas the probability of mortality from I. sexdentatus increased with diameter in P. sylvestris. The decrease in bark thickness along the stem significantly affected the probability of mortality in P. sylvestris but not in P. nigra. As tree size increased, bark thickness and resin duct area investment were lower in P. sylvestris than P. nigra, suggesting reduced defences in P. sylvestris. For both species, pines that survived had faster growth, higher resin duct area, and fewer but larger ducts than pines that died after the attack. However, resin duct area and density rather than growth or fire-caused injury were better predictors of tree resistance. Moreover, in unattacked trees resin duct area increased with diameter in P. nigra. Our study showed that bark beetles attacks were tree species and size specific, but ultimately resin duct characteristics determined host colonization. Our findings suggest managers can expect higher delayed mortality from bark beetles in P. sylvestris after burning; however, actions that encourage faster growth may increase resin duct-related defences in both pine species. [ABSTRACT FROM AUTHOR]

• No evidence of tradeoff between growth/defense among trees in natural forest stands. • Differences in resin chemistry support field observations of bark beetle activity. • Whitebark pines produce more compounds that beetles exploit to enhance attack success. • Lodgepole pines produce more compounds that beetles exploit to recognize hosts. • Engelmann spruce negatively influences constitutive monoterpene concentrations. Recent, widespread tree mortality in the western U.S. resulting from changes in climate, pathogens, insect activity, and forest management practices has led to concerns for many ecologically and culturally important species. Within conifers, resin-based defenses have long been recognized as a primary defense mechanism against a variety of insects and pathogens. Oleoresin produced by trees contain complex mixtures of terpenoids that have numerous insecticidal and fungicidal properties. Research has also identified links between resin duct characteristics and increased probability of survival during bark beetle outbreaks. Whitebark pine (Pinus albicaulis) is a culturally significant high elevation species that provides numerous ecological services within subalpine and alpine ecosystems. Whitebark pine has co-evolved with a suite of biotic and abiotic disturbances. Individual trees allocate resources towards growth and resin-based defenses, making it a good candidate species to evaluate growth and defense relationships and tradeoffs. In this study we compared constitutive resin chemistry, tree growth and resin duct anatomy between similarly aged whitebark and lodgepole pine (P. contorta var latifolia) growing in proximity within a disturbance-prone, mixed-conifer forest in northwestern Montana. These two host species have varying degrees of historical exposure to mountain pine beetle. Our research yields four important findings. First, we did not find evidence of a tradeoff between tree growth and tree defenses (resin duct morphology and resin chemistry). This suggests that trees growing under favorable field conditions can experience high growth rates and still allocate ample resources towards defense. Second, we found that resin ducts and constitutive mono- and sesqui- terpenes were not correlated in lodgepole pine while duct production and area were positively related to constitutive monoterpenes, and duct size and area were positively related to constitutive sesquiterpenes, in whitebark pine. The lack of distinct, consistent relationships between these defensive features suggests that both whitebark and lodgepole pine trees present beetles with numerous, complex combinations of resin-based defenses. Third, based on constitutive terpene profiles, bark beetles are more likely to enter lodgepole pine but more likely to successfully elicit mass attacks in whitebark pine, which agrees with beetle attack and success patterns observed in the field. Fourth, overstory competition, particularly by Engelmann spruce (Picea engelmannii), can influence tree defenses, specifically by reducing constitutive terpene concentrations in lodgepole and whitebark pine. Competitive tree interactions could lead to altered bark beetle-conifer interactions as host and nonhost species migrate in response to changing climate. Our results suggest that strategies designed to support whitebark pine populations can benefit from better understanding interactions among growth, competition and physical and chemical defenses in response to multiple disturbance. [ABSTRACT FROM AUTHOR]

• Combined cutting and burning treatments exerted greatest impacts on understory. • Peak responses in understory cover and richness dampened over time. • Native understory vegetation communities were resilient to restoration treatment. • Non-natives fell to < 5% cover, despite a large spike 3–5 years post-treatment. • Overstory density controlled native but not non-native understory species. Restoration of fire-prone forests is a common practice intended to increase resilience to wildfire, drought, and bark beetles. However, the long-term effects of restoration treatments on understory species, particularly non-native species, are poorly understood. We investigated long-term (23 years) effects of restoration treatments on native and non-native understory vegetation at the Lick Creek Demonstration-Research Forest, a ponderosa pine/Douglas-fir forest in western Montana, USA. Restoration treatments included shelterwood-with-reserves harvesting implemented alone or in combination with prescribed burning under either wet or dry conditions, plus a no-action control. Changes in understory vegetation cover and species richness (measured pre-treatment, 2, 3, 4, 5, 15, and 23 years post-treatment) were analyzed by growth form (forb, graminoid, and shrub) and origin (native vs. non-native) to test how understory dynamics respond to disturbance intensity. We also accounted for basal area of trees to consider how differential overstory conditions affected final treatment responses. Native vegetation cover responses followed predictable successional patterns: after initial declines by grasses and shrubs immediately after disturbance (43% and 40% reduction, respectively), all groups increased, peaked about five years after disturbance (74% greater than pre-treatment on average) and then generally declined to levels similar to pre-treatment. The magnitude of these changes was related to the disturbance intensity (i.e., cut-and-dry-burn > cut-and-wet-burn > cut-and-no-burn > control). For all native groups, variation in tree basal area at year 23 explained remaining differences among treatments. Non-native grasses and forbs followed similar overall trajectories, but cover of these groups increased immediately after treatment and rose more steeply to their peak five years post-treatment (12 times greater than pre-treatment on average), with differences persisting through year 23. While non-native cover responses in year 23 were negatively correlated with tree basal area, as seen for native taxa, overstory conditions alone did not fully account for differences between burned and unburned treatments. This suggests that effects of burning unrelated to tree regeneration favored non-natives. Nonetheless, in the last sampling year treatment-induced increases in non-native cover were modest, suggesting that after 23 years understory responses in this forest type were fairly resilient to restoration treatments. However, because forest structure and composition were reverting to pre-treatment conditions by year 23, retreatment may be necessary at intervals of <23-years in our system, which could exacerbate non-native responses. Results of our long-term experiment provide insight into how different forest restoration treatments and their timing may affect understory susceptibility to non-native plant invasion. [ABSTRACT FROM AUTHOR]

• Multiple disturbances select for complex growth/defense strategies in whitebark pine. • Mean resin duct size was greater in trees that survived recent disturbances. • Resin duct size and relative duct area were positively related to survivorship. • Variations in seasonal climate result in complex responses in growth and defense. Whitebark pine (Pinus albicaulis) is a critical forest species of Northern Rocky Mountain upper subalpine ecosystems, yet little is known about the physiological response of whitebark pine to disturbance (e.g. fire, bark beetles, and pathogens) across a range of diverse environmental gradients. Resin-based defenses have long been recognized as the primary mechanism by which conifers respond to attack by bark beetles and pathogens and several studies have linked resin duct properties to survivorship during periods of increased beetle activity. However, to our knowledge, no studies have compared axial resin ducts in the secondary xylem of whitebark pine across pairs of living and dead whitebark pine trees to better understand survivorship following multiple disturbances including mountain pine beetle and white pine blister rust. We found a clear distinction in growth and defense characteristics between live and dead whitebark pine. Across our study sites on the Flathead Indian Reservation in northwestern Montana, live whitebark pine produced larger resin ducts with a greater annual investment in resin-based defenses than whitebark pine that died. Resin duct size, duct area, and relative duct area were all greater in live whitebark pine (by 56%, 48%, and 57%, respectively) and these were the most important variables influencing whitebark pine survivorship. In contrast, whitebark pine that had died grew faster over time (22% larger ring widths) than their live counterparts and also produced more resin duct structures (20% more ducts on average). Whitebark pine at our study sites exhibit differing strategies in the allocation of resources toward growth and defense, with the majority of survivors of recent disturbance investing more in defensive structures than growth. Our results support the idea that maintaining genetic variability and the associated suite of differing physiological traits promotes diverse response strategies to a complex array of biophysical and biological stressors that might leave a species vulnerable to extinction across its range. [ABSTRACT FROM AUTHOR]