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Whitebark pine (Pinus albicaulis) is an important component of high‐elevation ecosystems throughout the northern Rocky Mountains; however, the species was recently listed as "threatened" under the Endangered Species Act due to widespread mortality throughout its native range. Land managers tasked with whitebark pine conservation and restoration require information on strategies that can positively influence whitebark pine establishment and promote growth and defense of preexisting trees on the landscape. Our study investigated the response of whitebark pine tree growth and resin duct defenses to thinning and removal of competing shade‐tolerant conifers in the Caribou‐Targhee National Forest, Idaho, USA. In the nine years following thinning treatments, we found that whitebark pine trees (>12.7 cm diameter at breast height [dbh]) responded favorably to thinning and experienced a 60% increase in basal area increment (52% increase in ring width index) while also producing 34% more resin ducts. Additionally, resin ducts were 23% larger with 49% increased area in the nine years post‐thinning compared with trees in the control. Small diameter (<5 cm dbh) whitebark pine in the treatment experienced a dramatic growth release with over 90% increase in basal area increment (74% increase in ring width index). Thinning also created conditions favorable for whitebark pine seedling and sapling establishment. In the treatment, 75% of seedlings and 93% of saplings were whitebark pine, compared with 32% seedlings and 14% saplings in the control. In addition, 93% of overstory trees in the treatment were whitebark pine, compared with 42% in the control. Whitebark pine tree mortality was similar across both the control (26%) and the treatment (25%), indicating that the thinning treatments did not have any adverse impacts on tree mortality. The positive effects of thinning on whitebark pine establishment and tree growth, coupled with the lack of elevated mortality in the treatment, provide strong evidence in support of thinning as a viable silvicultural technique in whitebark pine conservation efforts. [ABSTRACT FROM AUTHOR]

Fire and drought are expected to increase in frequency and severity in temperate forests due to climate change. To evaluate whether drought increases the likelihood of post‐fire tree mortality, we used a large database of tree survival and mortality from 32 years of wildland fires covering four dominant western North American conifers. We used Bayesian hierarchical modeling to predict the probability of individual tree mortality after fire based on species—Pinus contorta (lodgepole pine), Abies concolor (white fir), Pseudotsuga menziesii (Douglas‐fir), and Pinus ponderosa (ponderosa pine)—bark thickness, bark char, percentage live tree crown scorched or consumed crown volume scorch (CVS), and mean annual climatic water deficit (CWD) anomalies the year pre‐fire and fire year relative to the 1985–2015 reference period. Although crown injury was the primary determinant of tree mortality after fire, drought increased likelihood of death, with a 2‐SD increase in CWD (+115.7) resulting in a 78% increase in the probability of mortality. We assessed the crown scorch level expected to result in >50% probability of mortality under different CWD scenarios: observed CWD, CWD of +2, and +4°C warming scenarios. Increased climatic moisture stress amplified tree death, reducing the threshold that causes tree mortality across all conifers under +4°C warming, with more subtle and species‐specific reductions for the +2°C scenario. Models predicting post‐fire tree mortality are components of global and regional carbon estimates, habitat suitability assessments, and forest management planning and decision support systems. The amplifying effects of drought on post‐fire tree mortality and predicted future climates are likely to lead to higher tree mortality following fires in forested landscapes of western North America and may have cascading effects on ecosystem services and future forest resilience. [ABSTRACT FROM AUTHOR]

This scientific commentary discusses the mechanisms of tree death caused by fire. The authors highlight the importance of understanding how trees respond to fire, especially as fire regimes continue to change due to climate change. They explore various hypotheses for fire-induced tree mortality, including immediate causes such as necrosis or consumption of leaves and meristems, as well as delayed causes such as carbon imbalance/starvation and hydraulic dysfunction. The authors present a recent study that focuses on how fire can cause hydraulic dysfunction in trees and discuss the potential mechanisms involved. They also emphasize the need for further research to fully understand the complex physiological responses of trees to fire. [Extracted from the article]

Trees use nonstructural carbohydrates (NSCs) to support many functions, including recovery from disturbances. However, NSC's importance for recovery following fire and whether NSC depletion contributes to post-fire delayed mortality are largely unknown. We investigated how fire affects NSCs based on fire-caused injury from a prescribed fire in a young Pinus ponderosa (Lawson & C. Lawson) stand. We assessed crown injury (needle scorch and bud kill) and measured NSCs of needles and inner bark (i.e. secondary phloem) of branches and main stems of trees subject to fire and at an adjacent unburned site. We measured NSCs pre-fire and at six timesteps post-fire (4 days–16 months). While all trees initially survived the fire, NSC concentrations declined quickly in burned trees relative to unburned controls over the same post-fire period. This decline was strongest for trees that eventually died, but those that survived recovered to unburned levels within 14 months post-fire. Two months post-fire, the relationship between crown scorch and NSCs of the main stem inner bark was strongly negative (Adj- R 2 = 0.83). Our results support the importance of NSCs for tree survival and recovery post-fire and suggest that post-fire NSC depletion is in part related to reduced photosynthetic leaf area that subsequently limits carbohydrate availability for maintaining tree function. Crown scorch is a commonly measured metric of tree-level fire severity and is often linked to post-fire tree outcome (i.e. recovery or mortality). Thus, our finding that NSC depletion may be the mechanistic link between the fire-caused injury and tree outcome will help improve models of post-fire tree mortality and forest recovery. [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]

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]

This article comments on: Short‐ and long‐term effects of fire on stem hydraulics in Pinus ponderosa saplings [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]

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]

Mortality and reduced growth rates due to raking accumulated basal duff were evaluated for old, large-diameter ponderosa and Jeffrey pine trees on the Lassen National Forest, California. No fire treatments were included to isolate the effect of raking from fire. Trees were monitored annually for 5 years after the raking treatment for mortality and then cored to measure basal area increment. Results showed that raking basal duff and litter to mineral soil from the bole out to 60 cm had no effect on basal area increment or mortality for 5 years posttreatment. Results are pertinent to managers who question whether raking basal duff will decrease tree vigor or increase tree mortality of large and old ponderosa and Jeffrey pine trees in northern California. [ABSTRACT FROM AUTHOR]

The article presents the author's views on the complex nature of interactions of trees with bark beetles and offer information on tree physiology. Topics discussed include endemic and epidemic phases of bark beetles, the availability of stressed hosts limits beetle populations in the endemic stage and negative effects of drought, fire and wind-throw on tree physiology.

• 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]

Tree mortality associated with drought and concurrent bark beetle outbreaks is expected to increase with further climate change. When these two types of disturbance occur in concert it complicates our ability to accurately predict future forest mortality. The recent extreme California USA drought and bark beetle outbreaks resulted in extensive tree mortality and provides a unique opportunity to examine questions of why some trees die while others survive these co-occurring disturbances. We use plot-level data combined with a three-proxy tree-level approach using radial growth, carbon isotopes, and resin duct metrics to evaluate 1) whether variability in stand structure, tree growth or size, carbon isotope discrimination, or defenses precede mortality, 2) how relationships between these proxies differ for surviving and now-dead trees, and 3) whether generalizable risk factors for tree mortality exist across pinyon pine (Pinus monophylla), ponderosa pine (P. ponderosa), white fir (Abies concolor), and incense cedar (Calocedrus decurrens) affected by the combination of drought and beetle outbreaks. We find that risk factors associated with mortality differ between species, and that few generalizable patterns exist when bark beetle outbreaks occur in concert with a particularly long, hot drought. We see evidence that both long-term differences in physiology and shorter-term beetle-related selection and variability in defenses influence mortality susceptibility for ponderosa pine, whereas beetle dynamics may play a more prominent role in mortality patterns for white fir and pinyon pine. In contrast, incense cedar mortality appears to be attributable to long-term effects of growth suppression. Risk factors that predispose some trees to drought and beetle-related mortality likely reflect species-specific strategies for dealing with these particular disturbance types. The combined influence of beetles and drought necessitates the consideration of multiple, species-specific risk factors to more accurately model forest mortality in the face of similar extreme events more likely under future climates. Identified within-species risk factors for tree-level mortality due to the combination of severe drought and concurrent bark beetle outbreaks associated with the 2012–2016 California, USA drought. Risk factors are categorized by species and category of metric measured. While we do not attempt to disentangle the effects of drought and bark beetles, host-specific beetles were identified as contributing to mortality for ponderosa pine, pinyon pine, and white fir, while incense cedar showed no clear evidence of bark beetle attacks. BAI refers to basal area increment (i.e. radial tree growth), Δ13C refers to discrimination of the heavier (13C) carbon isotope measured from tree rings, and PDSI refers to the Palmer Drought Severity Index. Unlabelled Image • Recent drought and bark beetles caused extensive tree mortality in California, USA. • A three-proxy approach is used to elucidate risk factors for tree-level mortality. • Living and dead trees exhibit some differences in growth, isotopes (Δ13C) and defenses. • Differences between living and dead trees are not consistent across species. • Effects of drought with beetles mask generalizable risk factors of mortality. [ABSTRACT FROM AUTHOR]

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