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The four-eyed fir bark beetle Polygraphus proximus Blandford, 1894 (Coleoptera, Curculionidae: Scolytinae) is an invasive species that originates from the Far East. Since early 2000s, it became known as an aggressive pest causing massive dieback of Siberian fir (Abies sibirica Ledeb.) in Siberia and, to some extent, in the European part of Russia. Here, we report the first record of P. proximus in Sverdlovsk Region (the Middle Urals, Russia). In summer 2023, A. sibirica trees colonized by the pest were discovered on the territory of the “Olenyi Ruchyi” Natural Park. The dendrochronological analysis of the cores of fir trees killed by the beetle indicated that the majority (72%) of these trees died in 2021–2022, while the earliest tree death occured already in 2007 and 2009, meaning that the pest was present in the Natural Park for at least two decades. Mycological analysis of bark and wood samples of infested trees revealed the fungus colonies assigned to Grosmannia aoshimae (Ohtaka, Masuya & Yamaoka) Masuya & Yamaoka. Associated with P. proximus, this Far Eastern fungus species has spread with the beetle to Siberia and the Urals, contributing to tree mortality. Siberian fir is present on 69% of the forested area in the “Olenyi Ruchyi”. The colonization of fir trees by P. proximus recorded in the park in 2023 alarms about the high risk of the pest outbreak in the coming years, which may lead to subsequent tree death over a huge territory of the park affecting its ecological, cultural, and recreational values. To prevent the dramatic impact, it is highly urgent to take all the necessary measures to suppress the distribution and combat this alien pest.

Aim: Bark beetle outbreaks have recently affected extensive areas of western North American forests, and factors explaining landscape patterns of tree mortality are poorly understood. The objective of this study was to determine the relative importance of stand structure, topography, soil characteristics, landscape context (the characteristics of the landscape surrounding the focal stand) and beetle pressure (the abundance of local beetle population eruptions around the focal stand a few years before the outbreak) to explain landscape patterns of tree mortality during outbreaks of three species: the mountain pine beetle, which attacks lodgepole pine and whitebark pine; the spruce beetle, which feeds on Engelmann spruce; and the Douglas-fir beetle, which attacks Douglas-fir. A second objective was to identify common variables that explain tree mortality among beetle—tree host pairings during outbreaks. Location: Greater Yellowstone ecosystem, Wyoming, USA. Methods: We used field surveys to quantify stand structure, soil characteristics and topography at the plot level in susceptible stands of each forest type showing different severities of infestation (0—98% mortality; n = 129 plots). We then used forest cover and beetle infestation maps derived from remote sensing to develop landscape context and beetle pressure metrics at different spatial scales. Plot-level and landscape-level variables were used to explain outbreak severity. Results: Engelmann spruce and Douglas-fir mortality were best predicted using landscape-level variables alone. Lodgepole pine mortality was best predicted by both landscape-level and plot-level variables. Whitebark pine mortality was best — although poorly — predicted by plot-level variables. Models including landscape context and beetle pressure were much better at predicting outbreak severity than models that only included plot-level measures, except for whitebark pine. Main conclusions: Landscape-level variables, particularly beetle pressure, were the most consistent predictors of subsequent outbreak severity within susceptible stands of all four host species. These results may help forest managers identify vulnerable locations during ongoing outbreaks.

Forests dominated by Douglas‐fir and western hemlock in the Pacific Northwest of the United States have strongly influenced concepts and policy concerning old‐growth forest conservation. Despite the attention to their old‐growth characteristics, a tendency remains to view their disturbance ecology in relatively simple terms, emphasizing infrequent, stand‐replacing (SR) fire and an associated linear pathway toward development of those old‐growth characteristics. This study uses forest stand‐ and age‐structure data from 124 stands in the central western Cascades of Oregon to construct a conceptual model of stand development under the mixed‐severity fire regime that has operated extensively in this region. Hierarchical clustering of variables describing the age distributions of shade‐intolerant and shade‐tolerant species identified six groups, representing different influences of fire frequency and severity on stand development. Douglas‐fir trees >400 years old were found in 84% of stands, yet only 18% of these stands (15% overall) lack evidence of fire since the establishment of these old trees, whereas 73% of all stands show evidence of at least one non‐stand‐replacing (NSR) fire. Differences in fire frequency and severity have contributed to multiple development pathways and associated variation in contemporary stand structure and the successional roles of the major tree species. Shade‐intolerant species form a single cohort following SR fire, or up to four cohorts per stand in response to recurring NSR fires that left living trees at densities up to 45 trees/ha. Where the surviving trees persist at densities of 60–65 trees/ha, the postfire cohort is composed only of shade‐tolerant species. This study reveals that fire history and the development of old‐growth forests in this region are more complex than characterized in current stand‐development models, with important implications for maintaining existing old‐growth forests and restoring stands subject to timber management.

The natural range of variation (NRV) is an important reference for ecosystem management, but has been scarcely quantified for forest landscapes driven by infrequent, severe disturbances. Extreme events such as large, stand-replacing wildfires at multi-century intervals are typical for these regimes; however, data on their characteristics are inherently scarce, and, for land management, these events are commonly considered too large and unpredictable to integrate into planning efforts (the proverbial “Black Swan”). Here, we estimate the NRV of late-seral (mature/old-growth) and early-seral (post-disturbance, pre-canopy-closure) conditions in a forest landscape driven by episodic, large, stand-replacing wildfires: the Western Cascade Range of Washington, USA (2.7 million ha). These two seral stages are focal points for conservation and restoration objectives in many regions. Using a state-and-transition simulation approach incorporating uncertainty, we assess the degree to which NRV estimates differ under a broad range of literature-derived inputs regarding (1) overall fire rotations and (2) how fire area is distributed through time, as relatively frequent smaller events (less episodic), or fewer but larger events (more episodic). All combinations of literature-derived fire rotations and temporal distributions (i.e., “scenarios”) indicate that the largest wildfire events (or episodes) burned up to 10⁵–10⁶ ha. Under most scenarios, wildfire dynamics produced 5th–95th percentile ranges for late-seral forests of ~47–90% of the region (median 70%), with structurally complex early-seral conditions composing ~1–30% (median 6%). Fire rotation was the main determinant of NRV, but temporal distribution was also important, with more episodic (temporally clustered) fire yielding wider NRV. In smaller landscapes (20,000 ha; typical of conservation reserves and management districts), ranges were 0–100% because fires commonly exceeded the landscape size. Current conditions are outside the estimated NRV, with the majority of the region instead covered by dense mid-seral forests (i.e., a regional landscape with no historical analog). Broad consistency in NRV estimates among widely varied fire regime parameters suggests these ranges are likely relevant even under changing climatic conditions, both historical and future. These results indicate management-relevant NRV estimates can be derived for seral stages of interest in extreme-event landscapes, even when incorporating inherent uncertainties in disturbance regimes.

Land managers use hazard (susceptibility) and risk rating systems to guide the application of forest management treatments that aim to reduce future damages to forests. Rating systems are typically designed for individual damage agents, but tree mortality often results from multiple agents without a clear proximate cause. In interior Douglas‐fir (DF, Pseudotsuga menziesii var. glauca) forests in the Northern Rocky Mountains, USA, multiple damage agents are commonly associated with DF tree mortality, such as insects, disease, weather, and fire. We investigated how recent DF tree mortality from insects and diseases (excluding fire and harvest) shifted stand structure and composition in DF forests and was influenced by susceptibility (e.g., stand structure and composition, topography, and spatial variability in climate) and risk (biotic agent pressure). Our multi‐scale analysis used 884 plots remeasured after 10 years from the USDA Forest Inventory and Analysis program with support from spatial datasets. Across a large, forested landscape, 60% of the plots had no new DF mortality, and most plots (80%) experienced mortality <10% of DF basal area. However, severe tree mortality, defined as >25% loss of DF basal area, occurred in 6% of plots. Most of the dead DF trees (68%) were smaller diameter (12.7–29 cm at breast height) and mortality rates of smaller trees were significantly greater than those of larger diameter trees (>29 cm), a finding consistent with natural stand development processes. During the remeasurement period, average DF tree size increased in most plots (80%) and <4% of DF‐dominated plots with severe mortality shifted in dominance from DF to another tree species. Greater DF mortality (percentage of initial DF basal area) was associated with lower tree growth rates, larger average tree sizes, greater availability of DF tree hosts for biotic agents, cooler and wetter topo‐climatic locations, and higher Douglas‐fir beetle population pressure. The relative importance of each variable differed west and east of the Continental Divide. By identifying thresholds in susceptibility and risk variables associated with higher DF tree mortality, our results support adaptive forest management for multiple damage agents, when the goal is to reduce tree mortality of a widespread and abundant conifer.

Consequences of bark beetle outbreaks for forest wildfire potential are receiving heightened attention, but little research has considered ecosystems with mixed-severity fire regimes. Such forests are widespread, variable in stand structure, and often fuel limited, suggesting that beetle outbreaks could substantially alter fire potentials. We studied canopy and surface fuels in interior Douglas-fir ( Pseudotsuga menziesii v. glauca ) forests in Greater Yellowstone, Wyoming, USA, to determine how fuel characteristics varied with time since outbreak of the Douglas-fir beetle ( Dendroctonus pseudotsugae ). We sampled five stands in each of four outbreak stages, validated for pre-outbreak similarity: green (undisturbed), red (1-3 yr), gray (4-14 yr), and silver (25-30 yr). General linear models were used to compare variation in fuel profiles associated with outbreak to variation associated with the range of stand structures (dense mesic forest to open xeric parkland) characteristic of interior Douglas-fir forest. Beetle outbreak killed 38-83% of basal area within stands, generating a mix of live trees and snags over several years. Canopy fuel load and bulk density began declining in the red stage via needle drop and decreased by ∼50% by the silver stage. The dead portion of available canopy fuels peaked in the red stage at 41%. After accounting for background variation, there was little effect of beetle outbreak on surface fuels, with differences mainly in herbaceous biomass (50% greater in red stands) and coarse woody fuels (doubled in silver stands). Within-stand spatial heterogeneity of fuels increased with time since outbreak, and surface-to-crown continuity decreased and remained low because of slow/sparse regeneration. Collectively, results suggest reduced fire potentials in post-outbreak stands, particularly for crown fire after the red stage, although abundant coarse fuels in silver stands may increase burn residence time and heat release. Outbreak effects on fuels were comparable to background variation in stand structure. The net effect of beetle outbreak was to shift the structure of mesic closed-canopy stands toward that of parklands, and to shift xeric parklands toward very sparse woodlands. This study highlights the importance of evaluating outbreak effects in the context of the wide structural variation inherent to many forest types in the absence of beetle disturbance.

As temperatures continue rising, the direction, magnitude, and tempo of change in disturbance-prone forests remain unresolved. Even forests long resilient to stand-replacing fire face uncertain futures, and efforts to project changes in forest structure and composition are sorely needed to anticipate future forest trajectories. We simulated fire (incorporating fuels feedbacks) and forest dynamics on five landscapes spanning the Greater Yellowstone Ecosystem (GYE) to ask the following questions: (1) How and where are forest landscapes likely to change with 21st-century warming and fire activity? (2) Are future forest changes gradual or abrupt, and do forest attributes change synchronously or sequentially? (3) Can forest declines be averted by mid-21st-century stabilization of atmospheric greenhouse gas (GHG) concentrations? We used the spatially explicit individual-based forest model iLand to track multiple attributes (forest extent, stand age, tree density, basal area, aboveground carbon stocks, dominant forest types, species occupancy) through 2100 for six climate scenarios. Hot-dry climate scenarios led to more fire, but stand-replacing fire peaked in mid-century and then declined even as annual area burned continued to rise. Where forest cover persisted, previously dense forests were converted to sparse young woodlands. Increased aridity and fire drove a ratchet of successive abrupt declines (i.e., multiple annual landscape-level changes ≥20%) in tree density, basal area, and extent of older (>150 yr) forests, whereas declines in carbon stocks and mean stand age were always gradual. Forest changes were asynchronous across landscapes, but declines in stand structure always preceded reductions in forest extent and carbon stocks. Forest decline was most likely in less topographically complex landscapes dominated by fire-sensitive tree species (Picea engelmannii, Abies lasiocarpa, Pinus contorta var. latifolia) and where fire resisters (Pseudotsuga menziesii var. glauca) were not already prevalent. If current GHG emissions continue unabated (RCP 8.5) and aridity increases, a suite of forest changes would transform the GYE, with cascading effects on biodiversity and myriad ecosystem services. However, stabilizing GHG concentrations by mid-century (RCP 4.5) would slow the ratchet, moderating fire activity and dampening the magnitude and rate of forest change. Monitoring changes in forest structure may serve as an operational early warning indicator of impending forest decline.

The Chinese fir (Cunninghamia lanceolata) is the largest tree species used for afforestation in China. The purpose of this study was to explore the effects of site quality, stand density, and tree species composition on the growth and yield of mixed Chinese fir forests and to build prediction models for their stand average DBH (diameter at breast height), average height, and volume. Using 430 plots of mixed Chinese fir forests in the Fujian Province of China, the optimal base models for predicting stand average DBH, average height, and volume were selected from the Schumacher, Korf, Logistic, Mitscherlich, and Richards equations. On this basis, the site class index (SCI), stand density index (SDI), and tree species composition coefficient (TSCC) were introduced to improve the model’s performance, and the applicability of the different models was evaluated. The optimal base models for the average DBH, average height, and stand volume of mixed Chinese fir forests all used the Richards equation. The best fitting effect was obtained when the SCI was introduced into parameter a in the average height model, while the inclusion of the TSCC did not improve the model significantly. The fitting effects of the average DBH and stand volume models were both best in the form of y=a1SCIa2[1−exp⁡(−b1SDIb2)t]c when the SCI and SDI were introduced. When the TSCC was further included, the fitting effects of the stand average DBH and volume models were significantly improved, with their R2 increased by 47.47% and 58.45%, respectively, compared to the base models. The optimal models developed in this study showed good applicability; the residuals were small and distributed uniformly. We found that the SCI had an impact on the maximum values of the stand average DBH, average height, and volume; the SDI was closely related to the growth rate of the diameter and volume, while the TSCC influenced the maximum values of the stand average DBH and volume. The model system established in this study can provide a reference for the harvest prediction and mixing ratio optimization of mixed Chinese fir forests.

Background The establishment of mixed forest stands is nowadays seen as an opportunity to maintain forest services in the course of global climate change. Methods Thus, we determined forest floor and mineral soil pH, base saturation (BS) as well as exchangeable base cation stocks in adjacent groups of pure mature European beech (Fagus sylvatica), Douglas fir (Pseudotsuga menziesii) and Norway spruce (Picea abies) as well as single-tree mixtures of beech with either Douglas fir or spruce at two forest sites in Southern Germany that differ in site and soil properties. Results Spruce forest floors had lowest pH and BS, while beech favoured less acidic forest floors with higher BS. The impact of Douglas fir on soils varied depending on the site. Under beech–Douglas fir and beech–spruce mixtures, forest floor and mineral soil pH and BS were higher than under the respective pure conifer stands. While beech depletes soil exchangeable Ca and Mg stocks more than Douglas fir and spruce, respectively, total soil exchangeable K stocks under beech were among the highest. Again, beech–conifer mixtures were intermediate. Conclusions Mixed species stands might maintain forest soil fertility by mitigating soil acidification, nutrient leaching and concomitant soil base cation depletion compared to pure conifer stands.

Background and aimsFine root multi-element stoichiometric coupling is essential to maintain terrestrial ecosystem functions. Although forest conversion has been a major driver globally in recent decades, its effect on multi-element stoichiometric coupling remains poorly understood.MethodsThis study examined the ratios of fine root N or P relative to exchangeable cations (i.e., K, Ca, and Mg) along a chronosequence of secondary forests and Chinese fir plantations (5 to 41 years old) with reference to primary forests in subtropical China.ResultsOn average, the ratios of live root N or P to exchangeable cations of plantations were higher than those of secondary forests, and the live roots N:Ca and N:Mg ratios in plantations were also higher than those of primary forests. The dead root N:K and P:K ratios of plantations were higher than those of secondary forests, while the dead root N:K, N:Ca, and N:Mg ratios of plantations and secondary forests were lower than those of primary forests. The ratios of live root N or P to exchangeable cations did not vary remarkably with stand age within plantations and secondary forests, except for the P:K ratio. However, the dead root N:Ca ratio increased with stand age for secondary forests and plantations. Overall, the live and dead root N:Mg, P:K and P:Mg, or N:K ratios decreased with soil depth.ConclusionOur findings suggest that intensive forest management impairs fine root multi-element stoichiometric coupling in reference to primary forests and stand development has strong influences on fine root multi-element absorption and resorption.