Spruce Mortality Research Articles

Reaction of Wood Ants to a Large-Scale European Spruce Bark Beetle Outbreak in Temperate Forests

In the Białowieża Forest, Norway spruce is the preferred host tree species for wood ants, both in coniferous and mixed stands; thus, spruce mortality as a consequence of a continuous spruce bark beetle outbreak in the Białowieża Forest since 2012 could have severe consequences for wood ant colonies, as well as their vitality and distribution. The main aim of this study was to assess whether the bark beetle outbreak had any effects on wood ant nest density and abundance and whether we could find any factors affecting the dead spruce distribution around ant nests. A re-inventory of active and abandoned wood ant nests in our study area was conducted from April to July 2022, using the same procedure as the previous inventory performed in 2016. The wood ant nest density was 0.11 per ha and remained practically the same relative to that determined in 2016. Our results indicate that, despite the importance of spruce for wood ants, in situations when only part of the spruce trees die, natural forest disturbances such as bark beetle outbreaks can actually have positive effects because more light can reach the forest floor, thereby promoting the establishment of new nests.

Wood decay and Norway spruce vulnerability to wind-inflicted mortality in monospecific and mixed stands in hemiboreal forests

Pathogen-caused stem and root decay are becoming increasingly common in Norway spruce (Picea abies (L.) H. Karst) which is believed to contribute to greater stand instability and susceptibility to wind-inflicted damage. Thus, this study aims to assess the effect of root and stem decay on Norway spruce vulnerability to wind-inflicted mortality in monospecific and mixed stands, to widen our base of knowledge on potentially more resilient spruce forest management approaches in hemiboreal forest zone. In this study we used data from: i) the National Forest inventory (NFI), ii) 34 observation plots established in wind-affected stands, iii) two transects (9200 m long, 36.8 ha inspection area combined), iv) and monospecific spruce plantation thinning experiment (five thinning intensities with two repetitions of each) affected by wind. We found that the total mortality of spruce during the NFI four five-year re-measurement cycles (2003–2007, 2008–2012, 2013–2017, 2018–2022) was 2.28 %, during which the main disturbance-causing agents were wind at 0.86 %, pests at 0.41 %, intra/interspecific competition at 0.37 %, and diseases at 0.36 %. NFI data-based multinomial logistic regression model revealed that the probability of wind-inflicted spruce mortality is dictated by soil moisture regime, stand age, stand stocking level, and tree decay presence. Results from the 34 established observation plots show that wood decay is a potential risk factor associated with wind damage occurrence in spruce stands. In a spruce stand, when a group of trees are damaged by wind the proportion of undecayed trees increases. Whereas a single tree is more likely to be attributed to decay caused wind damage.Results point towards deciduous broadleaf admixture having a positive effect on mitigating wind damage among non-decayed spruce trees: however, decayed trees, are more likely to be affected in such stands. The thinning experiment portion of this study suggests that increasingly intensive thinning in monospecific spruce stands will lead to an increasing spread of root and stem decay and thus the risk of wind damage. Therefore, from this perspective, the main goal is to reduce the root and stem decay presence in the stand and thus increase wind stability. We suggest further research to be directed into finding the optimal initial spruce seedlings density with a combination of coniferous and deciduous broadleaf species in order to mitigate root and stem decay presence and wind-inflicted spruce mortality in the forest stands.

From single trees to country-wide maps: Modeling mortality rates in Germany based on the Crown Condition Survey

Most years in the period from 2018 to 2022 have been exceptionally dry in Central Europe. In Germany’s forests, this long-lasting drought has caused unprecedented tree mortality. Systematic ground-based surveys, such as the annual Crown Condition Survey, provide information on the vitality status of the different tree species and their mortality rates. However, models are needed to be able to map the spatial patterns of mortality for each tree species based on cause-effect relationships derived from field observations. In this study, logistic regression models were used to identify the most important drivers of mortality for the most important tree species in Germany. For this purpose, the dead and surviving trees from the Crown Condition Survey were combined with a large set of potential predictor variables from the domains of climate, topography, soil, land cover and deposition. After feature selection, the models were evaluated using the area under the curve (AUC) statistic. Norway spruce (Picea abies; AUC = 0.9) showed by far the greatest increase in mortality, with the country-wide average observed and predicted rates approaching almost 10% per year from 2020 to 2022, and much higher predicted rates at the regional level. Much of the spruce mortality was explained by the climatic water balance of the driest summer in previous years. The other main tree species also showed clear mortality responses to the drought conditions. However, in the case of European beech (Fagus sylvatica; AUC = 0.94) and Pedunculate and Sessile oak (Quercus robur and petraea; AUC = 0.88), the peaks in the time series of the country-wide mortality rates stayed below 1%. For these broadleaved species, mortality was more dependent on a range of site conditions, i.e., soil and topography. For Scots pine (Pinus sylvestris; AUC = 0.76), for which the observed mortality rate peaked at 1.2% in 2020, the given drivers could explain mortality only to a lesser degree than for the other species. The regression models were used for spatial prediction to produce country-wide maps of species-specific mortality rates at annual temporal and 100-m spatial resolution, covering all years from 1998 to 2022. The maps visualize the spatial patterns of mortality over time. The regions in western and central Germany, which were most seriously affected by spruce dieback can clearly be identified. The models and maps presented can be used for risk assessment, forest planning, and tree species selection, providing decision support for forest practitioners.

Effects of cattle grazing on young spruce trees in boreal production forest

The compatibility of forest livestock grazing with timber production is disputed, as livestock can damage young trees through browsing and trampling. At the same time, livestock grazing might reduce the growth of competing vegetation and thereby enhance conifer forest regeneration. We investigated the effects of cattle (Bos taurus) on young spruce (Picea abies) trees in boreal production forest. We compared data from young spruce plantations within and outside of the grazing area, and during and off the cattle grazing season. Overall, stocking rates were 0.01 cows per hectare across the grazing areas. Cattle did not distribute equally, but concentrated their area use to young spruce plantations, leading to locally high densities during periods of the grazing season. Cattle removed vegetation competing with young spruce trees: Within the grazing area, the sward height of the field layer vegetation was lower, and young willow (Salix spec.) and downy birch (Betula pubescens) trees were browsed more than outside the grazing area. The annual height growth of spruce increased once the spruce grew above sward height. Spruce trees surrounded by many competing trees grew slower than those surrounded by few trees. Bent spruce trees grew slower than undamaged spruce trees. Yet, no difference in annual spruce growth was found between inside and outside the cattle grazing area. In contrast, we found a higher spruce mortality risk inside than outside the cattle grazing area. This mortality risk was very low and positively correlated with a given spruce tree's bark being damaged. Cattle did not increase the risk of damaged stem bark. This study did not succeed in pointing out the mechanism by which cattle might increase spruce mortality risk. Interestingly, the probability of being browsed and the probability of being bent were higher after winter than after summer, indicating that wild ungulates and snow might have a stronger effect on young spruce trees than summer grazing cattle at these stocking rates. In conclusion, cattle had minor effects on young spruce trees. This study suggests that at low stocking rates, forestry and livestock production in boreal production systems are compatible.

Future impacts of climate change on black spruce growth and mortality: review and challenges

Black spruce ( Picea mariana (Mill.) B.S.P.) is the dominant conifer species across a large part of North American boreal forests, providing many goods and services essential to human activities, and playing a major climatic role through the global carbon cycle. However, a comprehensive synthesis of the effects of climate change on black spruce has not yet been undertaken. The dynamics of black spruce are influenced by various living (biotic) and non-living (abiotic) factors, as well as their combined effects, which are particularly responsive to changes in climate. Climate change predictions suggest that northern ecosystems will experience the world's most significant impact. Therefore, black spruce is likely to undergo profound disruptions in its growth and mortality rate in the next few decades, resulting in significant changes in forestry and carbon storage. However, these changes will not be uniform throughout the entire distribution of the species. Future changes in temperature and precipitation will create more stress for water availability in the boreal forests of western and central North America than in their eastern counterparts. Thus, significant longitudinal and latitudinal gradients in tree growth and mortality variability are expected throughout the range of the species. This literature review aims to summarise the impacts of climate change on individual tree growth and mortality of this major species. While enhanced black spruce productivity could occur through both increased air temperature and nitrogen mineralisation in the soil, moisture limitation in central and western North America will result in significant growth reduction and mortality events across these regions. Conversely, under the expected climate change scenarios, black spruce forests may be more resilient in eastern North America, where climatic conditions appear more suitable, particularly in their northernmost range. In this review, we identify current research gaps for some disturbances, which should be addressed to better understand the impact of climate change on black spruce. Finally, we identify issues associated with sustainable forest management and the maintenance of black spruce under projected future climate changes.

Spatial and remote sensing monitoring shows the end of the bark beetle outbreak on Belgian and north-eastern France Norway spruce (Picea abies) stands.

In 2022, Europe emerged from eight of the hottest years on record, leading to significant spruce mortality across Europe. The particularly dry weather conditions of 2018 triggered an outbreak of bark beetles (Ips typographus), causing the loss of thousands of hectares of Norway spruce stands, including in Wallonia and North-eastern France. A methodology for detecting the health status of spruce was developed based on a dense time series of satellite imagery (Sentinel-2). The time series of satellite images allowed the modelling of the spectral response of healthy spruce forests over the seasons: a decrease in photosynthetic activity of the forest canopy causes deviations from this normal seasonal vegetation index trajectory. These anomalies are caused by a bark beetle attack and are detected automatically. The method leads in the production of an annual spruce health map of Wallonia and Grand-Est. The goal of this paper is to assess the damage caused by bark beetle using the resulting spruce health maps. A second objective was to compare the influence of basic variables on the mortality of spruce trees in these two regions. Lasted 6years (2017-2022), bark beetle has destroyed 12.2% (23,674ha) of the spruce area in Wallonia and Grand-Est of France. This study area is composed of three bioclimatic areas: Plains, Ardennes and Vosges, which have not been equally affected by bark beetle attacks. The plains were the most affected, with 50% of spruce forests destroyed, followed by the Ardennes, which lost 11.3% of its spruce stands. The Vosges was the least affected bioclimatic area, with 5.6% of spruce stands lost. For the most problematic sites, Norway spruce forestry should no longer be considered.

The response of lichens inhabiting exposed wood of spruce logs to post-hurricane disturbances in Western Carpathian forests

We investigated which of the following environmental factors: the number of years since the windthrow of the tree (the age of dead wood), the phytocenosis (the type of forest community), altitude, exposure, wood hardness and the spatial scale of forest disturbances (small gaps with a few fallen spruces vs large-area windthrows) contributed to the diversity and abundance of lichens inhabiting the exposed wood of windthrown spruce trees in Polish Western Carpathian forests. Both Shannon H index and sum of coverage coefficients rose with increasing age of the wood, levelling off after 11–14 y (diversity) and 14–17 y (abundance). This factor appeared to be the most important for this group of lichens, but the significant positive impact of large-area windthrows on the lichen abundance was also demonstrated by using a GLM model. The age of the wood we precisely determined on the basis of data on Norway spruce mortality collected annually in permanent plots of the Gorce National Park since 2000. Using the Shore durometer we linked the course of the wood-inhabiting lichen succession with wood decay more precisely than before. The largest number of species was associated with medium hard wood, i.e., 51 < x ≤ 80 on the Shore scale. Based on the NMDS analysis, we distinguished four age groups of logs, differing in lichen abundance and defined by the dominance of distinctive species. A large number of usually corticolous lichen species used the wood of windthrown spruce logs as an optional habitat to survive large-scale, post-hurricane forest disturbances.

The cumulative impacts of droughts and N deposition on Norway spruce (Picea abies) in Switzerland based on 37 years of forest monitoring

Norway spruce is one of the most important tree species in Central Europe, however, it is facing major problems with recent droughts. In this study we present 37 years (1985–2022) of long-term forest observation data on 82 different forest sites across Switzerland including 134′348 tree observations. The sites consists of managed spruce or mixed forest stands with beech (Fagus sylvatica) and comprise large gradients in altitude (290–1870 m), precipitation (570–2448 mm a−1), temperature (3.6–10.9 °C) or total nitrogen deposition (8.5–81.2 kg N ha−1 a−1).Long-term tree mortality has increased more than fivefold due to the multiple drought years 2019, 2020 and 2022, which is more than double the increase following the 2003 drought. We used a Bayesian multilevel model including three years of lagged drought indicator to predict spruce mortality. Besides stand age, the most important factors were drought and N deposition. Especially under drought conditions spruce mortality was increased on sites with high N deposition. Moreover, N deposition increased the imbalance of foliar phosphorus concentrations, with negative impacts on tree mortality. Mortality was increased by a factor of 1.8 in spruce compared to mixed beech and spruce stands. Stands with high mortality rates showed previously an increased proportion of trees with damaged crowns, especially after the droughts of 2003 and 2018.Taken together, we found evidences of an increase in spruce mortality droughts amplified under high N depositions. The perennial drought of 2018–2020 resulted in a cumulative spruce mortality of 12.1 % (564 dead trees in 82 sites) in only three years. With a Bayesian change-point regression framework we estimated a critical empirical load for nitrogen of 10.9 ± 4.2 kg N ha−1 a−1, which is in line with current thresholds, above which future plantings of spruce in Switzerland may not be a sustainable option due to the observed interaction between drought and nitrogen deposition.

Ecological factors affecting the recent Picea abies decline in Slovenia: the importance of bedrock type and forest naturalness

Norway spruce (Picea abies [L.] Karst.) has been at the centre of controversy for many decades. Recent evidence of its profound disturbance-induced damage and consequent stock depletions across forest landscapes in Europe has reinforced doubts regarding the sustainability and prospects of this tree species in the future. Like many other European countries, Slovenia has experienced significant Norway spruce mortality and a decrease in growing stock primarily as the result of several disturbance agents (bark beetle outbreaks, an ice storm, windthrows). We investigated a countrywide spruce growing stock decline based on data between 2010 and 2018. Particular focus was placed on identifying the main ecological drivers of this decline, namely geological conditions, climatic parameters, soil attributes, topographic factors and forest stand characteristics. The effects of potential predictors on the relative change (%) in spruce volume (m3 ha-1) during the period 2010-2018 were analysed with Generalized Additive Models. Based on a national dataset including forest compartments (n = 6355) with a spruce growing stock decline > 10%, we found mixed support for ecology-based hypotheses. While spruce decline responded to bedrock type as predicted (i.e., greater relative decline in carbonate compared to silicate compartments), higher forest naturalness (preservation of tree species composition) was not associated with a lower decline. Spruce decline was amplified by higher potential evapotranspiration and soil clay content but showed a strong negative relationship with spruce proportion in the year 2010. General trends along the gradients of other selected predictors (stoniness/rockiness and heat load index) were less pronounced. The results suggest that most of these ecological predictors interact with geology and forest naturalness in affecting Norway spruce decline. Our analysis reveals that bedrock type can play an important role due to its mitigating effects. However, forest naturalness is of secondary significance as intensified large-scale forest disturbances likely override its buffering potential.

Within-site adaptation: Growth and mortality of Norway spruce, Scots pine and Silver birch seedlings in different planting positions across a soil moisture gradient

Adapting to site conditions is a central part of forest regeneration and can be done through selection of different planting positions. Requirements are tree species specific, and the use of soil moisture maps could be a way to support decision making in forest regeneration planning. At two experimental sites with varying soil moisture conditions in southern Sweden Norway spruce (Picea abies (L.) Karst.), Scots pine (Pinus sylvestris L.), and silver birch (Betula pendula Roth) seedlings were planted in four different planting positions following mounding site preparation; Depression, Hinge, Mound and Unscarified. Soil moisture estimates were obtained from a high-resolution depth-to-water raster for each planting spot. The effect of soil moisture, planting position and their interactions on mortality, height and diameter was evaluated for each tree species. In wet conditions mounds proved to be the best option to minimize seedling mortality for conifers, but with decreasing soil moisture, differences between the planting positions decreased. Birch on the other hand had the greatest survival in the hinge. The coniferous species displayed increased height and diameter when planted in mounds independent of the soil moisture conditions, whereas silver birch was less dependent on a specific planting position. Results from this study shows that a soil moisture map can explain mortality, height and diameter and thus can be a useful tool when choosing planting position in different soil moisture conditions.

Multi-Source Mapping of Forest Susceptibility to Spruce Budworm Defoliation Based on Stand Age and Composition across a Complex Landscape in Maine, USA

Spruce budworm (Choristoneura fumiferana; SBW) outbreaks in the northeastern USA and Canada are recurring phenomena leading to large-scale mortality of spruce (Picea sp.) and balsam fir (Abies balsamea (L.) Mill.) forests as susceptibility to SBW is primarily determined by the availability of host species and their maturity. Our study examined several satellite remote sensing (Sentinel-1 C-band synthetic aperture radar (SAR), PALSAR L-band SAR, and Sentinel-2 multispectral) and site variables over space and time to develop a method to produce large-scale SBW stand impact types and susceptibility maps in Maine, USA. We used two machine-learning algorithms (Random Forest, RF; Multi-Layer Perceptron, MLP) to map SBW host species where RF produced better results than MLP. Our best model with site (elevation and aspect) and Sentinel-2 data attained an overall accuracy (OA) of 83.4%. However, the addition of SAR variables did not improve the model further. Combining host species data with age data retrieved from Land Change Monitoring, Assessment, and Projection (LCMAP) products, we demonstrated that SBW susceptibility map (based on stand impact types) could be produced with an OA of 88.3%. The fine spatial resolution (20 m) maps derived from our study provide reliable products for landscape-level SBW interventions in the region.

Efficacy of insecticide aerial spraying programs to reduce tree mortality during a spruce budworm outbreak (1967–1992) in the province of Quebec

Abstract Spruce budworm (Choristoneura fumiferana) is the most important pest of conifer forests in eastern North America. The main approach for protecting forests against this defoliator is aerial spraying of insecticides. Despite the crucial role of aerial insecticide applications in the global forest protection strategy, little is known about its real effects on tree mortality or the stand characteristics that may affect its long‐term efficacy. We evaluated the efficacy of the protection program that was implemented during the 1967–1992 outbreak in Quebec (Canada) in tree mortality reduction. We established 422 plots in eastern Quebec based upon the following parameters: insecticide application; stand composition; drainage quality; and stand age. Unprotected plots exhibited 18 to 44% greater tree mortality than protected plots. Insecticide efficacy was affected by drainage condition. Protected plots that established on sites with poor quality drainage exhibited 67.53% lower balsam fir (Abies balsamea) mortality than did unprotected plots. Stand age did not affect tree mortality. Insecticide application had no substantial effect on spruce mortality. Our study also found that the return on protection investment in terms of mortality reduction may decline after 7 years of aerial spraying, and that it is not necessary to achieve the target protection goal (50% foliage protection) every year to reduce host tree mortality. Our results may be useful for decision‐makers to decide where and when to apply insecticides during a spruce budworm outbreak and may help to determine the appropriateness of treating the forest on an annual basis depending upon the protection goals.

Interaction of drought- and pathogen-induced mortality in Norway spruce and Scots pine.

Pathogenic diseases frequently occur in drought‐stressed trees. However, their contribution to the process of drought‐induced mortality is poorly understood. We combined drought and stem inoculation treatments to study the physiological processes leading to drought‐induced mortality in Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) saplings infected with Heterobasidion annosum s.s. We analysed the saplings' water status, gas exchange, nonstructural carbohydrates (NSCs) and defence responses, and how they related to mortality. Saplings were followed for two growing seasons, including an artificially induced 3‐month dormancy period. The combined drought and pathogen treatment significantly increased spruce mortality; however, no interaction between these stressors was observed in pine, although individually each stressor caused mortality. Our results suggest that pathogen infection decreased carbon reserves in spruce, reducing the capacity of saplings to cope with drought, resulting in increased mortality rates. Defoliation, relative water content and the starch concentration of needles were predictors of mortality in both species under drought and pathogen infection. Infection and drought stress create conflicting needs for carbon to compartmentalize the pathogen and to avoid turgor loss, respectively. Heterobasidion annosum reduces the functional sapwood area and shifts NSC allocation patterns, reducing the capacity of trees to cope with drought.

Emulating Succession of Boreal Mixedwood Forests in Alberta Using Understory Protection Harvesting

Understory protection harvesting is a form of partial cutting that can be used in aspen (Populus tremuloides Michx.)-dominated stands that have understories of white spruce (Picea glauca (Moench) Voss). This practice involves removing 75% to 85% of the merchantable aspen while minimizing damage to the advance spruce regeneration, in addition to leaving 15% to 25% of the aspen standing to reduce potential windthrow of the spruce understory. In this paper, we summarize results from 18 stands measured 10 to 12 years after understory protection harvest. Diameter growth of spruce increased during the first five years after harvest while height growth increased during the second five-year period (5 to 10 or 7 to 12 years after release). Consistent with other studies, mortality rates of aspen trees ≥7.1 cm DBH (diameter breast height, 1.3 m) averaged 45.0% over the 10–12 year period following harvesting. Spruce mortality averaged 27.5% over the same 10–12 year period. Substantial aspen regeneration was evident across most harvested blocks, with aspen sapling densities 10–12 years from harvest being higher in removal (14,637 stems⋅ha−1) than in buffer areas (6686 stems⋅ha−1) and in extraction trails (7654 stems⋅ha−1). Spruce sapling (&gt;1.3 m height and &lt;4 cm DBH) densities averaged 1140 stems⋅ha−1 in removal areas at ages 10–12, with these trees likely being present as seedlings at the time of harvest. Mixedwood Growth Model projections indicate merchantable volumes averaging 168 m3⋅ha−1 (conifer) and 106 m3⋅ha−1 (deciduous) 70 years from harvest, resulting in MAI (mean annual increment) for this period averaging 2.0 m3⋅ha−1⋅y−1 with MAI for a full 150-year rotation of approximately 2.5 m3⋅ha−1⋅y−1.

Effectiveness of Emamectin Benzoate and Propiconazole for Protecting Picea engelmannii (Pinales: Pinaceae) from Mortality Attributed to Dendroctonus rufipennis (Coleoptera: Curculionidae) in Wyoming

Spruce beetle, Dendroctonus rufipennis Kirby (Coleoptera: Curculionidae), causes significant mortality of mature spruce, Picea spp. (Pinales: Pinaceae), in western North America. We evaluated the effectiveness of two formulations of bole injections of emamectin benzoate (TREE-äge® G4 and TREE-äge R10; Arborjet, Inc., Woburn, MA) alone and combined with propiconazole (Propizol®; Arborjet, Inc.) for protecting Engelmann spruce, Picea engelmannii Parry ex Engelm., from mortality attributed to colonization by D. rufipennis. Treatments were injected 16–19 July 2017, and levels of Pi. engelmannii mortality were determined for 2018 and 2019. Each experimental tree (n = 30, including an untreated control group) was baited with aggregation pheromone (frontalin) June–September 2018. All surviving treated Pi. engelmannii and a second untreated control group were baited June–September 2019. Both TREE-äge G4 and TREE-äge R10 significantly reduced levels of Pi. engelmannii mortality compared with the untreated control; however, protection was limited to one field season. Protection was increased to two field seasons by combining TREE-äge G4 and TREE-äge R10 with Propizol. The addition of Propizol, a triazole fungicide, likely reduced the progression of blue stain within treated Pi. engelmannii increasing tree health and resistance to D. rufipennis.

Differential defoliation and mortality of white spruce and balsam fir by eastern spruce budworm

Eastern spruce budworm (Choristoneura fumiferana) is a native defoliating insect in Canada and the United States that has large impacts on forest health. Balsam fir (Abies balsamea) and white spruce (Picea glauca) are its primary host trees, where budworm larvae consume buds and needles during larval development. While Eastern spruce budworm consumes both species, it is generally reported to defoliate balsam fir at higher levels. Investigating patterns and conditions that differentiate species’ defoliation and mortality is informative for understanding susceptibility. We studied an outbreak of spruce budworm at plantations in northern Wisconsin, USA, with defoliation first detected in 2014 that appeared to have higher impacts on white spruce, which is counter to general expectations. In 2019, we quantified spruce budworm impacts on these two tree species and tested which tree characteristics were most associated with tree defoliation and mortality. We found mortality of 60% and 49% at the sites for white spruce trees, while 0% of the balsam fir trees surveyed were dead. We determined that tree species was the main factor related to spruce budworm defoliation, rather than tree diameter at breast height or canopy class and suggest that different timing of species-specific mast-seeding events were related to defoliation and tree mortality.

Understory plant community responses to widespread spruce mortality in a subalpine forest

AbstractAimsSpruce beetles (Dendroctonus rufipennis) are causing widespread spruce (Picea spp.) mortality in subalpine forests in western North America. Spruce beetles are changing forest structure and composition by killing a dominant overstory species, but we know little about how the understory community responds to the increase in resource availability brought about by spruce mortality, what mechanisms drive its response, or how its response affects other forest properties and processes.LocationGlacier Lakes Ecosystem Experiments Site, Medicine Bow Mountains, Wyoming, USA.MethodsWe measured understory community cover and richness in 75 permanent plots during and 10 years after an epidemic spruce beetle outbreak, and measured trait values for 46 common understory species. We used linear regression to determine how the understory community has changed over time and along a gradient of spruce mortality, and to evaluate the relative support for two mechanisms contributing to species responses.ResultsUnderstory cover nearly doubled between sampling periods and increased the most where spruce mortality was most severe. Understory richness doubled and showed a weak positive trend with spruce mortality. Understory species with the largest increases in cover were the most frequent across the landscape before the disturbance, were the tallest at maturity and had the lowest leaf turgor loss points. Fir seedling density decreased over time, with decreases lessening with increases in understory cover. Changes in spruce seedling density were not predicted by changes in understory cover.ConclusionsOur findings highlight some of the diverse ways in which understory communities can be altered by spruce beetle outbreaks, and how the direction and magnitude of change can depend on the amount of spruce mortality as well as on priority effects and traits of pre‐disturbance species. Our findings also highlight how understory community changes can have implications for other forest properties and processes, such as tree regeneration and forest recovery.

Mass outbreaks and factors related to the spatial dynamics of spruce bark beetle (Ips typographus) dieback considering diverse management regimes in the Białowieża forest

Dealing with tree mortality caused by forest pests is one of the most demanding challenges in forest protection and management. The current spruce bark beetle infestation in the Polish part of the Białowieża Forest (BF), which started in 2012, is considered the largest in history and has nearly eliminated Norway spruce (Picea abies (L.) Karst) as a major forest tree species there. This research analyzed the dynamics and spatial extent of the BF outbreak from 2015 to 2019. The extensive beetle epidemic caused a more than six times higher number of infested trees in 2019 as compared to 2015. Under the conditions of the unprecedented ongoing I. typographus outbreak, the dynamics of spruce stand mortality varied across the study area. A lower spruce mortality was observed in Białowieża National Park as compared to other forest districts. Host and environmental factors influenced the timing of spruce mortality. Spruce height, stand age, the proportion of spruce, the share of area covered by tree crowns, excluding spruce and dominant species in the stands, were the key factors of bark beetle infestation. The most intensive outbreak progressed in stands older than 100 years dominated by spruce, and areas with the lowest dynamics of spruce dieback were localized in mixed young forests or pine stands. The methods and results presented in this study serve as baseline information, supporting the efforts to model the spread of bark beetles and future decision-making.

Response and mortality of beech, fir, spruce and sycamore to rapid light exposure after large-scale disturbance

Physiological response to rapid light exposure due to canopy disintegration in young beech, fir, Norway spruce and sycamore trees was measured in three consecutive years after the severe ice storm in 2014 and after windthrow in 2017. Nitrogen amount (Ntot), maximum assimilation response to light (Amax) and quantum yield (Φ) were measured in three categories of different light intensities under closed canopy with indirect site factor (ISF) < 15%, at the forest edge (15% <ISF < 25%) and in the open (ISF > 25%). Tree responses with number of seedlings per hectare were compared between damaged and undamaged sites, with young trees gradually adapting to light conditions in the two years following the two disturbance events.Nitrogen levels were in the optimal range for all species studied. Rapid exposure to elevated light reduced efficiency in fir and increased efficiency in beech and especially sycamore. No differences in response were observed in spruce. Assimilation efficiency, where both tree species were equal, shifted towards shade. Recovery was similar for all species studied after both disturbances: better after the windstorm, compared to the ice storm, indicating the severity of the event. Reductions in seedling numbers followed the pattern observed for physiological traits.

Tree mortality of European beech and Norway spruce induced by 2018-2019 hot droughts in central Germany

Anthropogenic climate change pushes forest ecosystems globally beyond their limits. Widespread events of forest die-off have been attributed to direct and indirect impacts of increasingly frequent and intense droughts. Here, we focus on an extensive mortality event in Norway spruce (Picea abies (L.) H. Karst.) and European beech (Fagus sylvatica (L.)) forests in Germany, following the successive 2018 – 2019 hot droughts. To examine whether this die-off indeed attributed to observed trends in drought occurrence and intensity, we sampled 143 beech and 186 spruce trees at three low-elevation sites (Spessart, Hassberge, Fichtelberg) with different edaphic properties in northern Bavaria. We analysed long-term hydroclimatic sensitivity and growth responses to extreme events of five site- and species-specific tree-ring width chronologies, including a reference site for each species. Growth of beech was sensitive to drought in April to June, whereas spruce growth was strongly related to drought during June to August, except at slightly higher elevations at the Fichtelberg site, where a summer temperature signal was observed. Trees at the Spessart and Hassberge sites showed an increased response to hydroclimatic conditions in April following the extreme drought in 1976 and from the 1990s onwards at the Fichtelberg site. Spruce bark beetle (Ips typographus L.) outbreaks during the 2018 drought accelerated the high mortality rates in around 50% of the trees at the Spessart and Hassberge site. In 2018, around 7% of all beech trees died at the Hassberge site, the site with the highest clay content. Our results suggest that these widespread mortality events can be attributed to an increasing drought sensitivity and were accelerated by the consecutive recent drought years. Sustainable forest management practices for these ecologically and economically important tree species are required to mitigate the effects of global warming in the future.