Spruce Mortality Research Articles

Comparative drought sensitivity of co‐occurring white spruce and paper birch in interior Alaska

Abstract Numerous recent studies have argued that moisture limitation is leading to growth declines and mortality of black spruce (Picea mariana) and white spruce (Picea glauca) in the North American boreal forest. A parallel line of research suggests that increasingly common severe wildfires are altering successional pathways and leading to long‐term replacement of spruce forests with those dominated by paper birch (Betula papyrifera, Betula neoalaskana) and trembling aspen (Populus tremuloides). When both conifers and deciduous trees establish after fire, this biome shift hypothesis implicitly assumes that deciduous species will outcompete the conifers, owing to their more rapid vertical growth and because they might be less sensitive to warm and dry conditions. We established a research site in a white spruce‐paper birch forest on an east‐facing slope in interior Alaska and tested the hypothesis that Alaska paper birch are better adapted to warm and dry conditions than white spruce. Over 6 years (2013–2018), we made hourly measurements of microclimate and xylem sap flux of both species. We also collected increment cores and conducted climate‐growth analyses for both species. During our 6‐year study, growing seasons with low volumetric soil water content (SWC) were those that followed shallow winter snowpacks and had limited summer rainfall, not necessarily those with warm air temperature. Both species were sensitive to moisture limitation. The tree‐ring data revealed significant positive effects of cumulative water year precipitation on radial growth, with a steeper slope for paper birch than for white spruce. Radial growth of both species was also positively related to mean water year air temperature. Sap flux density declined progressively for white spruce over the range of observed SWC and abruptly for paper birch when SWC fell below ~15%. Synthesis. Our results show that, while paper birch might be less sensitive to mild drought than white spruce, it may be more sensitive to severe drought, raising questions about the ability of paper birch to outcompete co‐occurring white spruce in a drier climate.

Permafrost thaw in boreal peatlands is rapidly altering forest community composition

Abstract Boreal peatlands are frequently underlain by permafrost, which is thawing rapidly. A common ecological response to thaw is the conversion of raised forested plateaus to treeless wetlands, but unexplained spatial variation in responses, combined with a lack of stand‐level data, make it difficult to predict future trajectories of boreal forest composition and structure. We sought to characterize patterns and identify drivers of forest structure, composition, mortality and recruitment in a boreal peatland experiencing permafrost thaw. To do this, we established a large (10 ha) permanent forest plot (completed in 2014), located in the Northwest Territories, Canada, that includes 40,584 mapped and measured trees. In 2018, we conducted a comprehensive mortality and recruitment recensus. We also measured frost table depth, soil moisture, soil humification and organic layer thickness within the plot between 2012 and 2018, and used habitat association tests to link these variables to forest characteristics and dynamics. Forest composition and structure varied markedly throughout the plot and were strongly governed by patterns in permafrost presence and organic layer thickness. Overall, there was a net loss of trees from the plot at a rate of 0.7% year−1. Mortality of black spruce, the dominant tree species, was more than double that of recruitment and was strongly associated with permafrost thaw. In contrast, recruitment of larch was over four times greater than mortality, and occurred primarily in low‐lying, permafrost‐free wetlands with mineral soil near the surface. Synthesis. The trends in tree demography and underlying drivers suggest that spruce‐dominated permafrost plateaus will be converted into larch‐dominated wetlands as permafrost thaw progresses in boreal peatlands, particularly in areas where mineral soil is near the surface. In the longer term, thaw could increase the hydrologic connectivity of the landscape, resulting in widespread drainage and re‐vegetation by spruce, but we did not find evidence that this is occurring yet. Given the increasing rates of permafrost thaw, and positive feedbacks between thaw and forest change, we predict that larch abundance will continue to increase in boreal peatlands over the coming decades, leading to shifts in ecosystem function, wildlife habitat, albedo and snow dynamics.

Forest floor plant diversity drives the use of mature spruce forests by European bison.

The distribution of large ungulates in space is in large extent driven by the availability of forage, which in temperate forests depends on light availability, and associated plant diversity and cover. We hypothesized that the increased number of GPS fixes of European bison (Bison bonasus L.) in usually avoided spruce forests was an effect of higher plant species richness and cover of the forest floor, which developed owing to increased light availability enhanced by spruce mortality. We carried out 80 forest floor plant surveys combined with tree measurement on plots chosen according to the number of GPS locations of GPS‐collared European bison. The mean plant species richness per plot was higher on intensively visited plots (IV) than rarely visited (RV) plots (30 ± 5.75 (SD) versus. 26 ± 6.19 (SD)). The frequency of 34 plant species was higher on IV plots, and they were mainly herbaceous species (32 species), while a significant part of 13 species with higher frequency on RV plots was woody plants (5 species). The species richness of forbs was higher on IV plots, while other functional groups of plants did not differ. Tree stem density on the IV plots was lower than on the RV plots (17.94 ± 6.73 (SD) versus 22.9 ± 7.67 (SD)), and the mean value of Ellenberg's ecological indicator for light availability for all forest floor plant species was higher on IV plots. European bison visiting mature spruce forests was driven by higher forest floor plant cover and species richness, and high share and species richness of forbs. The two latter features may be translated into higher quality and diversity of forage. In spite of morphological characteristics suggesting that European bison is a species of mixed (mosaic) habitats, it seems to be well adapted to thrive in diverse forests.

Studying the probability of spruce beetle caused mortality in Colorado's spruce forests using Bayesian hierarchical models

AbstractThe association between environmental factors and Engelmann spruce mortality in western Colorado was investigated using Bayesian hierarchical zero‐and‐one inflated beta regressive model. The results indicated that the probability of mortality occurrence rate was positively associated with moderate to warm temperature zones, moderate precipitation, and single canopy stands with the smaller size class of spruce within stands. The probability of full mortality rate was positively associated with warm and humid climate zones with spruce as subdominant species in the vertically complex stands. On the other hand, higher mortality rate was more associated with local stand characteristics than climate factors that low mortality rate associated with stands dominated by spruce of a large size classes. The correlation model offers an approach to create predicted risk map for probability and rate of spruce mortality that could help manager in precedent planning for the upcoming outbreak events.Recommendations for Resource Managers This study aims to investigate the environmental association of spruce bark beetle caused mortality in landscape context with binomial‐beta hierarchical framework. Both regional‐scale climate and stand‐level characteristics should be considered to assess the risk and severity of outbreak caused mortality at a large spatial scale. The mortality risk is higher and more severe in warmer zones, especially the stands with spruce as subdominant tree species should be more concerned. Predicted map could improve the ability to spatially assess the risk of spruce mortality assisting managers to be better prepared for the future outbreaks, particularly in the risky area.

Preliminary indications for diverging heat and drought sensitivities in Norway spruce and Scots pine in Central Europe

Massive and increasing tree mortality is currently observed in the two conifer species Norway spruce and Scots pine in Central Europe. Consecutive dry years are made responsible for this phenomenon. Leaf trait measurements, in specific leaf osmotic potential (πosm) and leaf water potential at turgor loss (πtlp), indicate that the underlying mechanisms for tree mortality are most likely different between the two species. πtlp of spruce was highly negative, revealing a potentially high drought tolerance of the species. πtlp of Scots pine was less negative, suggesting a higher susceptibility to drought stress. I conclude that the mortality of Norway spruce might be caused by rising temperatures and that the summer temperatures in the past years were beyond the species thermal tolerance threshold. Overall, I want to highlight and enhance the discussion that the search for suitable species for a climate change adapted forest should go in both directions, i.e., species should be chosen to make the forest fit for both increasing drought and heat stress.

Growth and mortality of Norway spruce and European beech in monospecific and mixed-species stands under natural episodic and experimentally extended drought. Results of the KROOF throughfall exclusion experiment

Key messageUnder severe drought, growth of Norway spruce suffered much more than European beech. Norway spruce benefited from growing in the environment of beech, and both species acclimated slightly to 5 years of experimentally extended drought.Recent studies show that the detrimental effects of drought on stand growth are mitigated when the stand contains mixed tree species. We analysed the growth responses of Norway spruce and European beech to episodic and experimentally extended drought in intra- and inter-specific mature stands. We used annual diameter growth records dating back to 1998 to determine the impact of the natural episodic drought in 2003 and 2015. To analyse extended drought, spruce and beech trees were exposed to extreme drought under automatic throughfall exclusion roofs from 2014 to 2018. The growth of spruce in an inter-specific environment with beech was 20–50% less affected by natural episodic drought compared with an intra-specific constellation. When beech grew in an inter-specific environment, it was by 23% more affected by drought compared to intra-specific conditions, but seemed to recover faster. The induced drought from 2014 to 2018 resulted in a strong growth reduction in the first year particularly for spruce, followed by a slight acclimation to the dry conditions. Beech acclimated and recovered faster than spruce across all growing conditions, while spruce only acclimatized faster in the environment of beech. Both species showed a higher mortality under induced drought compared with the controls; for spruce, the mortality rate was fivefold higher compared to the long-term mortality. The long-term moderate-growth stabilization and the growth increase after the 5-year exposure to drought suggest a gradual acclimation to drought by beech. The resistance and acclimation to drought of spruce when growing in mixture should be considered when designing resource efficient and productive mixed conifer-broadleaved stands for future climates.

Temperature-induced growing season drought threatens survival and height growth of white spruce in southern Ontario, Canada

The adaptive capacity of boreal tree species to thrive in a warming, more variable climate has significant implications for boreal forest ecosystem structure and function. Dendrochronological and other empirical data indicate reduced growth and increased mortality of white spruce (Picea glauca [Moench] Voss) growing in warmer, drier portions of its natural range in western North America due to recent climate change. Comparatively, little is known of potential climate change effects on white spruce in the more mesic, eastern portion of its range where mean annual precipitation is relatively high. In this study, we examined the potential long-term consequences of climate warming on annual rates of mortality and height growth of mature white spruce by substituting space for time using data from 10, 31- to 40-year-old large common garden experiments (CGEs) established in Ontario, Canada. Across these CGEs, white spruce was exposed to differences of up to 5.0 °C in mean annual temperature and 405 mm in mean annual precipitation. During a 12- to 16-year measurement interval, mortality increased from the colder, more northern CGEs to the warmer CGEs near the species’ southern range margin. Along this climatic gradient, white spruce was also exposed to more frequent, severe growing season moisture deficits, with southern CGEs sometimes subject to extreme drought events with a low probability based on historical occurrence. Our results indicate that the severity of recent growing season drought events at more southern CGEs may have exceeded the drought tolerance capacity of white spruce and resulted in significantly increased mortality and reduced height growth rates. These results suggest that with projected increases in temperature and frequency/severity of growing season drought by the end of this century, white spruce is likely to experience maladaptation over a larger portion of its range in Ontario and gradually retract from its current southern range edge.

Stem- and stand-level growth and mortality following partial cutting in eastern boreal poplar – white spruce stands

Variable retention (VR) and partial cutting are both considered important silvicultural tools of natural disturbance or ecosystem based forest management approaches. Partial harvesting differs from VR in that post-treatment growth responses and stand regeneration are the primary objective rather than the maintenance of biodiversity. This partial cutting study is undertaken in mixed poplar (Populus spp.) – white spruce (Picea glauca (Moench) Voss) stands in the eastern Canadian boreal mixedwood forest. It compares, at the tree level, absolute growth rates (AGR) and relative growth rates (RGR) of basal area (BA) and stem survival; and at the stand level, it also compares absolute BA growth, mortality, and sapling density 10 years following treatment. The completely randomized experiment was established with four intensities of partial cutting (0, 50%, 65%, and 100% of poplar BA). All partial cutting intensities had a significant and similar positive effect on AGR of residual spruce stems. Complete poplar removal resulted not only in the highest increase in RGR of suppressed and intermediate spruce stems, but also in higher spruce mortality. Removal of 50% of the initial poplar stand BA provided the best trade-off between positive residual stem growth of spruce and poplar and limited post-treatment mortality.

Interactive effects of defoliation and water deficit on growth, water status, and mortality of black spruce (Picea mariana (Mill.) B.S.P.)

Defoliation followed by water deficit showed time-dependent effects on plant water status and growth in black spruce ( Picea mariana (Mill.) B.S.P.). Biotic stress negatively (during active defoliation by growing instars) and positively (after defoliation) affected plant water relations. However, water deficit, alone or combined with defoliation, prevails over defoliation-related stress for radial growth and sapling vitality. Tree vitality is influenced by multiple factors such as insect damage, water deficit, and the timing of these stresses. Under drought, positive feedback via the reduction of leaf area may improve the water status of defoliated trees. However, the effect on tree mortality remains largely unknown. We investigated the effects of defoliation followed by a water deficit on tree growth, plant water status, and mortality in black spruce (Picea mariana (Mill.) B.S.P.) saplings. In a controlled greenhouse setting, saplings were submitted to combined treatments of defoliation and water stress. To assess the impact of these stresses and their interaction, we measured phenology, twig development, secondary growth of the stem, water potential, and mortality of the saplings. Both defoliation and water deficits reduced growth; however, the effect was not additive. During active defoliation, we observed a higher evaporative demand and a lower midday leaf water potential Ψmd. We observed an opposite pattern of response post-stress. Drought alone increased sapling mortality immediately after the stress period, but after c.a. 20 days, mortality rates remained similar following combined drought and defoliation. Our results highlight two key periods during which defoliation affects plant water relations either negatively (during active defoliation) or positively (after defoliation). Mortality in defoliated saplings was reduced immediately following drought because available internal water increased in the stem.

Landscape-level spread of beetle infestations from windthrown- and beetle-killed trees in the non-intervention zone of the Tatra National Park, Slovakia (Central Europe)

The European spruce bark beetle (Ips typographus) causes widespread Norway spruce (Picea abies) mortality in European forests. The pattern of landscape-level tree mortality varies over the course of beetle outbreak and by the presence and location of active breeding sites. Increased understanding of rules governing the unmanaged spread of beetle-induced tree mortality over the landscape would help to optimise management control strategies on the borderline between highly valuable protected areas and surrounding managed forests.Our study aimed to quantify the dynamics of standing tree infestation patterns from two infestation sources: windthrow and previous-year beetle infestations. Specifically, we (i) evaluated dispersal distances between the nearest infestation source and new infestations, (ii) quantified size and shape of infestation spots, (iii) modelled an infestation gradient and (iv) probability of new infestation during the incipient, peak and decline phases of beetle outbreak. Based on one- and two-year records of colour-infrared aerial photography, taken between 2005 and 2015, we identified windthrown and beetle-killed trees in the non-intervention zone of Tatra National Park, Slovakia (Central Europe).The size and compactness of infestation spots evolved from small and simple to more extended and complex shapes during beetle epidemics. In total, 40% of infestations were smaller than 100 m2 and 79% smaller than 500 m2. Spot growth dominated over spot initiation, with the mean spot growth extent during peak epidemic (54.8 m). Beetle infestations reached the upper tree line (1605 m a.s.l.). In total, 71% of new infestations emerged within 100 m and 97% within 500 m from an infestations source. Emergence of new infestations varied between infestation sources and phases of beetle outbreak. New beetle infestations emerged near windthrown locations during incipient and peak phases and near source beetle infestations during peak and decline phases following inverse power-law function. We found that the forest within 100 m from the active infestation compared to more distant buffers had the highest risk of infestations. The distance to previous-year infestations should be considered as one of the main factors in determining the risk of subsequent tree mortality, especially if other predictors are absent. If the active wind disturbances or beetle infestations neighbour the border of the unmanaged protected area, the search and sanitation felling of active breeding trees should be concentrated within 500 m from the border of unmanaged protected areas in order to restrain the spread of beetle infestations from nature reserve to surrounding managed forests.

Gap regeneration patterns in Carpathian old-growth mixed beech forests – Interactive effects of spruce bark beetle canopy disturbance and deer herbivory

The combined effects of various disturbance types have lasting consequences on forest ecosystems and the services they provide. We examined inter-specific differences in the responses of European beech, silver fir and Norway spruce to canopy disturbance across four life stages (small seedlings, tall seedlings, saplings and pole-sized trees) in two Carpathian old-growth mixed beech forests located in the Gorce National Park (GNP) and Babia Gora National Park (BGNP), Poland. Both study sites were recently affected by spruce bark beetle outbreak. We compared the composition of tree regeneration between forest plots, canopy gaps and expanded areas (i.e. areas located under tree crowns). Moreover, we studied how various gap properties (size, expanded gap to canopy gap size ratio, shape) and other factors (browsing, plant and advanced regeneration cover) shaped regeneration patterns. Inter-specific differences in species abundance relative to gap properties and intra-specific differences in species response across various life stages to gap properties were found. Gap properties had more pronounced effects on saplings and pole-sized trees than on small and tall seedlings. Gap size had the most noticeable effect on beech across all regeneration classes. However, its effect varied from negative to positive depending on life stage. Pole-sized trees of all species responded positively to gap size. Thus, it seems that large gaps provided an opportunity for all species to recruit to canopy. Fir and spruce responded contrastingly to expanded gap to canopy gap size ratio, which reflects differences in their ability to adapt to high-intensity solar radiation. With the exception of beech, the ‘forest-gap’ gradient only affected trees in the advanced life stages (saplings and pole-sized trees). Beech and fir sapling densities and fir pole-sized tree densities were higher when under forest canopy, but spruce pole-sized trees were slightly more abundant in expanded areas. Browsing rates differed between the two study sites; they were more severe in BGNP, where deer densities in the last twenty years have been significantly higher. This intense deer browsing kept silver fir regeneration below the height of 0.5 m and prevented any recruitment to sapling stage. We conclude that even though the recent spruce mortality caused by bark beetle outbreak improved light conditions in both study areas, thus providing good regeneration opportunities to both beech and fir, deer browsing may have long-lasting effects on successional patterns by hindering fir recruitment in BGNP.

Effects of silicon-rich soil amendments on growth, mortality and bark feeding damage of Sitka spruce (Picea sitchensis) seedlings under field conditions

ABSTRACTAlternative methods of protection are required against feeding by the large pine weevil (Hylobius abietis) on the bark of conifer seedlings. Silicon (Si) has been shown to enhance the resistance of plants to insect herbivores. This study investigated the effects of low doses of Si-rich soil amendments on growth, mortality and bark feeding damage of Sitka spruce (Picea sitchensis) seedlings. Two-year old seedlings were grown, individually, in soil taken from a tree nursery treated with coal ash, peat ash, rice husk ash, slag, sodium metasilicate or a commercially available Si fertiliser (Pro-Tekt) and planted out on two reforestation sites in Ireland. Seedlings grew well (about 20% growth in terms of height, 66% in root collar diameter, after two growing seasons), and Si-rich amendments did not have a significant effect on growth or mortality. Bark feeding damage on Si-treated seedlings did not vary significantly from control seedlings. Bark Si concentrations were not significantly larger in treated seedlings than in control seedlings, but control seedlings already had comparatively high bark Si concentrations (560 mg kg−1 dry tissue). In conclusion, Sitka spruce seedlings grown in the presence of Si-rich soil amendments prior to planting did not show greater resistance to weevil feeding under the present conditions.

Влияние состава древостоев на усыхание ели

Plantions of pleurocarpous moss spruce forest type are dominant in the area of coniferous-deciduous (mixed) forests of the European part of the Russian Federation in Perm region. The problem of drying of spruce forests has aggravated dramatically in recent years. However, among scientists there is no consensus on the causes of the drying of spruce forests, which hinders the development of recommendations on improvement of forest management in spruce forests. In this regard spruce plantations in Perm region are not exceptions. On the basis of acts of forest pathology survey, drying of pleurocarpous moss spruce forest with different composition of forest stands is analyzed. The drying area for the period from 2010 to 2016 at Ocherskoe forestry in Perm region (forest area of coniferous-deciduous (mixed) forests of the European part of the Russian Federation) is mapped to a planted area of pleurocarpous moss spruce forest of different species composition. It was found that there was no drying out spots for the analyzed period in plantings of pleurocarpous moss spruce forest with a mixture of softwood species. As well as stands with admixture of fir and birch are characterized by high resistance. Among the identified drying out spots of spruce the most represented are plantations which contain of spruce, fir and pine (28.1 % of the total area of drying out spots), as well as spruce, fir, pine and birch (22.56 % of the total area of drying out spots). Data on the influence of the composition of the forest on mortality of spruce in the conditions of pleurocarpous moss spruce forest can be used in forming thinnings, as well as creation of forest crops.

Occurrence of Ips typographus (L.) after wind damage in the Kościeliska Valley of the Tatra National Park

Abstract At the end of 2013, Norway spruce stands in the area of the Tatra National Park were severely damaged by strong storms especially in the Kościeliska Valley region. In the following spring of 2014, a survey recording the occurrence of the spruce bark beetle Ips typographus (Coleoptera, Curculionidae, Scolytinae) was initiated in order to describe the dynamics of beetle reproduction in relation to protection measures executed in wind-damaged stands. Ten research plots with 20 trees each were established in a socalled active protection zone, where the broken and fallen trees had been processed and removed in 2014, and in a passive protection zone, where no actions were taken, and the dynamics of Norway spruce mortality due to bark beetle infestation including quantitative parameters (infestation density, sex ratio of beetle populations) were examined. The entomological analyses were performed on 25 × 25 cm large bark samples taken from four (active zone) or two (passive zone) tree sections. In the first year of the survey, no infested standing trees were recorded on the plots and the colonisation of fallen and broken trees was very weak. In the second year (2015), infestations appeared in larger numbers on the plots with passive compared to active protection but the infestation density was 0.89 mating chambers per 1 dm2 regardless of the protection status. In the third year (2016), most of the remaining living spruces had been infested with a mean density of 0.82 m.ch. per 1 dm2. In 2015, the proportion of females in the beetle population was 65.8% being higher in the active (68.4%) than the passive (64.0%) protection zone, while in 2016 the proportion was 63.5% and in this case slightly higher in the passive protection zone (63.9% as compared to 63.2%). These results are in accordance with patterns observed in wind-damaged Norway spruce stands of other areas in Poland and Europe and demonstrate the usefulness of forest management procedures in mitigating I. typographus outbreaks.

Gap dynamics in the Western Carpathian mixed beech old-growth forests affected by spruce bark beetle outbreak

Understanding how species-specific disturbances affect the dynamics of mixed forests is becoming increasingly important due to rapidly changing disturbance regimes. This study estimated the effect of Norway spruce (Picea abies (L.) Karst.) mortality on the disturbance processes in two mixed beech stands of the Western Carpathians that were affected by a bark beetle outbreak. We evaluated the size distribution, fraction of canopy and expanded gaps, the characteristics of gapmakers and the contribution of different species to gap size. The measured canopy gap fraction was <5%, and most canopy gaps were small (<100 m2). Spruce was the most abundant gapmaker, and its share among gapmakers was 3–6 times higher than its share in the canopy. We found that the increase in spruce mortality due to the outbreak resulted in a fine-scale mortality pattern. However, spruce gapmakers did not contribute much to gap area size, as shown by a weak correlation between the number of spruce gapmakers and the area of expanded gaps. Diameter distribution of living versus recently dead trees showed that beech mortality occurred disproportionately in large size classes. However, dead spruce trees were equally frequent in all diameter classes, which means beetles did not exclusively attack larger trees in these stands during the outbreak. We conclude that spruce mortality may have influenced successional processes by giving a competitive advantage to two other species that were not affected by the outbreak, provided that a high deer browsing intensity does not hinder the regeneration of new seedlings.

Landscape-Level Spruce Mortality Patterns and Topographic Forecasters of Bark Beetle Outbreaks in Managed and Unmanaged Forests of the Tatra Mountains

Bark beetle outbreaks and tree mortality patterns should be better understood to control outbreak impacts. We investigated landscape-level patterns of Norway spruce mortality caused by Ips typographus outbreaks across three periods from 1999–2012 in Tatra National Park (Poland) using high-resolution aerial orthophotos and satellite imagery. Shifts in tree mortality related to elevation, slope, and solar equinox radiation were analyzed with ANOVAs (Tukey's HSD tests). Boosted regression trees were employed to assess the forecasting effectiveness of these variables related to mortality period. Spruce mortality severity increased significantly across time in both managed and unmanaged forests. Management activities did not effectively reduce spruce mortality severity. Mortality severity increased significantly at higher elevations over time, while slope and radiation trends varied. Elevation and radiation were the best forecasters of mortality period, exhibiting moderate predictive ability. Beetle-induced spruce mortality increased significantly in Tatra National Park from 1999–2012, particularly at high elevations. Management strategies aimed at minimizing spruce mortality have been ineffective.

Białowieża Forest: what it used to be, what it is now and what we want it to be in the future

Abstract For many centuries, management of the Białowieża Forest has not focused on timber production. Therefore, despite hunting, grazing by domestic animals and sporadic cutting of valuable trees the forest has retained its natural character. After World War I, a small part of the Białowieża Forest was protected as a reserve that later became a national park, while the remainder was managed for timber. After World War II, the protection status of the Polish part of the Białowieża Forest was maintained with the national park at the center surrounded by managed stands. During the last few decades, the national park was enlarged and new reserves were established. However, the majority of the Białowieża Forest is still managed for timber. The forest management has been sustainable for decades and in the last few years logging has even been strongly reduced, to a level comparable with some national parks. In recent years the Białowieża Forest, like many areas in Central Europe, has been plagued by a high spruce mortality caused by bark beetles. In managed forests, cutting the infested spruces and removing them from the forest is a standard practice aimed at reducing the growth rate of the bark beetle population. This, however, raises the question of whether we expect the Białowieża Forest to remain a managed forest, in which case the fight against bark beetles would be justified, or whether we want it to be converted into a large national park? In the latter case, cutting trees to fight bark beetles would be inconsistent with the aim of conservation. Recent discussions concerning the Białowieża Forest have been dominated by two different ideologies for nature protection. The first approach aims at protecting nature to make it sustainable, beautiful and healthy. In the second approach, protecting nature is achieved by removing any direct human influence, even if the resulting natural environment does not meet our expectations.

Spectral evidence of early-stage spruce beetle infestation in Engelmann spruce

Spruce beetle (Dendroctonus rufipennis (Kirby)) outbreaks cause widespread mortality of Engelmann spruce (Picea engelmannii (Parry ex Engelm)) within the subalpine forests of the western United States. Early detection of infestations could allow forest managers to mitigate outbreaks or anticipate a response to tree mortality and the potential effects on ecosystem services of interest. However, the subtle changes in the foliage of infested spruce make early detection difficult. An experiment was conducted in southern Colorado to determine important wavelengths for detecting early-stage (i.e. recently infested) spruce beetle infestation in Engelmann spruce. Spectral reflectance from non-infested and recently infested spruce needles were obtained using the ASDi Field-Spec Pro spectroradiometer. After pre-processing, random forest analysis was used to identify hyperspectral bands and aggregations of hyperspectral bands corresponding to Landsat TM bands and vegetation indices that effectively discriminated between non-infested and infested trees. Results show that the shortwave infrared region of the electromagnetic spectrum was a key area for detecting early stages of spruce beetle infestation, likely due to the effects of beetle infestation on water transport within Engelmann spruce. The strong discriminability of bands in the shortwave infrared region indicates a potential for this spectral region to be used to detect early-stage spruce beetle infestation over larger areas using multispectral satellite imagery. In a preliminary trial, we found that a time series of reflectance in Landsat TM band 7 (shortwave infrared) was strongly correlated with the progression through time of a spruce beetle outbreak in southern Wyoming. These findings suggest that multispectral indicators of early-stage spruce beetle outbreak can be developed. These indicators are needed to better understand spatiotemporal dynamics of spruce beetle outbreaks, and can be used by forest managers to detect early stages of spruce beetle infestation and to potentially mitigate some spruce mortality.

Fine-scale spruce mortality dynamics driven by bark beetle disturbance in Babia Góra National Park, Poland

Bark beetle outbreaks have had major impacts on Norway spruce forests in Europe. The large majority of these forests are located in areas under forest management; thus, few studies have investigated outbreak-driven spruce mortality patterns unaffected by humans. Our study examined spruce mortality resultant from a beetle outbreak in a high-elevation, unmanaged forest over a 17-year span. We analyzed three tree-level survivorship and DBH datasets collected during pre-, mid-, and post-outbreak conditions to evaluate long-term mortality dynamics. We measured changes in mortality severity, topographic and stand structure characteristics, and stand complexity using ANOVAs, and we assessed five topographic and stand structure mortality predictors by employing boosted regression trees. Our results showed that though spruce mortality increased significantly over time, such increases were disproportionate with spatial synchrony. Moreover, the outbreak did not significantly alter the living stand structure and had little effect on stand complexity, exhibiting the effects of an outbreak that spread throughout the forest without causing major stand-level damage. Larger trees at higher elevations on south-facing slopes were targeted most frequently, particularly during the later stages of the outbreak. Aspect, elevation, and slope were the best predictors of mortality, demonstrating moderate forecasting ability. We showed that bark beetle outbreaks can operate on patch-scale gradients, affecting microhabitat conditions, without resulting in sweeping, stand-altering mortality. Small-scale outbreaks may increase forest resilience against more severe outbreaks in the future by creating canopy gaps that facilitate regeneration, which leads to more complex age and size structures within the stand.

Managed Mixtures of Aspen and White Spruce 21 to 25 Years after Establishment

Intimate mixtures of trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench) Voss) are a key feature of western Canadian boreal forests. These mixtures have the potential to produce high yields of merchantable fibre and provide numerous ecological services. Achievement of this potential has been difficult, and often expensive, to realize as a regeneration goal in managed forests. We report 21 to 25 year results of managed mixtures on two study sites where the white spruce was planted, and the density of aspen natural regeneration manipulated within five years of the stand initiation disturbance. On both sites, white spruce mortality did not increase with increasing aspen density. While height and diameter growth of white spruce declined with increasing aspen density, the effect was not entirely consistent across the two sites. Abrasion from aspen branches was the most common source of damage to spruce crowns. Mixed stands had greater merchantable volume production than pure spruce stands based on model projections. Application of aspen harvest at year 60, while protecting the spruce component for a second harvest entry at year 90, was projected to optimize combined yield for the mixedwood stands.