Eurasian Forests Research Articles

Cenozoic Dalbergia (Fabaceae) plant fossils from Southwest China: Biogeographic implications and plant-insect interactions

Dalbergia is a pantropical genus of Fabaceae with limited fossil records. In this study we describe diverse and well-preserved Dalbergia fossils distributed through Cenozoic (Eocene to Pliocene) strata in Yunnan Province, Southwest China. Comparison with newly collected modern samples from Eurasian forests and herbarium specimens allow accurate identification of our fossil materials, and we demonstrate that Yunnan Province represents an hitherto unrecognised hotspot for the diversification of Dalbergia. Our findings also include evidence of the earliest known mining damage reported on Dalbergia leaflets, which sheds light on their interactions with various arthropods (Leucoptera and other insects) during the early Neogene. As such, our results improve knowledge of Dalbergia fossils in East Asia and are crucial for understanding the evolutionary history of analogous mining behavior observed from modern Dalbergia.

Diurnal temperature fluctuations improve predictions of developmental rates in the spruce bark beetle Ips typographus

AbstractThe European spruce bark beetle Ips typographus is a widespread pest in Norway spruce-dominated forests in Eurasia. Predicting its phenology and voltinism is crucial to plan forest management measures and to mitigate mass outbreaks. Current phenology models are based on constant temperatures inferred from laboratory experiments; however, insect life cycles under natural conditions are rather driven by diurnal and seasonal temperature fluctuations. Therefore, phenology models based on fluctuating temperatures would reflect field conditions more realistically and might thus improve model predictions. In a laboratory experiment, we investigated the development of I. typographus, applying mean temperatures between 3 and 35 °C and diurnal temperature oscillations of up to ± 15 °C. Subsequently, we calibrated developmental rate models and applied them to climate data, in order to assess the effect of temperature fluctuations on voltinism under field conditions. Our results showed that diurnal temperature oscillations significantly affected developmental rates. Compared to constant temperatures, development was faster at temperature oscillations falling below the lower developmental threshold, and slower at temperature oscillations exceeding the developmental optimum. Furthermore, short exposures to suboptimal temperatures affected I. typographus less than expected from constant conditions. Natural temperature fluctuations thus accelerate development under cool, shaded conditions, whilst slowing it under hot, sun-exposed conditions, thereby ultimately affecting voltinism. Our findings highlight the importance to account for diurnal temperature fluctuations for more accurate predictions of developmental rates of I. typographus in natural thermal environments, and provide the fundament for improving current phenology models to support effective bark beetle management in a warming climate.

Engineered pine endophytic Bacillus toyonensis with nematocidal and colonization abilities for pine wilt disease control.

The pinewood nematode (PWN) is responsible for causing pine wilt disease (PWD), which has led to the significant decline of conifer species in Eurasian forests and has become a globally invasive quarantine pest. Manipulating plant-associated microbes to control nematodes is an important strategy for sustainable pest management. However, it has proven difficult to find pine-associated bacteria that possess both nematocidal activity and the ability to colonize pine tissues. The stress experiments with turpentine and pine tissue extract were carried out to screen for the desired target strain that could adapt to the internal environment of pine trees. This strain was used to construct an engineered nematocidal strain. Additionally, a fluorescent strain was constructed to determine its dispersal ability in Pinus massoniana seedlings through plate separation, PCR detection, and fluorescence microscopy observations. The engineered nematocidal strain was tested in the greenhouse experiment to assess its ability to effectively protect P. massoniana seedlings from nematode infection. This study isolated a Bacillus toyonensis strain Bxy19 from the healthy pine stem, which showed exceptional tolerance in stress experiments. An engineered nematocidal strain Bxy19P3C6 was constructed, which expressed the Cry6Aa crystal protein and exhibited nematocidal activity. The fluorescent strain Bxy19GFP was also constructed and used to test its dispersal ability. It was observed to enter the needles of the seedlings through the stomata and colonize the vascular bundle after being sprayed on the seedlings. The strain was observed to colonize and spread in the tracheid after being injected into the stems. The strain could colonize the seedlings and persist for at least 50 days. Furthermore, the greenhouse experiments indicated that both spraying and injecting the engineered strain Bxy19P3C6 had considerable efficacy against nematode infection. The evidence of the colonization ability and persistence of the strain in pine advances our understanding of the control and prediction of the colonization of exogenously delivered bacteria in pines. This study provides a promising approach for manipulating plant-associated bacteria and using Bt protein to control nematodes.

Approaching a thermal tipping point in the Eurasian boreal forest at its southern margin

Climate change is increasing the intensity and frequency of extreme heat events. Ecological responses to extreme heat will depend on vegetation physiology and thermal tolerance. Here we report that Larix sibirica, a foundation species across boreal Eurasia, is vulnerable to extreme heat at its southern range margin due to its low thermal tolerance (Tcrit of photosynthesis: ~ 37–48 °C). Projections from CMIP6 Earth System Models (ESMs) suggest that leaf temperatures might exceed the 25th percentile of Larix sibirica’s Tcrit by two to three days per year within the next two to three decades (by 2050) under high emission scenarios (SSP3-7.0 and SSP5-8.5). This degree of warming will threaten the biome’s continued ability to assimilate and sequester carbon. This work highlights that under high emission trajectories we may approach an abrupt ecological tipping point in southern boreal Eurasian forests substantially sooner than ESM estimates that do not consider plant thermal tolerance traits.

Research of methods and processing of databases on the biomass of Eurasian forests as neural networks. Part 2. New opportunities for artificial intelligence in predicting climate-driven changes

The assessment of the carbon storage capacity of forests has reached the global level, and the assessment of greenhouse gas absorption at carbon landfills is relevant. The authors have developed and published three author's databases on the biological productivity of Eurasian forests. It is shown that for databases, correct algorithms of alternative methods give close results, and an incorrect algorithm gives a significant shift in the result in relation to the model of the same ideology, but built according to the correct algorithm. The resulting models are used to predict changes in these indicators over time based on the principle of spatio-temporal substitution. It has been established that the climatic conditionality of the studied bioproduction indicators is of a general nature for both quantitative and qualimetric indicators of the biomass of trees and forest stands. The resulting models are applied in the construction of a neural network to predict changes in these indicators over time based on the principle of space-time substitution. In the process of machine learning and solution, it was found that the climatic conditionality of the studied bioproduction indicators is of a general nature for both quantitative and qualimetric indicators of the biomass of trees and forest stands.

Comparative evolutionary history of two closely related desert plant, Convolvulus tragacanthoide and Convolvulus gortschakovii (Convolvulaceae) from northwest China.

Desert ecosystems are one of the most fragile ecosystems on Earth. The study of the effects of paleoclimatic and geological changes on genetic diversity, genetic structure, and species differentiation of desert plants is not only helpful in understanding the strategies of adaptation of plants to arid habitats, but can also provide reference for the protection and restoration of vegetation in desert ecosystem. Northwest China is an important part of arid regions in the northern hemisphere. Convolvulus tragacanthoides and Convolvulus gortschakovii are closely related and have similar morphology. Through our field investigation, we found that the annual precipitation of the two species distribution areas is significantly different. Thus, C. tragacanthoides and C. gortschakovii provide an ideal comparative template to investigate the evolutionary processes of closely related species, which have adapted to different niches in response to changes in paleogeography and paleoclimate in northwest China. In this study, we employed phylogeographical approaches (two cpDNA spacers: rpl14–rpl36 and trnT–trnY) and species distribution models to trace the demographic history of C. tragacanthoides and C. gortschakovii, two common subshrubs and small shrubs in northwest China. The results showed the following: (1) Populations of C. tragacanthoides in northwest China were divided into three groups: Tianshan Mountains—Ili Valley, west Yin Mountains—Helan Mountains‐Qinglian Mountains, and Qinling Mountains—east Yin Mountains. There was a strong correlation between the distribution of haplotypes and the floristic subkingdom. The three groups corresponded to the Eurasian forest subkingdom, Asian desert flora subkingdom, and Sino‐Japanese floristic regions, respectively. Thus, environmental differences among different flora may lead to the genetic differentiation of C. tragacanthoides in China. (2) The west Yin Mountains—Helan Mountains‐Qinglian Mountains, and Qinling Mountains—east Yin Mountains were thought to form the ancestral distribution range of C. tragacanthoides. (3) C. tragacanthoides and C. gortschakovii adopted different strategies to cope with the Pleistocene glacial cycle. Convolvulus tragacanthoides contracted to the south during the glacial period and expanded to the north during the interglacial period; and there was no obvious north–south expansion or contraction of C. gortschakovii during the glacial cycle. (4) The interspecific variation of C. tragacanthoides and C. gortschakovii was related to the orogeny in northwest China caused by the uplift of the Tibetan Plateau during Miocene. (5) The 200 mm precipitation line formed the dividing line between the niches occupied by C. tragacanthoides and C. gortschakovii, respectively. In this study, from the perspective of precipitation, the impact of the formation of the summer monsoon limit line on species divergence and speciation is reported, which provides a new perspective for studying the response mechanism of species to the formation of the summer monsoon line, and also provides a clue for predicting how desert plants respond to future environmental changes.

Research of methods and processing of databases on the biomass of Eurasian forests as neural networks. Part 1. System analysis of a database for transformation into neural networks of artificial intelligence

The assessment of the carbon storage capacity of forests is now reaching the global level, and the assessment of greenhouse gas absorption at carbon landfills is relevant. The authors have developed and published three author's databases on the biological productivity of Eurasian forests. An algorithm for estimating the incoming part of the carbon cycle (NWP) of the forest cover of the Ural region has been developed, which forms the basis for generating a neural network from initial databases. The present work is devoted to the further development of models and algorithms for the use of these databases, transformed into neural networks to solve urgent global problems of our time related to climate change.

Climatic variation drives loss and restructuring of carbon and nitrogen in boreal forest wildfire

Abstract. The boreal forest landscape covers approximately 10 % of the earth's land area and accounts for almost 30 % of the global annual terrestrial sink of carbon (C). Increased emissions due to climate-change-amplified fire frequency, size, and intensity threaten to remove elements such as C and nitrogen (N) from forest soil and vegetation at rates faster than they accumulate. This may result in large areas within the region becoming a net source of greenhouse gases, creating a positive feedback loop with a changing climate. Meter-scale estimates of area-normalized fire emissions are limited in Eurasian boreal forests, and knowledge of their relation to climate and ecosystem properties is sparse. This study sampled 50 separate Swedish wildfires, which occurred during an extreme fire season in 2018, providing quantitative estimates of C and N loss due to fire along a climate gradient. Mean annual precipitation had strong positive effects on total fuel, which was the strongest driver for increasing C and N losses. Mean annual temperature (MAT) influenced both pre- and postfire organic layer soil bulk density and C : N ratio, which had mixed effects on C and N losses. Significant fire-induced loss of C estimated in the 50 plots was comparable to estimates in similar Eurasian forests but approximately a quarter of those found in typically more intense North American boreal wildfires. N loss was insignificant, though a large amount of fire-affected fuel was converted to a low C : N surface layer of char in proportion to increased MAT. These results reveal large quantitative differences in C and N losses between global regions and their linkage to the broad range of climate conditions within Fennoscandia. A need exists to better incorporate these factors into models to improve estimates of global emissions of C and N due to fire in future climate scenarios. Additionally, this study demonstrated a linkage between climate and the extent of charring of soil fuel and discusses its potential for altering C and N dynamics in postfire recovery.

Central European forest–steppe: An ecosystem shaped by climate, topography and disturbances

AbstractThe occurrence and origin of dry grasslands and their rich biota in the moderately humid Central‐European climate have fascinated scientists for over a century. Modern palaeoecological and phylogeographical data support earlier hypotheses that these grasslands are late Pleistocene relicts and can therefore be considered part of the Eurasian forest–steppe biome. However, it is still unclear which factors fostered the maintenance of steppe patches in Central Europe throughout the Holocene. Here, we provide an overview of the main hypotheses, which stress, respectively, the effects of climate, edaphic conditions and disturbances. We then develop a general conceptual framework on how these three factors interact to form forest–steppe mosaics. We thereby emphasize the role of topography as a crucial control on forest–steppe patterns at the landscape scale. Topography is related to several mechanistic drivers that influence vegetation processes, such as near‐surface microclimate and soil formation. Consequently, topographic variation allows both forest and steppe patches to occur beyond their macroclimatic niche, favouring the development of forest–steppe mosaics. To illustrate our framework, we demonstrate the interactive effect of macroclimate and topography on the occurrence of steppe patches at 108 selected Central European forest–steppe sites. Although we developed our framework focusing on the current distribution of Central European forest–steppe, we suggest that it contributes to the understanding of similar transitions between temperate forest and steppe biomes in the past as well as elsewhere in the world.

Asian gypsy moth (Lymantria dispar L.) populations: Tolerance of eggs to extreme winter temperatures.

Gypsy moth Lymantria dispar (GM) is a polyphagous insect and one of the most significant pests in the forests of Eurasia and North America (U.S. and Canada). Accurate information on GM cold-hardiness is needed to improve methods for the prediction of population outbreaks, as well as for forecasting possible GM range displacements due to climate change. As a result of laboratory and field studies, we found that the lower lethal temperature (at which all eggs die) range from -29.0°C to -29.9°C for three studied populations of L. dispar asiatica, and no egg survived cooling to -29.9°C. These limits agree, to within one degree, with the previously established cold-hardiness limits of the European subspecies L. dispar, which is also found in North America. This coincidence indicates that the lower lethal temperature of L. dispar is conservative. Thus, we found that the Siberian populations of GM inhabit an area where winter temperatures go beyond the limits of egg physiological tolerance, because temperatures often fall below -30°C. Apparently, it is due to the flexibility of ovipositional behavior that L. dispar asiatica survives in Siberia: the lack of physiological tolerance of eggs is compensated by choosing warm biotopes for oviposition. One of the most important factors contributing to the survival of GM eggs in Siberia is the stability of the snow cover.

High temperatures constrain latewood formation in Larix gmelinii xylem in boreal forests

In trees, both radial growth and earlywood or latewood differentiation of xylem can reflect responses not only to internal stimuli but also to changes in the external environment. Here, we examined the effect of age and environment on the proportion of latewood to earlywood (PLE) in Larix gmelinii (Dahurian larch) from boreal forests in Eurasia to gain a better understanding of the influence of climate change on genetically controlled characteristics of tree growth. Distinctive differentiation and interannual changes in growth divergences were dependent on age, whereas the influence of environmental factors was also limited. The low-frequency change in the PLE has a juvenile effect, and the variation is unstable. The proportions of latewood to earlywood changed significantly with age during the first 100 years of life (p < 0.05), and environmental factors had a limited impact on the PLE during this stage. However, environmental effects on the PLE became statistically significant when trees were over 100 years old. The average temperature in June–August had a significantly negative correlation with the PLE with temporal changes. The PLE increased significantly with decreased annual mean temperature (p < 0.05) with geospatial variations. After 100 years of tree age, temperature played a crucial role in PLE changes. This suggests that the abnormal radial growth of Larix gmelinii, as indicated by the PLE, will occur in response to rising temperatures. Therefore, PLE-related supporting structures and intrinsic physiological processes in trees can be expected to undergo complex changes, the nature of which is uncertain, as temperatures rise globally. The variation in the PLE could lead to significant changes in the growth of the dominant tree species and hence in the structure and function of boreal forests in cold temperate zones.

Introgression of phylogeography lineages of Convolvulus gortschakovii (Convolvulaceae) in the northwest China

The paleo-climatic and paleo-geological changes aggravated the aridification of northwest China during the Pleistocene. Thus, nowadays most regions of northwest China belong to the Asian desert flora subkingdom. However, Tianshan Mountains and Altay Mountains show relatively humid climate and belong to the Eurasian forest subkingdom. In order to investigate the effects of these two climate conditions on distribution of desert plants in northwest China, we selected the aridity adapted species Convolvulus gortschakovii (Convolvulaceae) to study its phylogeographical structure. Two plastid regions (rpl14 - rpl36 and trnT - trnY intron) and the nuclear ribosomal internal transcribed spacer (ITS) region were sequenced in individuals from 25 natural populations across the distribution range of this species in northwest China. Results revealed that all populations of C. gortschakovii were divided into three groups: Junggar Basin, Alataw Mountains – central Tianshan Mountains - Tarim Basin and Alxa Desert. Tianshan Mountains were considered to be great geographic barriers to gene flow between the Junggar Basin and Tarim Basin groups. Junggar Basin is thought to be the ancestral distribution range of C. gortschakovii. In addition, at least two dispersal routes were revealed. The eastern dispersal routes were from Junggar Basin to Alxa Desert, and from Junggar Basin to the central Tianshan Mountains. The western dispersal routes were from the Junggar Basin to the Alataw Mountains, from the Alataw Mountains to the central Tianshan Mountains, and from Tarim Basin to central Tianshan Mountains. In addition, a genetic intersection among the three phylogeographic lineages was identified in the Tianshan Mountains and southeastern Gurbantunggut Desert. According to the influence of mountains on the present conformation of genetic structure of C. gortschakovii, it is inferred that Eurasian forest subkingdom plays an important role in the genetic diversity and dispersal of xerophytes of Asian desert flora subkingdom. In addition, estimation of divergence times suggested that genetic structure in C. gortschakovii has been heavily affected by glaciation and climatic shifts during the Pleistocene.

Functional investigation of monoterpenes for improved understanding of the relationship between hosts and bark beetles

AbstractSpruce bark beetle (Ips typographus L.) is the most destructive insect pest of spruce forests in Eurasia. However, contact toxicity, in vivo metabolism, and ecological functions of host monoterpenes are poorly understood at the spruce tree–bark beetle–predator tritrophic level. Spruce monoterpenes including S‐(–)‐α‐pinene, R‐(+)‐α‐pinene, and myrcene showed contact toxicity to I. typographus, with LD50 values ranging from 22–32 μg/mg. When topically treated with S‐(–)‐α‐pinene or R‐(+)‐α‐pinene, the amounts of volatile metabolites, including 4S‐(–)‐cis‐verbenol, 4S‐(+)‐/4R‐(–)‐trans‐verbenol, R‐(+)‐/S‐(–)‐verbenone and 1R‐(–)‐/1S‐(+)‐myrtenol, in the hindgut extracts of I. typographus varied significantly between sexes, and their quality (enantiomeric composition) varied significantly with the chirality of α‐pinene. More importantly, S‐(–)‐α‐pinene induced male adults to produce large amounts of 4S‐(–)‐cis‐verbenol and S‐(–)‐verbenone. When topically treated with myrcene, the expected semiochemicals such as E‐myrcenol, ipsenol and ipsdienol were not detected in the beetle hindguts, indicating that the pheromone biosynthetic system of I. typographus does not participate in the metabolism of host myrcene. In trap tests, S‐(–)‐α‐pinene and R‐(+)‐α‐pinene increased the catches of I. typographus and its predator Thanasimus substriatus in pheromone‐baited traps, whereas myrcene exhibited a strong repellent (or inhibitory) effect on I. typographus but not on its predator. I. typographus seems to adopt different ecological strategies (e.g. avoidance to myrcene and preference for α‐pinene) to adapt to and tolerate different host monoterpenes. Extensive investigation of these monoterpenes will help us understand their roles in manipulating the arms race between host trees and bark beetles, and potentially improve the efficacy of controlling I. typographus via the push‐pull strategy using host kairomones.

Comparison of methods to obtain and maintain cultures of the pinewood nematode, Bursaphelenchus xylophilus

ABSTRACTThe pinewood nematode Bursaphelenchus xylophilus, native from North America, is the causal agent of the pine wilt disease, being considered one of the most important invasive pests in Eurasian pine forests. The establishment of laboratory isolates of B. xylophilus has been crucial to research on this important pathogen of pine trees. However, studies on the effects of in vitro serial passage on the pathogen evolution, and suitability of standard culture media to the wild populations have been surprisingly few. In the present work, we tested the suitability of different culture media for obtaining B. xylophilus lab populations from dauer larvae that have been isolated from pine sawyer beetles (Monochamus spp.), which serve as their vector. We conducted studies in North America and Portugal. Results showed that adding groundwood (xylem from pine trees) to standard agar medium dramatically improves the survival and growth of wild populations of B. xylophilus in the laboratory. Further, malt extract agar proved to be more suitable than potato dextrose agar. Once in culture, B. xylophilus grew well feeding on either of two fungus species: Botrytis cinerea or Ophiostoma minus. Growth rates were highest on B. cinerea, the most widely used fungus to maintain lab cultures of B. xylophilus, but O. minus has the advantage of being a natural associate and putative diet of wild populations. There remains a need for better understanding of how to obtain and maintain laboratory populations of this pathogen such that their natural characteristics are maintained.

Bacterial Community Structure of Pinus Thunbergii Naturally Infected by the Nematode Bursaphelenchus Xylophilus.

Pine wilt disease (PWD) caused by the nematode Bursaphelenchus xylophilus is a devastating disease in conifer forests in Eurasia. However, information on the effect of PWD on the host microbial community is limited. In this study, the bacterial community structure and potential function in the needles, roots, and soil of diseased pine were studied under field conditions using Illumina MiSeq coupled with Phylogenetic Investigation of Communities by Reconstruction of Unobserved states (PICRUSt) software. The results showed that the community and functional structure of healthy and diseased trees differed only in the roots and needles, respectively (p < 0.05). The needles, roots, and soil formed unique bacterial community and functional structures. The abundant phyla across all samples were Proteobacteria (41.9% of total sequence), Actinobacteria (29.0%), Acidobacteria (12.2%), Bacteroidetes (4.8%), and Planctomycetes (2.1%). The bacterial community in the healthy roots was dominated by Acidobacteria, Planctomycetes, and Rhizobiales, whereas in the diseased roots, Proteobacteria, Firmicutes, and Burkholderiales were dominant. Functionally, groups involved in the cell process and genetic information processing had a higher abundance in the diseased needles, which contributed to the difference in functional structure. The results indicate that PWD can only affect the host bacteria community structure and function in certain anatomical regions of the host tree.

Populations dynamics in Northern Eurasian forests: a long-term perspective from Northeast Asia.

The 'Northern Eurasian Greenbelt' (NEG) is the northern forest zone stretching from the Japanese Archipelago to Northern Europe. The NEG has created highly productive biomes for humanity to exploit since the end of the Pleistocene. This research explores how the ecological conditions in northern Eurasia contributed to and affected human migrations and cultural trajectories by synthesizing the complimentary viewpoints of environmental archaeology, Geographic Information Science (GIS), genetics and linguistics. First, the environmental archaeology perspective raises the possibility that the NEG functioned as a vessel fostering people to develop diverse cultures and engage in extensive cross-cultural exchanges. Second, geographical analysis of genomic data on mitochondrial DNA using GIS reveals the high probability that population dynamics in the southeastern NEG promoted the peopling of the Americas at the end of the Pleistocene. Finally, a linguistic examination of environmental- and landscape-related vocabulary of the proto-Turkic language groups enables the outline of their original cultural landscape and natural conditions, demonstrating significant cultural spheres, i.e. from southern Siberia to eastern Inner Mongolia during Neolithization. All of these results combine to suggest that the ecological complex in the southern edge of the NEG in northeast Asia played a significant role in peopling across the continents during prehistory.

Somatic embryogenesis of scots pine: current status and perspective of application in forestry

In the review it is come up with information on the possibilities of breeding programs and the creation of seed orhards of Scots pine (Pinus sylvestris L) as one of the key species of forests in Eurasia, which has significant economic value. Somatic embryogenesis is a promising biotechnological method that allows mass propagation of valuable individuals obtained, including during the implementation of breeding programs. Using the example of various species of the Pinus genus, the main stages of somatic embryogenesis are described. The influence of the conditions and period of cultivation on the effectiveness of somatic embryogenesis and the subsequent conversion of somatic embryos into the plants are considered. Particular attention is paid to the growth and physiology of clones in the field.

Sudden Aspen Decline: A Review of Pattern and Process in a Changing Climate

The American quaking aspen (Populus tremuloides Michx.) and its close relative, the Eurasian quaking aspen (Populus tremula L.), cover a realm that is perhaps the most expansive of all tree species in the world. In North America, sudden aspen decline (SAD) is a growing concern that marks the rapid decline of quaking aspen trees leading to mortality at the stand and landscape scale. Research suggests that drought and water stress are the primary causes of SAD. Predisposing factors (age, structure, and landscape position), as well as associated stressors (i.e., pests and pathogens), have been linked to mortality in affected stands. The conflation of multiple interacting factors across the aspen’s broad geographic range in North America has produced significant debate over the classification of SAD as a disease and the proper management of affected stands. Interestingly, no such effects have been reported for the Eurasian aspen. We here review and synthesize the growing body of literature for North America and suggest that SAD is a novel decline disease resulting from multiple inciting and interacting factors related to climate, land-use history, and successional dynamics. We suggest that the range of aspen observed at the onset of the 21st Century was bolstered by a wet period in western North America that coincided with widespread regional cutting and clearing of late-successional forests for timber and grazing. No comparable land-use history, successional status, or age-class structure is apparent or linked for Eurasian forests. Eurasian aspen is either absent or young in managed forests, or old and decadent in parks in Fenno-Scandinavia, or it grows more intimately with a more diverse mixture of tree species that have arisen from a longer period of frequent timber cutting in Russia. Based on these insights we provide recommendations for practical management techniques that can promote stand resilience and recovery across a range of stand conditions in North America. Managers should attempt to identify SAD-prone stands using the presence of predisposing conditions and focus treatments such as coppice or prescribed fire on stands with suitable topographies, elevations, and climates. We conclude that SAD will persist throughout the coming decades, given the enormity of past cutting history, fire exclusion, and current changes in climate until a more active restoration agenda is implemented.

Carbon exchange in a hemiboreal mixed forest in relation to tree species composition

Hemiboreal forests form a transitional belt between boreal and temperate forests in Eurasia, and due to long-term climate warming they could potentially expand in a northerly direction. However, carbon (C) exchange studies in this transitional forest type are scarce. In 2014–2015 we analyzed CO2 exchange in a hemiboreal mixed forest at SMEAR Estonia (Station for Measuring Ecosystem-Atmosphere Relations) using eddy covariance (EC), soil chamber technique and biometric methods. Employing two plots that differ in forest stand composition (mixed (MP) and coniferous (CP)) located within the footprint of the EC tower, we evaluated heterotrophic and total soil respiration (Rh and Rs, respectively). Measurements showed that C, nitrogen (N), and soil organic matter (SOM) contents in topsoil were significantly higher in the coniferous plot, and the total C stock until 20 cm of mineral soil was higher by a factor of two (i.e., 170.6 and 94.8 t C ha−1 for the CP and MP, respectively). Only in 2015 there was a significant difference in Rs between the plots, while Rh exhibited no detectable difference. The cumulative Rs during the growing season was similar for both plots (CP and MP) and years: 532 and 548 g m−2, respectively, in 2014; 636 and 592 g m−2, respectively, in 2015. The same applied to Rh: 418 and 430 g m−2, respectively, in 2014; 406 and 387 g m−2, respectively, in 2015.The forest as a whole was a C sink. In 2015, net ecosystem exchange (NEE) was -585.62 ± 45.16 g C m−2 yr−1 (the average and SD of two gap-filling methods: MDS and nonlinear regressions). Gross primary production (GPP), and ecosystem respiration (RE) were -1280.62 ± 53.36, and 696.00 ± 98.51 g C m−2 yr−1, respectively (the average and SD of two flux partitioning methods: nighttime data-based and daytime data-based). NEE and GPP suggested a greater similarity to temperate forests, while RE was closer to more boreal forest stands.The mean value of net ecosystem production (NEP) of both plots, estimated using forest stand biometric, litter and chamber data was similar to the annual NEE value: 6.3 and −5.9 t C ha−1 yr−1, respectively.

Legacies of Historical Exploitation of Natural Resources Are More Important Than Summer Warming for Recent Biomass Increases in a Boreal–Arctic Transition Region

Eurasian forest cover at high northern latitudes (> 67°N) has increased in recent decades due to stimulatory effects of global warming, but other factors may be important. The objective of this study is to compare the importance of historical human exploitation and climate change. Periodic information on forest and tundra resources along with human and domestic animal populations and forest harvesting was collected from sources like official statistics and maps and compiled for joint analysis. Our results show that the northernmost birch and Scots pine forests of the world often presumed as pristine were repeatedly exploited by logging, agriculture and grazing in the last century. In addition, repeated moth outbreaks have also had regulatory impacts on birch forest development. Despite these disturbances, forested area quadrupled during the period, largely because of reduced human activities in recent decades. Linear modelling confirms that the most important predictors for the variation in Scots pine and birch biomass and area were logging, grazing and farming activity, and not climatic changes. The dynamics in the forest cover over the last century seem to follow the ‘repeated human perturbation’ scenario. This study’s application of legacy data, and historical and long-term data and evaluation of how the different drivers impacted some of the northernmost forests are essential to understand whether the greening of the boreal and arctic regions is a result of recent climate change or a recovery from earlier human impacts.