Expansion Of Boreal Forest Research Articles

Grounding our Understanding of the Impacts of Boreal Forest Expansion on Shallow Cumulus Clouds with a Simple Modeling Framework

Abstract The expansion of the boreal forest poleward is a potentially important driver of feedbacks between the land surface and Arctic climate. A growing body of work has highlighted the importance of differences in evaporative resistance between different possible future Arctic land covers, which in turn alters humidity and cloudiness in the boundary layer, for these feedbacks. While thus far this problem has been studied primarily with complex Earth system models, we turn to a locally focused, idealized model capable of diagnosing and testing the sensitivity of first-order processes connecting vegetation, the atmospheric boundary layer, and low clouds in this critical region. This allows us to benchmark the mechanisms and results at the center of predictions from larger-scale simulations. A surface dominated by broadleaf trees, characterized by higher albedo and lower surface evaporative resistance, drives cooling and moistening of the boundary layer relative to a surface of needleleaf trees, characterized by lower albedo and higher surface evaporative resistance. Differences in evaporative resistance between these hypothetical Arctic vegetation covers are of equal importance to changes in albedo for the initial response of the boundary layer to boreal expansion, even with our idealized approach. However, compensation between the elevation of the lifting condensation level (LCL) and more rapid growth of the mixed layer over higher evaporative resistance surfaces can minimize changes in the favorability of shallow clouds over different land cover types under some conditions. We then perform two tests on the sensitivity of this compensating effect, to changes in water availability, represented first by a reduction in boundary layer humidity and then by both a reduction in humidity and soil moisture available to our vegetation surface. Finally, given the importance of this potential LCL–mixed-layer height compensation in our idealized modeling results, we look to determine its relevance in observational data from a field campaign in boreal Finland. These observations do confirm that such a coupling plays an important role in cumulus-topped boundary layers over a needleleaf forest surface. While our results confirm some underlying mechanisms at the center of prior work with Earth system models, they also provide motivation for future work to constrain the impact of boreal forest expansion. This will include both large eddy simulations to examine the impact of processes and feedbacks not resolved by a mixed-layer model, as well as a more systematic evaluation and comparison of relevant observations at the site in Finland and sites from prior boreal field campaigns. Significance Statement Clouds and vegetation are both important components of the climate system that interact across a range of scales. These interactions are central to understanding how changes at the land surface feedback on climate. For example, if a forest expands or recedes, diagnosing how that will impact clouds will determine whether you predict warming or cooling temperatures from that shift in the forest area. These predictions are often made with complex Earth system models, but we look to a more idealized representation of the land–atmosphere system to diagnose how shallow clouds should respond to changes in surface properties with different scenarios of boreal forest expansion at a more foundational level. This both grounds our understanding of previous analysis and provides helpful direction for future studies of this relevant and impactful land cover change.

Mammal extinction facilitated biome shift and human population change during the last glacial termination in East-Central Europe

The study of local extinction times, together with the associated environmental and human population changes in the last glacial termination, provides insights into the causes of mega- and microfauna extinctions. In East-Central (EC) Europe, groups of Palaeolithic humans were present throughout the last glacial maximum, but disappeared suddenly around 15,200 cal BP. In this study cave sediment profiles dated using radiocarbon techniques and a large set of mammal bones dated directly by AMS 14C were used to determine local extinction times. These were, in turn, compared to changes in the total megafauna population of EC Europe derived from coprophilous fungi, the Epigravettian population decline, quantitative climate models, pollen and plant macrofossil inferred climate, as well as to biome reconstructions. The results suggest that the population size of large herbivores decreased in the area after 17,700 cal BP, when temperate tree abundance and warm continental steppe cover both increased in the lowlands. Boreal forest expansion started around 16,200 cal BP. Cave sediments show the decline of narrow-headed vole and arctic lemming populations specifically associated with a tundra environment at the same time and the expansion of the common vole, an inhabitant of steppes. The last dated appearance of arctic lemming was at ~ 16,640 cal BP, while that of the narrow-headed vole at ~ 13,340, and the estimated extinction time of woolly mammoth was either at 13,830 (GRIWM) or 15,210 (PHASE), and reindeer at 11,860 (GRIWM) or 12,550 cal BP (PHASE). The population decline of the large herbivore fauna slightly preceded changes in terrestrial vegetation, and likely facilitated it via a reduction in the intensity of grazing and the concomitant accumulation of plant biomass. Furthermore, it is possible to conclude that the Late Epigravettian population had high degree of quarry-fidelity; they left the basin when these mammals vanished.

Climate impacts on vegetation and fire dynamics since the last deglaciation at Moossee (Switzerland)

Abstract. Since the Last Glacial Maximum (LGM; end ca. 19 000 cal BP) central European plant communities have been shaped by changing climatic and anthropogenic disturbances. Understanding long-term ecosystem reorganizations in response to past environmental changes is crucial to draw conclusions about the impact of future climate change. So far, it has been difficult to address the post-deglaciation timing and ecosystem dynamics due to a lack of well-dated and continuous sediment sequences covering the entire period after the LGM. Here, we present a new paleoecological study with exceptional chronological time control using pollen, spores and microscopic charcoal from Moossee (Swiss Plateau, 521 m a.s.l.) to reconstruct the vegetation and fire history over the last ca. 19 000 years. After lake formation in response to deglaciation, five major pollen-inferred ecosystem rearrangements occurred at ca. 18 800 cal BP (establishment of steppe tundra), 16 000 cal BP (spread of shrub tundra), 14 600 cal BP (expansion of boreal forests), 11 600 cal BP (establishment of the first temperate deciduous tree stands composed of, e.g., Quercus, Ulmus, Alnus) and 8200 cal BP (first occurrence of mesophilous Fagus sylvatica trees). These vegetation shifts were caused by climate changes at ca. 19 000, 16 000, 14 700, 11 700 and 8200 cal BP. Vegetation responses occurred with no apparent time lag to climate change when the mutual chronological uncertainties are considered. This finding is in agreement with further evidence from southern and central Europe and might be explained by the proximity to the refugia of boreal and temperate trees (<400 km) and rapid species spreads. Our palynological record sets the beginning of millennial-scale land use with periodically increased fire and agricultural activities of the Neolithic period at ca. 7000 cal BP. Subsequently, humans rather than climate triggered changes in vegetation composition and structure. We conclude that Fagus sylvatica forests were resilient to long-term anthropogenic and climatic impacts of the Mid and the Late Holocene. However, future climate warming and in particular declining moisture availability may cause unprecedented reorganizations of central European beech-dominated forest ecosystems.

Genus delimitation, biogeography and diversification ofChoristoneuraLederer (Lepidoptera: Tortricidae) based on molecular evidence

AbstractWidely known for pest species that include major modulators of temperate forests, the genusChoristoneurais part of the species‐rich tribe Archipini of leafroller moths (Tortricidae). Delimitation of the genus has remained unresolved because no phylogeny has included species endemic to Africa and studies have often omitted the type species of the genus. Further taxonomic confusion has been generated by the transfer ofArchips occidentalis(Walsingham) toChoristoneura, creating a homonym withChoristoneura occidentalisFreeman, an important defoliator of North American forests. To define the limits of the genus, we reconstructed a phylogeny using DNA sequences for mitochondrial cytochrome oxidase subunit I and nuclear ribosomal 28S genes. Our ingroup included 23Choristoneuraspecies‐level taxa, complemented by a large sample of outgroups comprising 82 species of Archipini and other Tortricidae. We generated a time‐calibrated tree using fossil and secondary calibrations and we inferred biogeographic and diversification processes inChoristoneura. Our analysis recovered the genus as polyphyletic, withArchips occidentalis,Choristoneura simonyiandChoristoneura evanidanaexcluded from the main clade. Based on the recovered phylogenies and a redefinition, we restrictChoristoneuraprimarily to species with a northern hemisphere distribution. Our analysis supportsA. occidentalisas the sister group ofCacoecimorpha pronubana,C. simonyias the sister of ‘Xenotemna’pallorana, andC. evanidanaas the sister ofArchips purpurana. A new combination is proposed:Archips evanidanacomb.n.; the availability of ‘Xenotemna’ as a valid name is discussed andA. occidentalisis considered as an orphaned name within the Archipini. We found support for a Holarctic origin ofChoristoneuraabout 23 Ma, followed by early divergence in the Palearctic region. The main divergence occurred at 16 Ma, with one clade in the Nearctic and another in the Palearctic. Subsequent cladogenetic events were synchronous and related to herbivorous specialization, with each clade divided into coniferophagous and polyphagous lineages. Their specialization as conifer feeders temporally matched the expansion of boreal forest during the Miocene.

The climatic control of sedimentary environment changes during the Weichselian – An example from the Middle Vistula Region (eastern Poland)

This study presents the sedimentary facies, stratigraphic framework and palaeoenvironmental conditions of the Weichselian sediments in the Middle Vistula Region. Based on sedimentological, palynological, palaeopedological and absolute age studies (luminescence and radiocarbon dating), a succession of three lithofacial complexes and the pattern of their evolution was distinguished. The lowermost, fluvial complex is composed of two parts separated by an extensive erosional surface. They were deposited in a braided river system during two distinct periods of Weichselian climate cooling: first at the end of the Marine Isotope Stage 5a and during the entire Marine Isotope Stage (MIS) 4, and then at the end of MIS 2 (Late Glacial). The middle, fluvio-aeolian complex was deposited during the Late Glacial as an effect of alternating fluvial and aeolian processes on abandoned, higher river terraces under periglacial conditions (discontinuous permafrost). It represents a record of climatic amelioration, culminating in the succession of tundra vegetation and sparse pine-birch forests, causing the formation of a soil in the top part of the middle complex. The uppermost, aeolian complex was deposited during the Older and Younger Dryas as a result of permafrost degradation. The deposition was interrupted by the expansion of boreal forest during the Allerød and the development of soils. Climate has been recognized as a major factor determining changes in sedimentary environments. The cooling of the climate during the Weichselian was conducive to the aggradation of sediments and permafrost. In contrast, climate warming resulted in increased erosion in the valley and creation of terraces.

Lightning as a major driver of recent large fire years in North American boreal forests

Changes in climate and fire regimes are transforming the boreal forest, the world’s largest biome. Boreal North America recently experienced two years with large burned area: 2014 in the Northwest Territories and 2015 in Alaska. Here we use climate, lightning, fire and vegetation data sets to assess the mechanisms contributing to large fire years. We find that lightning ignitions have increased since 1975, and that the 2014 and 2015 events coincided with a record number of lightning ignitions and exceptionally high levels of burning near the northern treeline. Lightning ignition explained more than 55% of the interannual variability in burned area, and was correlated with temperature and precipitation, which are projected to increase by mid-century. The analysis shows that lightning drives interannual and long-term ignition and burned area dynamics in boreal North America, and implies future ignition increases may increase carbon loss while accelerating the northward expansion of boreal forest. The boreal forest is being transformed by changes in its climate–fire regime. Analysis now shows that lightning drives year-to-year and long-term ignition and burned area trends in boreal North America.

Boreal carbon loss due to poleward shift in low-carbon ecosystems

Climate change can be thought of in terms of geographical shifts in climate properties. Tracking the geographical movement of analogous climate conditions between historical and future climate model simulations, and calculating the impact of such shifts on vegetation carbon storage, suggests that boreal forests will lose carbon as low-carbon ecosystems shift in. Climate change can be thought of in terms of geographical shifts in climate properties. Examples include assessments of shifts in habitat distributions1, of the movement needed to maintain constant temperature or precipitation2, and of the emergence and disappearance of climate zones3. Here I track the movement of analogue climates within climate models. From the model simulations, I define a set of vectors that link a historical reference climate for each location to the location in a changed climate whose seasonal temperature and precipitation cycles best match the reference climate. I use these vectors to calculate the change in vegetation carbon storage with climate change due to ecosystems following climate analogues. Comparing the derived carbon content change to direct carbon projections by coupled carbon–climate models reveals two regions of divergence. In the tropical forests, vector projections are fundamentally uncertain because of a lack of close climatic analogues. In the southern boreal forest, carbon losses are projected in the vector perspective because low-carbon ecosystems shift polewards. However, the majority of carbon–climate models—typically without explicit simulation of the disturbance and mortality processes behind such shifts—instead project vegetation carbon gains throughout the boreal region. Southern boreal carbon loss as a result of ecosystem shift is likely to offset carbon gains from northern boreal forest expansion.

The simulated atmospheric response to expansion of the Arctic boreal forest biome

Over the last century, the Arctic has warmed at twice the rate of the planet as a whole. Observational evidence indicates that this rapid warming is affecting the tundra and boreal forest biomes by changing their structure and geographic distribution. A global climate model (GCM) was used to explore the atmospheric response to boreal forest expansion by applying a one-grid cell shift of the forest into tundra. This subtle shift is meant to represent the expansion that would occur this century rather than more extreme scenarios predicted by dynamic vegetation models. Results show that this shift causes an average annual warming of 0.3 °C over the region because of a reduction in the surface albedo and an increase in net radiation. A warming of ~1.0 °C occurs in spring when the forest masks the higher albedo snow-covered surface and results in snowmelt and a reduction in cloud cover. Results fail to show a larger-scale dynamical response although some warming of the lower and mid troposphere occurs in July. No changes were found in the position or strength of the Arctic frontal zone as some studies have indicated will occur with a shift in the boreal forest-tundra boundary. These findings suggest that coupled model simulations that predict larger changes in vegetation distribution are likely overemphasizing the amount of Arctic warming that will occur this century. These findings also indicate that a realistic dynamical response to subtle land cover change might not be correctly simulated by GCMs run at coarse spatial resolutions.

Timing of Lateglacial vegetation dynamics and respective palaeoenvironmental conditions in southern Estonia: evidence from the sediment record of Lake Nakri

AbstractThis paper investigates a detailed well‐dated Lateglacial floristic colonisation in the eastern Baltic area, ca. 14 000–9000 cal. a BP, using palynological, macrofossil, loss‐on‐ignition, and 14C data. During 14 000–13 400 cal. a BP, primarily treeless pioneer tundra vegetation existed. Tree birch (Betula sect. Albae) macro‐remains and a high tree pollen accumulation rate indicate the presence of forest‐tundra with birch and possibly pine (Pinus sylvestris L.) trees during 13 400–12 850 cal. a BP. Palaeobotanical data indicate that the colonisation and development of forested areas were very rapid, arising within a period of less than 50 years. Thus far, there are no indications of conifer macrofossils in Estonia to support the presence of coniferous forests in the Lateglacial period. Signs of Greenland Interstadial 1b cooling during 13 100 cal. a BP are distinguishable. Biostratigraphic evidence indicates that the vegetation was again mostly treeless tundra during the final colder episode of the Lateglacial period associated with Greenland Stadial 1, approximately 12 850–11 650 cal. a BP. This was followed by onset of the Holocene vegetation, with the expansion of boreal forests, in response to rapid climatic warming. Copyright © 2011 John Wiley & Sons, Ltd.

The effects of boreal forest expansion on the summer Arctic frontal zone

Over the last 100 years, Arctic warming has resulted in a longer growing season in boreal and tundra ecosystems. This has contributed to a slow northward expansion of the boreal forest and a decrease in the surface albedo. Corresponding changes to the surface and atmospheric energy budgets have contributed to a broad region of warming over areas of boreal forest expansion. In addition, mesoscale and synoptic scale patterns have changed as a result of the excess energy at and near the surface. Previous studies have identified a relationship between the positioning of the boreal forest-tundra ecotone and the Arctic frontal zone in summer. This study examines the climate response to hypothetical boreal forest expansion and its influence on the summer Arctic frontal zone. Using the Weather Research and Forecasting model over the Northern Hemisphere, an experiment was performed to evaluate the atmospheric response to expansion of evergreen and deciduous boreal needleleaf forests into open shrubland along the northern boundary of the existing forest. Results show that the lower surface albedo with forest expansion leads to a local increase in net radiation and an average hemispheric warming of 0.6°C at and near the surface during June with some locations warming by 1–2°C. This warming contributes to changes in the meridional temperature gradient that enhances the Arctic frontal zone and strengthens the summertime jet. This experiment suggests that continued Northern Hemisphere high-latitude warming and boreal forest expansion might contribute to additional climate changes during the summer.

Resilience of Alaska’s boreal forest to climatic changeThis article is one of a selection of papers from The Dynamics of Change in Alaska’s Boreal Forests: Resilience and Vulnerability in Response to Climate Warming.

This paper assesses the resilience of Alaska’s boreal forest system to rapid climatic change. Recent warming is associated with reduced growth of dominant tree species, plant disease and insect outbreaks, warming and thawing of permafrost, drying of lakes, increased wildfire extent, increased postfire recruitment of deciduous trees, and reduced safety of hunters traveling on river ice. These changes have modified key structural features, feedbacks, and interactions in the boreal forest, including reduced effects of upland permafrost on regional hydrology, expansion of boreal forest into tundra, and amplification of climate warming because of reduced albedo (shorter winter season) and carbon release from wildfires. Other temperature-sensitive processes for which no trends have been detected include composition of plant and microbial communities, long-term landscape-scale change in carbon stocks, stream discharge, mammalian population dynamics, and river access and subsistence opportunities for rural indigenous communities. Projections of continued warming suggest that Alaska’s boreal forest will undergo significant functional and structural changes within the next few decades that are unprecedented in the last 6000 years. The impact of these social–ecological changes will depend in part on the extent of landscape reorganization between uplands and lowlands and on policies regulating subsistence opportunities for rural communities.

Vegetation dynamics and plant CO2 responses as positive feedbacks in a greenhouse world

An atmosphere‐ocean‐vegetation coupled model is used to quantify the biogeophysical feedback that emerges as vegetation adjusts dynamically to a quadrupling of atmospheric CO2. This feedback amplifies global warming by 13%. About half of it is due to climatically induced expansion of boreal forest into tundra, reinforced by reductions in snow and sea ice cover. The other half represents a global climatic effect of increased vegetative cover (an indirect consequence of plant physiological responses to CO2) in the semi‐arid subtropics. Enhanced absorption of shortwave radiation in these regions produces a net surface warming, which the atmosphere communicates poleward. The greatest vegetation‐induced warming is co‐located with large, vulnerable carbon stores in the north. These lose carbon, so that in the long term, the biospheric response to CO2 and climate change becomes dominated by positive feedbacks that overwhelm the effect of CO2 fertilization on terrestrial carbon stocks.

Spatially explicit fire-climate history of the boreal forest-tundra (Eastern Canada) over the last 2000 years

Across the boreal forest, fire is the main disturbance factor and driver of ecosystem changes. In this study, we reconstructed a long-term, spatially explicit fire history of a forest-tundra region in northeastern Canada. We hypothesized that current occupation of similar topographic and edaphic sites by tundra and forest was the consequence of cumulative regression with time of forest cover due to compounding fire and climate disturbances. All fires were mapped and dated per 100 year intervals over the last 2,000 years using several fire dating techniques. Past fire occurrences and post-fire regeneration at the northern forest limit indicate 70% reduction of forest cover since 1800 yr BP and nearly complete cessation of forest regeneration since 900 yr BP. Regression of forest cover was particularly important between 1500s-1700s and possibly since 900 yr BP. Although fire frequency was very low over the last 100 years, each fire event was followed by drastic removal of spruce cover. Contrary to widespread belief of northward boreal forest expansion due to recent warming, lack of post-fire recovery during the last centuries, in comparison with active tree regeneration more than 1,000 years ago, indicates that the current climate does not favour such expansion.

Effect of ice sheet interactions in anthropogenic climate change simulations

We investigate the effect of ice sheets on climate change under elevated atmospheric CO2 concentrations with an atmosphere ocean general circulation model (AOGCM) coupled to a thermomechanical ice sheet model and a vegetation model. The effect of increased meltwater fluxes from ice sheets turned out to be negligible in the phase of initial weakening of the North Atlantic meridional overturning circulation (AMOC), and more important during the recovery in subsequent centuries. Lower surface height of the Greenland ice sheet (GRIS) leads locally to a warming, especially in winter, and remotely to a cooling over northern Eurasia due to modified atmospheric circulation. With quadrupling of the atmospheric CO2 concentration the entire GRIS is exposed to surface melt in summer. On formerly ice‐covered grid points climate locally warms strongly via increased albedos, with positive feedbacks due to boreal forest expansion.

Last Interglacial Arctic warmth confirms polar amplification of climate change

The warmest millennia of at least the past 250,000 years occurred during the Last Interglaciation, when global ice volumes were similar to or smaller than today and systematic variations in Earth's orbital parameters aligned to produce a strong positive summer insolation anomaly throughout the Northern Hemisphere. The average insolation during the key summer months (M, J, J) was ca 11% above present across the Northern Hemisphere between 130,000 and 127,000 years ago, with a slightly greater anomaly, 13%, over the Arctic. Greater summer insolation, early penultimate deglaciation, and intensification of the North Atlantic Drift, combined to reduce Arctic Ocean sea ice, allow expansion of boreal forest to the Arctic Ocean shore across vast regions, reduce permafrost, and melt almost all glaciers in the Northern Hemisphere. Insolation, amplified by key boundary condition feedbacks, collectively produced Last Interglacial summer temperature anomalies 4–5 °C above present over most Arctic lands, significantly above the average Northern Hemisphere anomaly. The Last Interglaciation demonstrates the strength of positive feedbacks on Arctic warming and provides a potentially conservative analogue for anticipated future greenhouse warming.

Coupled atmosphere-ocean-vegetation simulations for modern and mid-Holocene climates: role of extratropical vegetation cover feedbacks

A full global atmosphere-ocean-land vegetation model is used to examine the coupled climate/vegetation changes in the extratropics between modern and mid-Holocene (6,000 year BP) times and to assess the feedback of vegetation cover changes on the climate response. The model produces a relatively realistic natural vegetation cover and a climate sensitivity comparable to that realized in previous studies. The simulated mid-Holocene climate led to an expansion of boreal forest cover into polar tundra areas (mainly due to increased summer/fall warmth) and an expansion of middle latitude grass cover (due to a combination of enhanced temperature seasonality with cold winters and interior drying of the continents). The simulated poleward expansion of boreal forest and middle latitude expansion of grass cover are consistent with previous modeling studies. The feedback effect of expanding boreal forest in polar latitudes induced a significant spring warming and reduced snow cover that partially countered the response produced by the orbitally induced changes in radiative forcing. The expansion of grass cover in middle latitudes worked to reinforce the orbital forcing by contributing a spring cooling, enhanced snow cover, and a delayed soil water input by snow melt. Locally, summer rains tended to increase (decrease) in areas with greatest tree cover increases (decreases); however, for the broad-scale polar and middle latitude domains the climate responses produced by the changes in vegetation are relatively much smaller in summer/fall than found in previous studies. This study highlights the need to develop a more comprehensive strategy for investigating vegetation feedbacks.

Simulated and Observed Preindustrial to Modern Vegetation and Climate Changes*

Abstract Rising levels of carbon dioxide since the preindustrial era have likely contributed to an observed warming of the global surface, and observations show global greening and an expansion of boreal forests. This study reproduces observed climate and vegetation trends associated with rising CO2 using a fully coupled atmosphere–ocean–land surface GCM with dynamic vegetation and decomposes the effects into physiological and radiative components. The simulated warming trend, strongest at high latitudes, was dominated by the radiative effect, although the physiological effect of CO2 on vegetation (CO2 fertilization) contributed to significant wintertime warming over northern Europe and central and eastern Asia. The net global greening of the model was primarily due to the physiological effect of increasing CO2, while the radiative and physiological effects combined to produce a poleward expansion of the boreal forests. Observed and simulated trends in tree ring width are consistent with the enhancement of vegetation growth by the physiological effect of rising CO2.

Il riscaldamento globale determinerà una espansione delle foreste boreali?

Il riscaldamento globale determinerà una espansione delle foreste boreali?

Stability of boreal forest stands during recent climate change: evidence from Landsat satellite imagery

AimTo detect possible expansion of boreal forest stands in response to recent warming. Previous modelling studies have concluded that major shifts in vegetation patterns, including changes in boreal forest extent, could arise during the next two centuries under global warming scenarios. However, field investigations of tree stands at ecotones have so far revealed little indication of stand response to warming during the last 100 years. This study uses a c. 25‐year record of Landsat satellite observations to quantify changes in forest stand extent in two areas of northern Canada.LocationTwo regions of northern Canada, near Richmond Gulf, Quebec, and Great Slave Lake, north‐west Territories.MethodsNormalized‐difference vegetation index (NDVI) plots across forest‐tundra boundaries were obtained from radiometrically corrected Landsat imagery acquired during the 1970s and 1990s. These curves were evaluated to look for changes over the c. 25‐year period related to forest stand expansion.ResultsAlthough forest‐tundra boundaries could be clearly mapped from the satellite data, no obvious change in forest boundaries was apparent during the duration of the image time series, constraining recent geographical expansion rates to <200–300 m per century. Also, no evidence for local expansion of forest stands (e.g. within sheltered valleys) was found.Main conclusionsThe results are consistent with field observations, and suggest that, at the moment, boreal forest extents remain basically stable. This may reflect inherent lags between forest response and climate change, or competitive pressures between tree stands and surrounding tundra and herbaceous vegetation.