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Local adaptations to environmental conditions are of high ecological importance as they determine distribution ranges and likely affect species responses to climate change. Increased environmental stress (warming, extreme drought) due to climate change in combination with decreased genetic mixing due to isolation may lead to stronger local adaptations of geographically marginal than central populations. We experimentally observed local adaptations of three marginal and four central populations of Fagus sylvatica L., the dominant native forest tree, to frost over winter and in spring (late frost). We determined frost hardiness of buds and roots by the relative electrolyte leakage in two common garden experiments. The experiment at the cold site included a continuous warming treatment; the experiment at the warm site included a preceding summer drought manipulation. In both experiments, we found evidence for local adaptation to frost, with stronger signs of local adaptation in marginal populations. Winter frost killed many of the potted individuals at the cold site, with higher survival in the warming treatment and in those populations originating from colder environments. However, we found no difference in winter frost tolerance of buds among populations, implying that bud survival was not the main cue for mortality. Bud late frost tolerance in April differed between populations at the warm site, mainly because of phenological differences in bud break. Increased spring frost tolerance of plants which had experienced drought stress in the preceding summer could also be explained by shifts in phenology. Stronger local adaptations to climate in geographically marginal than central populations imply the potential for adaptation to climate at range edges. In times of climate change, however, it needs to be tested whether locally adapted populations at range margins can successfully adapt further to changing conditions. Seedlings of F. sylvatica showed evidence for local adaptations to winter and spring frosts which were stronger developed in three marginal than in four central populations referring to the range of the species. We confirmed that preceding summer drought can improve frost tolerance and found no difference in the sensitivity of populations in this drought effect. The response of bud frost tolerance in winter to continuous warming, however, differed between central and marginal populations, thereby raising the question how marginal populations will be able to adapt to ongoing climate change.

Spatial variation in plant chemical defence towards herbivores can help us understand variation in herbivore top–down control of shrubs in the Arctic and possibly also shrub responses to global warming. Less defended, non‐resinous shrubs could be more influenced by herbivores than more defended, resinous shrubs. However, sparse field measurements limit our current understanding of how much of the circum‐Arctic variation in defence compounds is explained by taxa or defence functional groups (resinous/non‐resinous). We measured circum‐Arctic chemical defence and leaf digestibility in resinous (Betula glandulosa, B. nana ssp. exilis) and non‐resinous (B. nana ssp. nana, B. pumila) shrub birches to see how they vary among and within taxa and functional groups. Using liquid chromatography–mass spectrometry (LC–MS) metabolomic analyses and in vitro leaf digestibility via incubation in cattle rumen fluid, we analysed defence composition and leaf digestibility in 128 samples from 44 tundra locations. We found biogeographical patterns in anti‐herbivore defence where mean leaf triterpene concentrations and twig resin gland density were greater in resinous taxa and mean concentrations of condensing tannins were greater in non‐resinous taxa. This indicates a biome‐wide trade‐off between triterpene‐ or tannin‐dominated defences. However, we also found variations in chemical defence composition and resin gland density both within and among functional groups (resinous/non‐resinous) and taxa, suggesting these categorisations only partly predict chemical herbivore defence. Complex tannins were the only defence compounds negatively related to in vitro digestibility, identifying this previously neglected tannin group as having a potential key role in birch anti‐herbivore defence. We conclude that circum‐Arctic variation in birch anti‐herbivore defence can be partly derived from biogeographical distributions of birch taxa, although our detailed mapping of plant defence provides more information on this variation and can be used for better predictions of herbivore effects on Arctic vegetation.

Shrub expansion in alpine and arctic areas is a process with possibly profound implications for ecosystem functioning. The recent shrub expansion has been mainly documented by remote sensing techniques, but the drivers for this process largely remain hypotheses. Here, we outline a dendrochronological method, adapted to shrubs, to address these hypotheses and then present a mechanism for the current shrub expansion by linking recent climate change to shrub growth performance in northern Sweden. A pronounced increase in radial and vertical growth during recent decades along an elevational gradient from treeline to shrubline indicates an ongoing shrub expansion. Age distribution of the shrub population indicates the new colonization of shrubs at high elevations. Shrub growth is correlated with warm summers and winter snow cover and suggests the potential for large-scale ecosystem changes if climate change continues as projected.

In the 21st century, most of the world's glaciers are expected to retreat due to further global warming. The range of this predicted retreat varies widely as a result of uncertainties in climate and glacier models. To calibrate and validate glacier models, past records of glacier mass balance are necessary, which often only span several decades. Long-term reconstructions of glacier mass balance could increase the precision of glacier models by providing the required calibration data. Here we show the possibility of applying shrub growth increments as an on-site proxy for glacier summer mass balance, exemplified by Salix shrubs in Finse, Norway. We further discuss the challenges which this method needs to meet and address the high potential of shrub growth increments for reconstructing glacier summer mass balance in remote areas.

QUESTIONS: Which climatic variables are the main determinants of radial growth and to what extent does their effect on growth vary among species? What are the similarities between the temporal radial growth patterns of the two common shrub species, Alnus viridis and Salix glauca? Do changing growth conditions over the past 120 yr and their predicted impact on growth match shrub expansion observed in the region? LOCATION: Arsuk Fjord and Disko Bay regions, W Greenland. METHODS: Alnus viridis and S. glauca specimens were sampled in the field and radial growth was analysed using standard dendrochronological methods (‘response functions’). The identified climatic variables were applied to model radial growth using a linear mixed model and predict the growth for 1890–2010. RESULTS: The main determinants of radial growth were summer temperatures and, although not significant in the final models, spring precipitation. The empirical chronologies showed only somewhat similar growth patterns. They responded to similar sets of climatic variables, but their similarity was weakened because of the low number of replicates and local differences in growth conditions. The similarity between predicted (modelled) chronologies was higher, which was related to the response to similar sets of climatic variables and high correlation between climatic variables across long distances. CONCLUSION: Overall, estimated growth did not increase over the past 120 yr, but considerable variations in growth are conspicuous and match known historical patterns of the Atlantic Multidecadal Oscillation. Perspectives regarding the observed shrub expansion in W Greenland are discussed, and based on the estimated growth patterns, we consider it unlikely that the recent 10–15‐yr period of favourable climate is the main responsible cause.

The expansion of shrubs into tundra areas is a key terrestrial change underway in the Arctic in response to elevated temperatures during the twentieth century. Repeat photography permits a glimpse into greening satellite pixels, and it shows that, since 1950, some shrub patches have increased rapidly (hereafter expanding), while others have increased little or not at all (hereafter stable). We characterized and compared adjacent expanding and stable shrub patches across Arctic Alaska by sampling a wide range of physical and chemical soil and vegetation properties, including shrub growth rings. Expanding patches of Alnus viridis ssp. fruticosa (Siberian alder) contained shrub stems with thicker growth rings than in stable patches. Alder growth in expanding patches also showed strong correlation with spring and summer warming, whereas alder growth in stable patches showed little correlation with temperature. Expanding patches had different vegetation composition, deeper thaw depth, higher mean annual ground temperature, higher mean growing season temperature, lower soil moisture, less carbon in mineral soil, and lower C:N values in soils and shrub leaves. Expanding patches—higher resource environments—were associated with floodplains, stream corridors, and outcrops. Stable patches—lower resource environments—were associated with poorly drained tussock tundra. Collectively, we interpret these differences as implying that preexisting soil conditions predispose parts of the landscape to a rapid response to climate change, and we therefore expect shrub expansion to continue penetrating the landscape via dendritic floodplains, streams, and scattered rock outcrops.

The tundra is warming more rapidly than any other biome on Earth, and the potential ramifications are far-reaching because of global feedback effects between vegetation and climate. A better understanding of how environmental factors shape plant structure and function is crucial for predicting the consequences of environmental change for ecosystem functioning. Here we explore the biome-wide relationships between temperature, moisture and seven key plant functional traits both across space and over three decades of warming at 117 tundra locations. Spatial temperature–trait relationships were generally strong but soil moisture had a marked influence on the strength and direction of these relationships, highlighting the potentially important influence of changes in water availability on future trait shifts in tundra plant communities. Community height increased with warming across all sites over the past three decades, but other traits lagged far behind predicted rates of change. Our findings highlight the challenge of using space-for-time substitution to predict the functional consequences of future warming and suggest that functions that are tied closely to plant height will experience the most rapid change. They also reveal the strength with which environmental factors shape biotic communities at the coldest extremes of the planet and will help to improve projections of functional changes in tundra ecosystems with climate warming.

Recent research using repeat photography, long-term ecological monitoring and dendrochronology has documented shrub expansion in arctic, high-latitude and alpine tundra ecosystems. Here, we (1) synthesize these findings, (2) present a conceptual framework that identifies mechanisms and constraints on shrub increase, (3) explore causes, feedbacks and implications of the increased shrub cover in tundra ecosystems, and (4) address potential lines of investigation for future research. Satellite observations from around the circumpolar Arctic, showing increased productivity, measured as changes in 'greenness', have coincided with a general rise in high-latitude air temperatures and have been partly attributed to increases in shrub cover. Studies indicate that warming temperatures, changes in snow cover, altered disturbance regimes as a result of permafrost thaw, tundra fires, and anthropogenic activities or changes in herbivory intensity are all contributing to observed changes in shrub abundance. A large-scale increase in shrub cover will change the structure of tundra ecosystems and alter energy fluxes, regional climate, soil–atmosphere exchange of water, carbon and nutrients, and ecological interactions between species. In order to project future rates of shrub expansion and understand the feedbacks to ecosystem and climate processes, future research should investigate the species or trait-specific responses of shrubs to climate change including: (1) the temperature sensitivity of shrub growth, (2) factors controlling the recruitment of new individuals, and (3) the relative influence of the positive and negative feedbacks involved in shrub expansion.

Rapid climate warming has been linked to increasing shrub dominance in the Arctic tundra. Research now shows that climate–shrub growth relationships vary spatially and according to site characteristics such as soil moisture and shrub height. Rapid climate warming in the tundra biome has been linked to increasing shrub dominance 1 , 2 , 3 , 4 . Shrub expansion can modify climate by altering surface albedo, energy and water balance, and permafrost 2 , 5 , 6 , 7 , 8 , yet the drivers of shrub growth remain poorly understood. Dendroecological data consisting of multi-decadal time series of annual shrub growth provide an underused resource to explore climate–growth relationships. Here, we analyse circumpolar data from 37 Arctic and alpine sites in 9 countries, including 25 species, and ∼42,000 annual growth records from 1,821 individuals. Our analyses demonstrate that the sensitivity of shrub growth to climate was: (1) heterogeneous, with European sites showing greater summer temperature sensitivity than North American sites, and (2) higher at sites with greater soil moisture and for taller shrubs (for example, alders and willows) growing at their northern or upper elevational range edges. Across latitude, climate sensitivity of growth was greatest at the boundary between the Low and High Arctic, where permafrost is thawing 4 and most of the global permafrost soil carbon pool is stored 9 . The observed variation in climate–shrub growth relationships should be incorporated into Earth system models to improve future projections of climate change impacts across the tundra biome.