Facilitation and competition in the high Arctic: the importance of the experimental approach

In the last decade, plant ecologists have focussed more on the occurrence of positive plant–plant interactions than ever before. Especially in severe environments, such as the Arctic, species removal experiments tend to find facilitative rather than competitive effects, casting doubt on the importance of competition under extreme growing conditions. Two approaches to measure plant–plant interactions presented here reveal that competition affects plant growth even in the high Arctic. Luzula confusa and Salix polaris show a reduced growth in mixed stands compared with pure. This competition effect is not detected in a removal experiment, which inevitably also alters site microclimate. Indeed, in the latter experiment, facilitative effects of Luzula on Salix were found. Evidently, both facilitation and competition are acting and important. Causes and implications are discussed using a conceptual model derived from that of Brooker and Callaghan (1998).     

Balancing positive and negative plant interactions: how mosses structure vascular plant communities

Our understanding of positive and negative plant interactions is primarily based on vascular plants, as is the prediction that facilitative effects dominate in harsh environments. It remains unclear whether this understanding is also applicable to moss–vascular plant interactions, which are likely to be influential in low-temperature environments with extensive moss ground cover such as boreal forest and arctic tundra. In a field experiment in high-arctic tundra, we investigated positive and negative impacts of the moss layer on vascular plants. Ramets of the shrub Salix polaris, herb Bistorta vivipara, grass Alopecurus borealis and rush Luzula confusa were transplanted into plots manipulated to contain bare soil, shallow moss (3 cm) and deep moss (6 cm) and harvested after three growing seasons. The moss layer had both positive and negative impacts upon vascular plant growth, the relative extent of which varied among vascular plant species. Deep moss cover reduced soil temperature and nitrogen availability, and this was reflected in reduced graminoid productivity. Shrub and herb biomass were greatest in shallow moss, where soil moisture also appeared to be highest. The relative importance of the mechanisms by which moss may influence vascular plants, through effects on soil temperature, moisture and nitrogen availability, was investigated in a phytotron growth experiment. Soil temperature, and not nutrient availability, determined Alopecurus growth, whereas Salix only responded to increased temperature if soil nitrogen was also increased. We propose a conceptual model showing the relative importance of positive and negative influences of the moss mat on vascular plants along a gradient of moss depth and illustrate species-specific outcomes. Our findings suggest that, through their strong influence on the soil environment, mat-forming mosses structure the composition of vascular plant communities. Thus, for plant interaction theory to be widely applicable to extreme environments such as the Arctic, growth forms other than vascular plants should be considered.     

The nature of spatial transitions in the Arctic

Aim Describe the spatial and temporal properties of transitions in the Arctic and develop a conceptual understanding of the nature of these spatial transitions in the face of directional environmental change. Location Arctic tundra ecosystems of the North Slope of Alaska and the tundra‐forest region of the Seward Peninsula, Alaska Methods We synthesize information from numerous studies on tundra and treeline ecosystems in an effort to document the spatial changes that occur across four arctic transitions. These transitions are: (i) the transition between High‐Arctic and Low‐Arctic systems, (ii) the transition between moist non‐acidic tundra (MNT) and moist acidic tundra (MAT, also referred to as tussock tundra), (iii) the transition between tussock tundra and shrub tundra, (iv) the transition between tundra and forested systems. By documenting the nature of these spatial transitions, in terms of their environmental controls and vegetation patterns, we develop a conceptual model of temporal dynamics of arctic ecotones in response to environmental change. Results Our observations suggest that each transition is sensitive to a unique combination of controlling factors. The transition between High and Low Arctic is sensitive primarily to climate, whereas the MNT/MAT transition is also controlled by soil parent material, permafrost and hydrology. The tussock/shrub tundra transition appears to be responsive to several factors, including climate, topography and hydrology. Finally, the tundra/forest boundary responds primarily to climate and to climatically associated changes in permafrost. There were also important differences in the demography and distribution of the dominant plant species across the four vegetation transitions. The shrubs that characterize the tussock/shrub transition can achieve dominance potentially within a decade, whereas spruce trees often require several decades to centuries to achieve dominance within tundra, and Sphagnum moss colonization of non‐acidic sites at the MNT/MAT boundary may require centuries to millennia of soil development. Main conclusions We suggest that vegetation will respond most rapidly to climatic change when (i) the vegetation transition correlates more strongly with climate than with other environmental variables, (ii) dominant species exhibit gradual changes in abundance across spatial transitions, and/or (iii) the dominant species have demographic properties that allow rapid increases in abundance following climatic shifts. All three of these properties characterize the transition between tussock tundra and low shrub tundra. It is therefore not surprising that of the four transitions studied this is the one that appears to be responding most rapidly to climatic warming.     

Direct effects of warming increase woody plant abundance in a subarctic wetland

Both the direct effects of warming on a species’ vital rates and indirect effects of warming caused by interactions with neighboring species can influence plant populations. Furthermore, herbivory mediates the effects of warming on plant community composition in many systems. Thus, determining the importance of direct and indirect effects of warming, while considering the role of herbivory, can help predict long‐term plant community dynamics. We conducted a field experiment in the coastal wetlands of western Alaska to investigate how warming and herbivory influence the interactions and abundances of two common plant species, a sedge, Carex ramenskii, and a dwarf shrub, Salix ovalifolia. We used results from the experiment to model the equilibrium abundances of the species under different warming and grazing scenarios and to determine the contribution of direct and indirect effects to predict population changes. Consistent with the current composition of the landscape, model predictions suggest that Carex is more abundant than Salix under ambient temperatures with grazing (53% and 27% cover, respectively). However, with warming and grazing, Salix becomes more abundant than Carex (57% and 41% cover, respectively), reflecting both a negative response of Carex and a positive response of Salix to warming. While grazing reduced the cover of both species, herbivory did not prevent a shift in dominance from sedges to the dwarf shrub. Direct effects of climate change explained about 97% of the total predicted change in species cover, whereas indirect effects explained only 3% of the predicted change. Thus, indirect effects, mediated by interactions between Carex and Salix, were negligible, likely due to use of different niches and weak interspecific interactions. Results suggest that a 2°C increase could cause a shift in dominance from sedges to woody plants on the coast of western Alaska over decadal timescales, and this shift was largely a result of the direct effects of warming. Models predict this shift with or without goose herbivory. Our results are consistent with other studies showing an increase in woody plant abundance in the Arctic and suggest that shifts in plant–plant interactions are not driving this change.     

Successional changes in ectomycorrhizal fungi associated with the polar willow Salix polaris in a deglaciated area in the High Arctic, Svalbard

Polar willow (Salix polaris Wahlenb.), a mycorrhizal dwarf shrub, colonizes recently deglaciated areas in the High Arctic, Svalbard. To clarify successional changes in ECM fungi associated with S. polaris after glacier retreat, we examined the diversity and density of ECM fungi in culture and field conditions. Plant and soil samples were collected from three sites of different successional stages in the deglaciated area of Austre Br?ggerbreen, near Ny-?lesund, Svalbard. The successional stages were early stage with newly exposed bare ground (site I), transient stage with scattered colonization of Salix (sites IIa and IIb), and late stage with well-developed vegetation (site III). No ECM colonization on Salix was observed in soils collected from bare ground in early and transient stages (sites I and IIa). However, most Salix individuals showed ECM colonization in soils collected from sites close to Salix colonies in transient and late stages (sites IIb and III). Based on molecular analyses and operational taxonomic unit (OTU: >95% ITS sequence similarity) delimitations, we identified 15 OTUs/species in eight genera. The dominant OTU/species of ECM fungi identified in the transient and late stages was Geopora sp.1 and Cenococcum sp.1, respectively. In the culture experiment, ECM diversity was greater in late stage (eight OTUs/species) than in transient stage (three OTUs/species). This pattern was consistent with field observations, i.e., late-stage sites contained more OTUs/species of ECM fungi. These results indicate that species diversity of ECM fungi increases and the dominant species changes with the progress of succession after glacier retreat in the High Arctic.     

Long‐term experimental manipulation of climate alters the ectomycorrhizal community of Betula nana in Arctic tundra

Climate warming is leading to shrub expansion in Arctic tundra. Shrubs form ectomycorrhizal (ECM) associations with soil fungi that are central to ecosystem carbon balance as determinants of plant community structure and as decomposers of soil organic matter. To assess potential climate change impacts on ECM communities, we analysed fungal internal transcribed spacer sequences from ECM root tips of the dominant tundra shrub Betula nana growing in treatments plots that had received long‐term warming by greenhouses and/or fertilization as part of the Arctic Long‐Term Ecological Research experiment at Toolik Lake Alaska, USA. We demonstrate opposing effects of long‐term warming and fertilization treatments on ECM fungal diversity; with warming increasing and fertilization reducing the diversity of ECM communities. We show that warming leads to a significant increase in high biomass fungi with proteolytic capacity, especially Cortinarius spp., and a reduction of fungi with high affinities for labile N, especially Russula spp. In contrast, fertilization treatments led to relatively small changes in the composition of the ECM community, but increased the abundance of saprotrophs. Our data suggest that warming profoundly alters nutrient cycling in tundra, and may facilitate the expansion of B. nana through the formation of mycorrhizal networks of larger size.     

High Arctic summer warming tracked by increased Cassiope tetragona growth in the world's northernmost polar desert

Rapid climate warming has resulted in shrub expansion, mainly of erect deciduous shrubs in the Low Arctic, but the more extreme, sparsely vegetated, cold and dry High Arctic is generally considered to remain resistant to such shrub expansion in the next decades. Dwarf shrub dendrochronology may reveal climatological causes of past changes in growth, but is hindered at many High Arctic sites by short and fragmented instrumental climate records. Moreover, only few High Arctic shrub chronologies cover the recent decade of substantial warming. This study investigated the climatic causes of growth variability of the evergreen dwarf shrub Cassiope tetragona between 1927 and 2012 in the northernmost polar desert at 83°N in North Greenland. We analysed climate–growth relationships over the period with available instrumental data (1950–2012) between a 102‐year‐long C. tetragona shoot length chronology and instrumental climate records from the three nearest meteorological stations, gridded climate data, and North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) indices. July extreme maximum temperatures (JulT_emx), as measured at Alert, Canada, June NAO, and previous October AO, together explained 41% of the observed variance in annual C. tetragona growth and likely represent in situ summer temperatures. JulT_emx explained 27% and was reconstructed back to 1927. The reconstruction showed relatively high growing season temperatures in the early to mid‐twentieth century, as well as warming in recent decades. The rapid growth increase in C. tetragona shrubs in response to recent High Arctic summer warming shows that recent and future warming might promote an expansion of this evergreen dwarf shrub, mainly through densification of existing shrub patches, at High Arctic sites with sufficient winter snow cover and ample water supply during summer from melting snow and ice as well as thawing permafrost, contrasting earlier notions of limited shrub growth sensitivity to summer warming in the High Arctic.     

Compositional differentiation,vegetation‐environment relationships and classification of willow‐characterised vegetation in the western Eurasian Arctic

Question: How does willow‐characterised tundra vegetation of western Eurasia vary, and what are the main vegetation types? What are the ecological gradients and climatic regimes underlying vegetation differentiation? Location: The dataset was collected across a wide spectrum of tundra habitats at 12 sites in subarctic and arctic areas spanning from NW Fennoscandia to West Siberia. Methods: The dataset, including 758 vegetation sample plots (relevés), was analysed using a TWINSPAN classification and NMDS ordination that also included analyses of vegetation‐environment correlations. Results: Based on the TWINSPAN classification, eight vegetation types characterised by willow (cover of upright willows >10%) were discerned: (1) Salix glauca‐Carex aquatilis type, (2) Aulacomnium‐Tomentypnum type, (3) Salix‐Betula‐Hylocomium type, (4) Salix lanata‐Brachythecium mildeanum type, (5) Salix‐Pachypleurum type, (6) S. lanata‐Myosotis nemorosa type, (7) Salix‐Trollius‐Geranium type and (8) Salix‐Comarum palustre‐Filipendula ulmaria type. Willow‐characterised vegetation types were compositionally differentiated from other tundra vegetation and were confined to relatively moist valley and sloping tundra sites, from mire to mineral soils. These vegetation types were encountered across a broad latitudinal zone in which July mean temperature ranged from 6 to 10°C. Conclusions: Willow‐characterised tundra vegetation forms a broad category of ecologically and geographically differentiated vegetation types that are linked to dwarf shrub tundra, shrub tundra or mire. Because of complex ecological gradients underlying compositional differentiation, predicting the responses of willow‐characterised tundra vegetation to a warming climate may be complicated.     

Taller and larger: shifts in Arctic tundra leaf traits after 16 years of experimental warming

Understanding plant trait responses to elevated temperatures in the Arctic is critical in light of recent and continuing climate change, especially because these traits act as key mechanisms in climate‐vegetation feedbacks. Since 1992, we have artificially warmed three plant communities at Alexandra Fiord, Nunavut, Canada (79°N). In each of the communities, we used open‐top chambers (OTCs) to passively warm vegetation by 1–2 °C. In the summer of 2008, we investigated the intraspecific trait responses of five key species to 16 years of continuous warming. We examined eight traits that quantify different aspects of plant performance: leaf size, specific leaf area (SLA), leaf dry matter content (LDMC), plant height, leaf carbon concentration, leaf nitrogen concentration, leaf carbon isotope discrimination (LCID), and leaf δ¹⁵N. Long‐term artificial warming affected five traits, including at least one trait in every species studied. The evergreen shrub Cassiope tetragona responded most frequently (increased leaf size and plant height/decreased SLA, leaf carbon concentration, and LCID), followed by the deciduous shrub Salix arctica (increased leaf size and plant height/decreased SLA) and the evergreen shrub Dryas integrifolia (increased leaf size and plant height/decreased LCID), the forb Oxyria digyna (increased leaf size and plant height), and the sedge Eriophorum angustifolium spp. triste (decreased leaf carbon concentration). Warming did not affect δ¹⁵N, leaf nitrogen concentration, or LDMC. Overall, growth traits were more sensitive to warming than leaf chemistry traits. Notably, we found that responses to warming were sustained, even after many years of treatment. Our work suggests that tundra plants in the High Arctic will show a multifaceted response to warming, often including taller shoots with larger leaves.     

Phenology and species determine growing‐season albedo increase at the altitudinal limit of shrub growth in the sub‐Arctic

Arctic warming is resulting in reduced snow cover and increased shrub growth, both of which have been associated with altered land surface–atmospheric feedback processes involving sensible heat flux, ground heat flux and biogeochemical cycling. Using field measurements, we show that two common Arctic shrub species (Betula glandulosa and Salix pulchra), which are largely responsible for shrub encroachment in tundra, differed markedly in albedo and that albedo of both species increased as growing season progressed when measured at their altitudinal limit. A moveable apparatus was used to repeatedly measure albedo at six precise spots during the summer of 2012, and resampled in 2013. Contrary to the generally accepted view of shrub‐covered areas having low albedo in tundra, full‐canopy prostrate B. glandulosa had almost the highest albedo of all surfaces measured during the peak of the growing season. The higher midsummer albedo is also evident in localized MODIS albedo aggregated from 2000 to 2013, which displays a similar increase in growing‐season albedo. Using our field measurements, we show the ensemble summer increase in tundra albedo counteracts the generalized effect of earlier spring snow melt on surface energy balance by approximately 40%. This summer increase in albedo, when viewed in absolute values, is as large as the difference between the forest and tundra transition. These results indicate that near future (<50 years) changes in growing‐season albedo related to Arctic vegetation change are unlikely to be particularly large and might constitute a negative feedback to climate warming in certain circumstances. Future efforts to calculate energy budgets and a sensible heating feedback in the Arctic will require more detailed information about the relative abundance of different ground cover types, particularly shrub species and their respective growth forms and phenology.     

Arctic warming on two continents has consistent negative effects on lichen diversity and mixed effects on bryophyte diversity

Little is known about the impact of changing temperature regimes on composition and diversity of cryptogam communities in the Arctic and Subarctic, despite the well‐known importance of lichens and bryophytes to the functioning and climate feedbacks of northern ecosystems. We investigated changes in diversity and abundance of lichens and bryophytes within long‐term (9–16 years) warming experiments and along natural climatic gradients, ranging from Swedish subarctic birch forest and subarctic/subalpine tundra to Alaskan arctic tussock tundra. In both Sweden and Alaska, lichen diversity responded negatively to experimental warming (with the exception of a birch forest) and to higher temperatures along climatic gradients. Bryophytes were less sensitive to experimental warming than lichens, but depending on the length of the gradient, bryophyte diversity decreased both with increasing temperatures and at extremely low temperatures. Among bryophytes, Sphagnum mosses were particularly resistant to experimental warming in terms of both abundance and diversity. Temperature, on both continents, was the main driver of species composition within experiments and along gradients, with the exception of the Swedish subarctic birch forest where amount of litter constituted the best explanatory variable. In a warming experiment in moist acidic tussock tundra in Alaska, temperature together with soil ammonium availability were the most important factors influencing species composition. Overall, dwarf shrub abundance (deciduous and evergreen) was positively related to warming but so were the bryophytes Sphagnum girgensohnii, Hylocomium splendens and Pleurozium schreberi; the majority of other cryptogams showed a negative relationship to warming. This unique combination of intercontinental comparison, natural gradient studies and experimental studies shows that cryptogam diversity and abundance, especially within lichens, is likely to decrease under arctic climate warming. Given the many ecosystem processes affected by cryptogams in high latitudes (e.g. carbon sequestration, N₂‐fixation, trophic interactions), these changes will have important feedback consequences for ecosystem functions and climate.     

Origin of volatile organic compound emissions from subarctic tundra under global warming

Warming occurs in the Arctic twice as fast as the global average, which in turn leads to a large enhancement in terpenoid emissions from vegetation. Volatile terpenoids are the main class of biogenic volatile organic compounds (VOCs) that play crucial roles in atmospheric chemistry and climate. However, the biochemical mechanisms behind the temperature‐dependent increase in VOC emissions from subarctic ecosystems are largely unexplored. Using ¹³CO₂‐labeling, we studied the origin of VOCs and the carbon (C) allocation under global warming in the soil–plant–atmosphere system of contrasting subarctic heath tundra vegetation communities characterized by dwarf shrubs of the genera Salix or Betula. The projected temperature rise of the subarctic summer by 5°C was realistically simulated in sophisticated climate chambers. VOC emissions strongly depended on the plant species composition of the heath tundra. Warming caused increased VOC emissions and significant changes in the pattern of volatiles toward more reactive hydrocarbons. The ¹³C was incorporated to varying degrees in different monoterpene and sesquiterpene isomers. We found that de novo monoterpene biosynthesis contributed to 40%–44% (Salix) and 60%–68% (Betula) of total monoterpene emissions under the current climate, and that warming increased the contribution to 50%–58% (Salix) and 87%–95% (Betula). Analyses of above‐ and belowground ^12/13C showed shifts of C allocation in the plant–soil systems and negative effects of warming on C sequestration by lowering net ecosystem exchange of CO₂ and increasing C loss as VOCs. This comprehensive analysis provides the scientific basis for mechanistically understanding the processes controlling terpenoid emissions, required for modeling VOC emissions from terrestrial ecosystems and predicting the future chemistry of the arctic atmosphere. By changing the chemical composition and loads of VOCs into the atmosphere, the current data indicate that global warming in the Arctic may have implications for regional and global climate and for the delicate tundra ecosystems.     

Responses of High Arctic wet sedge tundra to climate warming since 1980

The global climate is changing rapidly and Arctic regions are showing responses to recent warming. Responses of tundra ecosystems to climate change have been examined primarily through short‐term experimental manipulations, with few studies of long‐term ambient change. We investigated changes in above‐ and belowground biomass of wet sedge tundra to the warming climate of the Canadian High Arctic over the past 25 years. Aboveground standing crop was harvested from five sedge meadow sites and belowground biomass was sampled from one of the sites in the early 1980s and in 2005 using the same methods. Aboveground biomass was on average 158% greater in 2005 than in the early 1980s. The belowground biomass was also much greater in 2005: root biomass increased by 67% and rhizome biomass by 139% since the early 1980s. Dominant species from each functional group (graminoids, shrubs and forbs) showed significant increases in aboveground biomass. Responsive species included the dominant sedge species Carex aquatilis stans, C. membranacea, and Eriophorum angustifolium, as well as the dwarf shrub Salix arctica and the forb Polygonum viviparum. However, diversity measures were not different between the sample years. The greater biomass correlated strongly with increased annual and summer temperatures over the same time period, and was significantly greater than the annual variation in biomass measured in 1980–1983. Increased decomposition and mineralization rates, stimulated by warmer soils, were likely a major cause of the elevated productivity, as no differences in the mass of litter were found between sample periods. Our results are corroborated by published short‐term experimental studies, conducted in other wet sedge tundra communities which link warming and fertilization with elevated decomposition, mineralization and tundra productivity. We believe that this is the first study to show responses in High Arctic wet sedge tundra to recent climate change.     

Mammalian herbivores confer resilience of Arctic shrub‐dominated ecosystems to changing climate

Climate change is resulting in a rapid expansion of shrubs in the Arctic. This expansion has been shown to be reinforced by positive feedbacks, and it could thus set the ecosystem on a trajectory toward an alternate, more productive regime. Herbivores, on the other hand, are known to counteract the effects of simultaneous climate warming on shrub biomass. However, little is known about the impact of herbivores on resilience of these ecosystems, that is, the capacity of a system to absorb disturbance and still remain in the same regime, retaining the same function, structure, and feedbacks. Here, we investigated how herbivores affect resilience of shrub‐dominated systems to warming by studying the change of shrub biomass after a cessation of long‐term experimental warming in a forest–tundra ecotone. As predicted, warming increased the biomass of shrubs, and in the absence of herbivores, shrub biomass in tundra continued to increase 4 years after cessation of the artificial warming, indicating that positive effects of warming on plant growth may persist even over a subsequent colder period. Herbivores contributed to the resilience of these systems by returning them back to the original low‐biomass regime in both forest and tundra habitats. These results support the prediction that higher shrub biomass triggers positive feedbacks on soil processes and microclimate, which enable maintaining the rapid shrub growth even in colder climates. Furthermore, the results show that in our system, herbivores facilitate the resilience of shrub‐dominated ecosystems to climate warming.     

Representation of tundra vegetation by pollen in lake sediments of northern Alaska

Aim To understand better the representation of arctic tundra vegetation by pollen data, we analysed pollen assemblages and pollen accumulation rates (PARs) in the surface sediments of lakes. Location Modern sediment samples were collected from seventy‐eight lakes located in the Arctic Foothills and Arctic Coastal Plain regions of northern Alaska. Methods For seventy of the lakes, we analysed pollen and spores in the upper 2 cm of the sediment and calculated the relative abundance of each taxon (pollen percentages). For eleven of the lakes, we used ²¹⁰Pb analysis to determine sediment accumulation rates, and analysed pollen in the upper 10–15 cm of the sediment to estimate modern PARs. Using a detailed land‐cover map of northern Alaska, we assigned each study site to one of five tundra types: moist dwarf‐shrub tussock‐graminoid tundra (DST), moist graminoid prostrate‐shrub tundra (PST) (coastal and inland types), low‐shrub tundra (LST) and wet graminoid tundra (WGT). Results Mapped pollen percentages and multivariate comparison of the pollen data using discriminant analysis show that pollen assemblages vary along the main north–south vegetational and climatic gradients. On the Arctic Coastal Plain where climate is cold and dry, graminoid‐dominated PST and WGT sites were characterized by high percentages of Cyperaceae and Poaceae pollen. In the Arctic Foothills where climate is warmer and wetter, shrub‐dominated DST, PST and LST were characterized by high percentages of Alnus and Betula pollen. Small‐scale variations in tundra vegetation related to edaphic variability are also represented by the pollen data. Discriminant analysis demonstrated that DST sites could be distinguished from foothills PST sites based on their higher percentages of Ericales and Rubus chamaemorus pollen, and coastal PST sites could be distinguished from WGT sites based on their higher percentages of Artemisia. PARs appear to reflect variations in overall vegetation cover, although the small number of samples limits our understanding of these patterns. For coastal sites, PARs were higher for PST than WGT, whereas in the Arctic Foothills, PARs were highest in LST, intermediate in DST, and lowest in PST. Main conclusion Modern pollen data from northern Alaska reflect patterns of tundra vegetation related to both regional‐scale climatic gradients and landscape‐scale edaphic heterogeneity.     

CO2 exchange in three Canadian High Arctic ecosystems: response to long-term experimental warming

Carbon dioxide exchange, soil C and N, leaf mineral nutrition and leaf carbon isotope discrimination (LCID‐Δ) were measured in three High Arctic tundra ecosystems over 2 years under ambient and long‐term (9 years) warmed (～2°C) conditions. These ecosystems are located at Alexandra Fiord (79°N) on Ellesmere Island, Nunavut, and span a soil water gradient; dry, mesic, and wet tundra. Growing season CO₂ fluxes (i.e., net ecosystem exchange (NEE), gross ecosystem photosynthesis (GEP), and ecosystem respiration (R_e)) were measured using an infrared gas analyzer and winter C losses were estimated by chemical absorption. All three tundra ecosystems lost CO₂ to the atmosphere during the winter, ranging from 7 to 12 g CO₂‐C m^?2 season^?1 being highest in the wet tundra. The period during the growing season when mesic tundra switch from being a CO₂ source to a CO₂ sink was increased by 2 weeks because of warming and increases in GEP. Warming during the summer stimulated dry tundra GEP more than R_e and thus, NEE was consistently greater under warmed as opposed to ambient temperatures. In mesic tundra, warming stimulated GEP with no effect on R_e increasing NEE by ～10%, especially in the first half of the summer. During the ～70 days growing season (mid‐June–mid‐August), the dry and wet tundra ecosystems were net CO₂‐C sinks (30 and 67 g C m^?2 season^?1, respectively) and the mesic ecosystem was a net C source (58 g C m^?2 season^?1) to the atmosphere under ambient temperature conditions, due in part to unusual glacier melt water flooding that occurred in the mesic tundra. Experimental warming during the growing season increased net C uptake by ～12% in dry tundra, but reduced net C uptake by ～20% in wet tundra primarily because of greater rates of R_e as opposed to lower rates of GEP. Mesic tundra responded to long‐term warming with ～30% increase in GEP with almost no change in R_e reducing this tundra type to a slight C source (17 g C m^?2 season^?1). Warming caused LCID of Dryas integrafolia plants to be higher in dry tundra and lower in Salix arctic plants in mesic and wet tundra. Our findings indicate that: (1) High Arctic ecosystems, which occur in similar mesoclimates, have different net CO₂ exchange rates with the atmosphere; (2) long‐term warming can increase the net CO₂ exchange of High Arctic tundra by stimulating GEP, but it can also reduce net CO₂ exchange in some tundra types during the summer by stimulating R_e to a greater degree than stimulating GEP; (3) after 9 years of experimental warming, increases in soil carbon and nitrogen are detectable, in part, because of increases in deciduous shrub cover, biomass, and leaf litter inputs; (4) dry tundra increases in GEP, in response to long‐term warming, is reflected in D. integrifolia LCID; and (5) the differential carbon exchange responses of dry, mesic, and wet tundra to similar warming magnitudes appear to depend, in part, on the hydrologic (soil water) conditions. Annual net ecosystem CO₂‐C exchange rates ranged from losses of 64 g C m^?2 yr^?1 to gains of 55 g C m^?2 yr^?1. These magnitudes of positive NEE are close to the estimates of NPP for these tundra types in Alexandra Fiord and in other High Arctic locations based on destructive harvests.     

Shrub–tree interactions and environmental changes drive treeline dynamics in the Subarctic

Treelines have drawn persistent research interest as they can respond markedly to climate. However, the mechanisms that determine tree seedling recruitment and the response of the forest‐tundra ecotone to environmental changes remain poorly understood. We hypothesise that treeline tree seedling performance depends on the interplay between climatic and soil nutritional changes and facilitative and competitive interactions between trees and shrubs. We conducted a seedling transplantation experiment with Betula pubescens at a subarctic treeline, in northern Sweden, which followed a full factorial design with four treatment factors relating to environmental regimes of stress and resource availability: site (forest vs treeline); temperature (+/? passive warming); shrub presence (+/?Vaccinium myrtillus removal); and nutrient availability (+/? NPK addition). During three growing seasons we assessed the establishment and performance of Betula. The experimental manipulations caused highly significant effects on seedling performance. Although Vaccinium enhanced seedling survival and reduced the effects of excessive solar radiation and insect herbivory, the seedlings growing with the shrub had a poorer performance by the end of the experimental period. Also, seedlings in the forest had a poorer performance than those at the treeline. Betula seedlings showed a very pronounced and positive response to passive warming and to nutrient addition, but such effects were more evident at the treeline site and often interacted with the presence of Vaccinium. This experiment shows that shrub–tree interactions are important drivers of subarctic treeline dynamics and that they vary with time and space. Facilitation, competition, herbivory and environmental changes at the tree seedling stage act as important filters in structuring the forest–tundra ecotone. We demonstrate that changes in this ecotone cannot be simply predicted from changing temperature patterns alone, and that complex interactions need to be considered, not only between shrubs and trees, but also with herbivores and between warming and soil nutrient availability.     

Aphid–willow interactions in a high Arctic ecosystem: responses to raised temperature and goose disturbance

Recently, there have been several studies using open top chambers (OTCs) or cloches to examine the response of Arctic plant communities to artificially elevated temperatures. Few, however, have investigated multitrophic systems, or the effects of both temperature and vertebrate grazing treatments on invertebrates. This study investigated trophic interactions between an herbivorous insect (Sitobion calvulum, Aphididae), a woody perennial host plant (Salix polaris) and a selective vertebrate grazer (barnacle geese, Branta leucopsis). In a factorial experiment, the responses of the insect and its host to elevated temperatures using open top chambers (OTCs) and to three levels of goose grazing pressure were assessed over two summer growing seasons (2004 and 2005). OTCs significantly enhanced the leaf phenology of Salix in both years and there was a significant OTC by goose presence interaction in 2004. Salix leaf number was unaffected by treatments in both years, but OTCs increased leaf size and mass in 2005. Salix reproduction and the phenology of flowers were unaffected by both treatments. Aphid densities were increased by OTCs but unaffected by goose presence in both years. While goose presence had little effect on aphid density or host plant phenology in this system, the OTC effects provide interesting insights into the possibility of phenological synchrony disruption. The advanced phenology of Salix effectively lengthens the growing season for the plant, but despite a close association with leaf maturity, the population dynamics of the aphid appeared to lack a similar phenological response, except for the increased population observed.     

Experimental icing affects growth,mortality, and flowering in a high Arctic dwarf shrub

Effects of climate change are predicted to be greatest at high latitudes, with more pronounced warming in winter than summer. Extreme mid‐winter warm spells and heavy rain‐on‐snow events are already increasing in frequency in the Arctic, with implications for snow‐pack and ground‐ice formation. These may in turn affect key components of Arctic ecosystems. However, the fitness consequences of extreme winter weather events for tundra plants are not well understood, especially in the high Arctic. We simulated an extreme mid‐winter rain‐on‐snow event at a field site in high Arctic Svalbard (78°N) by experimentally encasing tundra vegetation in ice. After the subsequent growing season, we measured the effects of icing on growth and fitness indices in the common tundra plant, Arctic bell‐heather (Cassiope tetragona). The suitability of this species for retrospective growth analysis enabled us to compare shoot growth in pre and postmanipulation years in icing treatment and control plants, as well as shoot survival and flowering. Plants from icing treatment plots had higher shoot mortality and lower flowering success than controls. At the individual sample level, heavily flowering plants invested less in shoot growth than nonflowering plants, while shoot growth was positively related to the degree of shoot mortality. Therefore, contrary to expectation, undamaged shoots showed enhanced growth in ice treatment plants. This suggests that following damage, aboveground resources were allocated to the few remaining undamaged meristems. The enhanced shoot growth measured in our icing treatment plants has implications for climate studies based on retrospective analyses of Cassiope. As shoot growth in this species responds positively to summer warming, it also highlights a potentially complex interaction between summer and winter conditions. By documenting strong effects of icing on growth and reproduction of a widespread tundra plant, our study contributes to an understanding of Arctic plant responses to projected changes in winter climatic conditions.     

Long‐term deepened snow promotes tundra evergreen shrub growth and summertime ecosystem net CO2 gain but reduces soil carbon and nutrient pools

Arctic climate warming will be primarily during winter, resulting in increased snowfall in many regions. Previous tundra research on the impacts of deepened snow has generally been of short duration. Here, we report relatively long‐term (7–9 years) effects of experimentally deepened snow on plant community structure, net ecosystem CO₂ exchange (NEE), and soil biogeochemistry in Canadian Low Arctic mesic shrub tundra. The snowfence treatment enhanced snow depth from 0.3 to ~1 m, increasing winter soil temperatures by ~3°C, but with no effect on summer soil temperature, moisture, or thaw depth. Nevertheless, shoot biomass of the evergreen shrub Rhododendron subarcticum was near‐doubled by the snowfences, leading to a 52% increase in aboveground vascular plant biomass. Additionally, summertime NEE rates, measured in collars containing similar plant biomass across treatments, were consistently reduced ~30% in the snowfenced plots due to decreased ecosystem respiration rather than increased gross photosynthesis. Phosphate in the organic soil layer (0–10 cm depth) and nitrate in the mineral soil layer (15–25 cm depth) were substantially reduced within the snowfences (47–70 and 43%–73% reductions, respectively, across sampling times). Finally, the snowfences tended (p = .08) to reduce mineral soil layer C% by 40%, but with considerable within‐ and among plot variation due to cryoturbation across the landscape. These results indicate that enhanced snow accumulation is likely to further increase dominance of R. subarcticum in its favored locations, and reduce summertime respiration and soil biogeochemical pools. Since evergreens are relatively slow growing and of low stature, their increased dominance may constrain vegetation‐related feedbacks to climate change. We found no evidence that deepened snow promoted deciduous shrub growth in mesic tundra, and conclude that the relatively strong R. subarcticum response to snow accumulation may explain the extensive spatial variability in observed circumpolar patterns of evergreen and deciduous shrub growth over the past 30 years.