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Plant growth responses to environmental changes may be linked to xylem anatomical adjustments. The study of such links is essential for improving our understanding of plant functioning under global change. We investigated the xylem anatomy and above-ground growth of the dwarf shrub Vaccinium myrtillus in the understorey of Larix decidua and Pinus uncinata at the Swiss treeline after 9 years of free-air CO2 enrichment (+200 ppm) and 6 years of soil warming (+4 °C). We aimed to determine the responses of xylem anatomical traits and growth to these treatments, and to analyse xylem anatomy–growth relationships. We quantified anatomical characteristics of vessels and ray parenchyma and measured xylem ring width (RW), above-ground biomass and shoot elongation as growth parameters. Our results showed strong positive correlations between theoretical hydraulic conductivity (Kh) and shoot increment length or total biomass across all treatments. However, while soil warming stimulated shoot elongation and RW, it reduced vessel size (Dh) by 14%. Elevated CO2 had smaller effects than soil warming: it increased Dh (5%) in the last experimental years and only influenced growth by increasing basal stem size. The abundance of ray parenchyma, representing storage capacity, did not change under any treatment. Our results demonstrate a link between growth and stem Kh in V. myrtillus, but its growth responses to warming were not explained by the observed xylem anatomical changes. Smaller Dh under warming may increase resistance to freezing events frequently occurring at treeline and suggests that hydraulic efficiency is not limiting for V. myrtillus growing on moist soils at treeline. Our findings suggest that future higher atmospheric CO2 concentrations will have smaller effects on V. myrtillus growth and functioning than rising temperatures at high elevations; further, growth stimulation of this species under future warmer conditions may not be synchronized with xylem adjustments favouring hydraulic efficiency.

Globally accelerating trends in societal development and human environmental impacts since the mid-twentieth century 1–7 are known as the Great Acceleration and have been discussed as a key indicator of the onset of the Anthropocene epoch 6 . While reports on ecological responses (for example, changes in species range or local extinctions) to the Great Acceleration are multiplying 8, 9 , it is unknown whether such biotic responses are undergoing a similar acceleration over time. This knowledge gap stems from the limited availability of time series data on biodiversity changes across large temporal and geographical extents. Here we use a dataset of repeated plant surveys from 302 mountain summits across Europe, spanning 145 years of observation, to assess the temporal trajectory of mountain biodiversity changes as a globally coherent imprint of the Anthropocene. We find a continent-wide acceleration in the rate of increase in plant species richness, with five times as much species enrichment between 2007 and 2016 as fifty years ago, between 1957 and 1966. This acceleration is strikingly synchronized with accelerated global warming and is not linked to alternative global change drivers. The accelerating increases in species richness on mountain summits across this broad spatial extent demonstrate that acceleration in climate-induced biotic change is occurring even in remote places on Earth, with potentially far-ranging consequences not only for biodiversity, but also for ecosystem functioning and services.

Summary Trees continuously adjust their axial xylem structure to meet changing needs imposed by ontogenetic and environmental changes. These axial structure-function responses need to be coordinated among competing biophysical constraints to avoid failure of the xylem system. Here, we investigated if ontogeny or experimental manipulation of CO2 and soil temperature influence these structure-function responses. We performed detailed xylem cell anatomical quantification along the axis of 40-year-old Larix decidua trees planted at the Swiss treeline and exposed to a combination of elevated CO2 (+200 ppm) and soil warming (+4 °C) between 2001 and 2012. We assessed how mean hydraulic tracheid diameter (Dh), the cell wall reinforcement ((t/b)2), tracheid wall thickness (CWT) and the percent area of ray parenchyma (PERPAR) – proxies for hydraulic efficiency, hydraulic safety, biomechanical support and metabolic xylem functions, respectively – co-vary along the tree axis. Dh increased from the stem apex to base, strictly following a power function (R2=0.81), independent from ontogeny and experimental treatments. In contrast, axial trends of (t/b)2 and CWT were either influenced by treatment and/or ontogeny, or showed no axial trend (PERPAR). Additionally, we found that a larger Dh only at the stem apex promoted primary and secondary growth. Our approach of analyzing xylem anatomical traits along the tree axis and across tree-rings provides novel insights into xylem functional architecture and allows reconstructing xylem function over time. We conclude that the maintenance of hydraulic efficiency during ontogeny is very robust, that the conduit diameter undergoes a strong apical control, and plays a fundamental role for assimilation and tree growth. Instead, the other functional traits more plastically vary with ontogeny and environmental changes. This article is protected by copyright. All rights reserved.

Summary Global warming may accelerate nitrogen (N) transformations in the soil, with potentially large effects in N-poor high-elevation ecosystems. To gain insight into the partitioning of inorganic and organic N inputs within the plant–soil system and how warming influences these patterns, we applied a 15N label (15NH4Cl or 15N-glycine) shortly after snowmelt during the sixth year of experimental soil warming (+4 °C) at treeline in the Swiss Alps. Seven weeks after labelling, approximately 60% of the applied label remained in the soil organic layer to 10 cm depth, whereas label recovery summed over all measured plant pools was

Climate warming and elevated CO2 can modify nutrient cycling mediated by enzymes in soils, especially in cold-limited ecosystems with a low availability of nutrients and a high temperature sensitivity of decomposition and mineralization. We estimated responses of soil extracellular enzyme activities (EEAs) to 6 years of soil warming and 9 years of CO2 enrichment at an Alpine treeline site. EEAs were measured in the litter (L), fermentation (F) and humified (H) horizons under Larix decidua and Pinus uncinata trees. Soil warming indirectly affected EEAs through altered soil moisture, fine root biomass, and C:N ratio of the organic horizons. Warming increased β-glucosidase and β-xylosidase activities in the F horizon but led to reduced laccase activity in the L horizon, probably caused by drying of the litter horizon associated with the treatment. In the H horizon, previous CO2 enrichment altered the activity of leucine amino peptidase, N-acetylglucosaminidase, and phosphatase. No interactive effects between warming and CO2 enrichment were detected. Warming affected the temperature sensitivity of β-xylosidase but not of the other enzymes. Altered EEAs after six years of soil warming indicate a sustained stimulation of carbon, nitrogen and nutrient cycling under climatic warming at the alpine treeline.

Climate warming may alter ecosystem nitrogen (N) cycling by accelerating N transformations in the soil, and changes may be especially pronounced in cold regions characterized by N-poor ecosystems. We investigated N dynamics across the plant-soil continuum during 6 years of experimental soil warming (2007-2012; +4 °C) at a Swiss high-elevation treeline site (Stillberg, Davos; 2180 m a.s.l.) featuring Larix decidua and Pinus uncinata. In the soil, we observed considerable increases in the NH4+ pool size in the first years of warming (by >50%), but this effect declined over time. In contrast, dissolved organic nitrogen (DON) concentrations in soil solutions from the organic layer increased under warming, especially in later years (maximum of +45% in 2012), suggesting enhanced DON leaching from the main rooting zone. Throughout the experimental period, foliar N concentrations showed species-specific but small warming effects, whereas δ15 N values showed a sustained increase in warmed plots that was consistent for all species analysed. The estimated total plant N pool size at the end of the study was greater (+17%) in warmed plots with Pinus but not in those containing Larix, with responses driven by trees. Irrespective of plot tree species identity, warming led to an enhanced N pool size of Vaccinium dwarf shrubs, no change in that of Empetrum hermaphroditum (dwarf shrub) and forbs, and a reduction in that of grasses, nonvascular plants, and fine roots. In combination, higher foliar δ15 N values and the transient response in soil inorganic N indicate a persistent increase in plant-available N and greater cumulative plant N uptake in warmer soils. Overall, greater N availability and increased DON concentrations suggest an opening of the N cycle with global warming, which might contribute to growth stimulation of some plant species while simultaneously leading to greater N losses from treeline ecosystems and possibly other cold biomes.

Treeline responses to climate change ultimately depend on successful seedling recruitment, which requires dispersal of viable seeds and establishment of individual propagules in novel environments. In this study, we evaluated the effects of several abiotic and biotic drivers of early tree seedling recruitment across an alpine treeline ecotone. In two consecutive years, we sowed seeds of low- and high-elevation provenances of Larix decidua (European larch) and Picea abies (Norway spruce) below, at, and above the current treeline into intact vegetation and into open microsites with artificially removed surface vegetation, as well as into plots protected from seed predators and herbivores. Seedling emergence and early establishment in treatment and in control plots were monitored over two years. Tree seedling emergence occurred at and several hundred metres above the current treeline when viable seeds and suitable microsites for germination were available. However, dense vegetation cover at lower elevations and winter mortality at higher elevations particularly limited early recruitment. Post-dispersal predation, species, and provenance also affected emergence and early establishment. This study demonstrates the importance of understanding multiple abiotic and biotic drivers of early seedling recruitment that should be incorporated into predictions of treeline dynamics under climate change., Scientific Reports, 8, ISSN:2045-2322

Plant growth responses to environmental changes may be linked to xylem anatomical adjustments. The study of such links is essential for improving our understanding of plant functioning under global change. We investigated the xylem anatomy and above-ground growth of the dwarf shrub Vaccinium myrtillus in the understorey of Larix decidua and Pinus uncinata at the Swiss treeline after 9 years of free-air CO

The impact of a warmer climate on CH 4 fluxes from soils is highly uncertain, because soil warming may affect methanotrophic bacteria in two opposed ways: CH 4 assimilation in soils might be increased by the decreasing soil moisture often associated with soil warming. In contrast, CH 4 oxidation might be suppressed by higher NH 4 + concentrations in warmed soils resulting from an accelerated nitrogen mineralization. We investigated effects of soil warming on soil-atmosphere CH 4 fluxes in the last two years of a six-year long field experiment at a Swiss alpine treeline. Specifically, we measured CH 4 fluxes using static chambers, and characterized N cycling by quantifying soil N 2 O emissions and NH 4 + and NO 3 − concentrations. We further labeled intact soil cores with C 14 H 4 and traced the labeled bacteria using an auto-radiographic technique to study the potential warming-related changes in the micro-distribution of methanotrophic bacteria within the soils. Our results did not show a significant effect of soil warming on net CH 4 fluxes after five and six years of soil warming. In general, soils were a net sink for CH 4 but CH 4 emissions were observed occasionally. One reason for the unaltered CH 4 fluxes might be the negligible warming effects on soil water contents in the treeline environment with frequent rainfalls. In the warmed soils, soil moisture was lower in the litter layer, but not deeper in the soils. Therefore, soil warming did not affect gas transport rates into deeper soil layers where methanotrophic bacteria were located. Another reason might be the general absence of substantial warming effects on mineral N, with NH 4 + concentrations being marginally significantly higher in warmed soils only in ion exchange resin bags (P 14 C distribution and a warming-induced shift of methanotrophic bacteria toward the soil surface. The absence of responses of CH 4 fluxes to warming in this alpine treeline ecosystem is likely related to the rather minimal changes in the putative drivers soil moisture and NH 4 + concentration.

Human societies depend on an Earth system that operates within a constrained range of nutrient availability, yet the recent trajectory of terrestrial nitrogen (N) availability is uncertain. Examining patterns of foliar N concentrations and isotope ratios (δ 15 N) from more than 43,000 samples acquired over 37 years, here we show that foliar N concentration declined by 9% and foliar δ 15 N declined by 0.6–1.6‰. Examining patterns across different climate spaces, foliar δ 15 N declined across the entire range of mean annual temperature and mean annual precipitation tested. These results suggest declines in N supply relative to plant demand at the global scale. In all, there are now multiple lines of evidence of declining N availability in many unfertilized terrestrial ecosystems, including declines in δ 15 N of tree rings and leaves from herbarium samples over the past 75–150 years. These patterns are consistent with the proposed consequences of elevated atmospheric carbon dioxide and longer growing seasons. These declines will limit future terrestrial carbon uptake and increase nutritional stress for herbivores. Fil: Craine, Joseph M.. Jonah Ventures; Estados Unidos Fil: Elmore, Andrew J.. University of Maryland; Estados Unidos Fil: Wang, Lixin. Indiana University; Estados Unidos. Purdue University; Estados Unidos Fil: Aranibar, Julieta Nelida. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Bauters, Marijn. University of Ghent; Bélgica Fil: Boeckx, Pascal. University of Ghent; Bélgica Fil: Crowley, Brooke E.. University of Cincinnati; Estados Unidos Fil: Dawes, Melissa A.. Swiss Federal Institute for Forest, Snow and Landscape Research; Suiza Fil: Delzon, Sylvain. Universite de Bordeaux; Francia. Institut National de la Recherche Agronomique; Francia Fil: Fajardo, Alex. Centro de Investigación en Ecosistemas de la Patagonia; Chile Fil: Fang, Yunting. Chinese Academy of Sciences; República de China Fil: Fujiyoshi, Lei. Research Institute for Humanity and Nature; Japón Fil: Gray, Alan. North American Electric Reliability Corporation. Centre for Ecology and Hydrology; Estados Unidos Fil: Guerrieri, Rossella. Consejo Superior de Investigaciones Científicas. Centre de Recerca Ecológica I Aplicacions Forestals; España Fil: Gundale, Michael J.. Swedish University of Agricultural Sciences; Suecia Fil: Hawke, David J.. Ara Institute of Canterbury. Department of Science and Primary Industries; Nueva Zelanda Fil: Hietz, Peter. University of Natural Resources and Life Sciences. Institute of Botany; Austria Fil: Jonard, Mathieu. Université Catholique de Louvain; Bélgica Fil: Kearsley, Elizabeth. University of Ghent; Bélgica Fil: Kenzo, Tanaka. Forestry and Forest Products Research Institute. Department of Plant Ecology; Japón Fil: Makarov, Mikhail. Moscow M.V. Lomonosov State University; Rusia Fil: Marañón Jiménez, Sara. Universidad de Granada; España. Consejo Superior de Investigaciones Científicas. Centre de Recerca Ecológica I Aplicacions Forestals; España Fil: McGlynn, Terrence P.. University of California; Estados Unidos. Natural History Museum of Los Angeles County. Department of Entomology; Estados Unidos Fil: McNeil, Brenden E.. West Virginia University. Department of Geology and Geography; Estados Unidos Fil: Mosher, Stella G.. University of Cincinnati; Estados Unidos Fil: Nelson, David M.. University of Maryland; Estados Unidos Fil: Peri, Pablo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Universidad Nacional de la Patagonia Austral; Argentina. Instituto Nacional de Tecnología Agropecuaria; Argentina Fil: Roggy, Jean Christophe. Institut National de la Recherche Agronomique; Francia. Ecologie des Forêts de Guyane; Francia Fil: Sanders Demott, Rebecca. Boston University; Estados Unidos Fil: Song, Minghua. Chinese Academy of Sciences; República de China Fil: Szpak, Paul. Trent University. Department of Anthropology; Canadá Fil: Templer, Pamela H.. Boston University; Estados Unidos Fil: Van der Colff, Dewidine. South African National Biodiversity Institute; Sudáfrica Fil: Werner, Christiane. Albert Ludwigs University of Freiburg; Alemania Fil: Xu, Xingliang. Chinese Academy of Sciences; República de China Fil: Yang, Yang. Chinese Academy of Sciences; República de China Fil: Yu, Guirui. Chinese Academy of Sciences; República de China. University of Chinese Academy of Sciences. College of Resources and Environment; República de China Fil: Zmudczyńska Skarbek, Katarzyna. University of Gdańsk; Polonia

Increased CO2 emissions and global warming may alter the composition of fungal communities through the removal of temperature limitation in the plant–soil system, faster nitrogen (N) cycling and changes in the carbon (C) allocation of host plants to the rhizosphere. At a Swiss treeline featuring Larix decidua and Pinus uncinata, the effects of multiple years of CO2 enrichment and experimental soil warming on the fungal community composition in the organic horizons were analysed using 454-pyrosequencing of ITS2 amplicons. Sporocarp production and colonization of ectomycorrhizal root tips were investigated in parallel. Fungal community composition was significantly altered by soil warming, whereas CO2 enrichment had little effect. Tree species influenced fungal community composition and the magnitude of the warming responses. The abundance of ectomycorrhizal fungal taxa was positively correlated with N availability, and ectomycorrhizal taxa specialized for conditions of high N availability proliferated with warming, corresponding to considerable increases in inorganic N in warmed soils. Traits related to N utilization are important in determining the responses of ectomycorrhizal fungi to warming in N-poor cold ecosystems. Shifts in the overall fungal community composition in response to higher temperatures may alter fungal-driven processes with potential feedbacks on ecosystem N cycling and C storage at the alpine treeline.

Summary How will carbon source–sink relations of 35-yr-old larch trees (Larix decidua) at the alpine treeline respond to changes in atmospheric CO2 and climate? We evaluated the effects of previously elevated CO2 concentrations (9 yr, 580 ppm, ended the previous season) and ongoing soil warming (4 yr, + 4°C). Larch branches were pulse labeled (50 at% 13CO2) in July 2010 to trace fresh assimilates through tissues (buds, needles, bark and wood) and non-structural carbon compounds (NCC; starch, lipids, individual sugars) using compound-specific isotope analysis. Nine years of elevated CO2 did not lead to increased NCC concentrations, nor did soil warming increase NCC transfer velocities. By contrast, we found slower transfer velocities and higher NCC concentrations than reported in the literature for lowland larch. As a result of low dilution with older carbon, sucrose and glucose showed the highest maximum 13C labels, whereas labels were lower for starch, lipids and pinitol. Label residence times in needles were shorter for sucrose and starch (c. 2 d) than for glucose (c. 6 d). Although our treatments showed no persistent effect on larch carbon relations, low temperature at high altitudes clearly induced a limitation of sink activities (growth, respiration, root exudation), expressed in slower carbon transfer and higher NCC concentrations.

Understanding the interplay between environmental factors contributing to treeline formation and how these factors influence different life stages remains a major research challenge. We used an afforestation experiment including 92 000 trees to investigate the spatial and temporal dynamics of tree mortality and growth at treeline in the Swiss Alps. Seedlings of three high-elevation conifer species (Larix decidua, Pinus mugo ssp. uncinata, and Pinus cembra) were systematically planted along an altitudinal gradient at and above the current treeline (2075 to 2230 m above sea level [a.s.l.]) in 1975 and closely monitored during the following 30 years. We used decision-tree models and generalized additive models to identify patterns in mortality and growth along gradients in elevation, snow duration, wind speed, and solar radiation, and to quantify interactions between the different variables. For all three species, snowmelt date was always the most important environmental factor influencing mortality, and elevation was always the most important factor for growth over the entire period studied. Individuals of all species survived at the highest point of the afforestation for more than 30 years, although mortality was greater above 2160 m a.s.l., 50-100 m above the current treeline. Optimal conditions for height growth differed from those for survival in all three species: early snowmelt (ca. day of year 125-140 [where day 1 is 1 January]) yielded lowest mortality rates, but relatively later snowmelt (ca. day 145-150) yielded highest growth rates. Although snowmelt and elevation were important throughout all life stages of the trees, the importance of radiation decreased over time and that of wind speed increased. Our findings provide experimental evidence that tree survival and height growth require different environmental conditions and that even small changes in the duration of snow cover, in addition to changes in temperature, can strongly impact tree survival and growth patterns at treeline. Further, our results show that the relative importance of different environmental variables for tree seedlings changes during the juvenile phase as they grow taller.

Climate change and elevated atmospheric CO2 levels could increase the vulnerability of plants to freezing. We analyzed tissue damage resulting from naturally occurring freezing events in plants from a long-term in situ CO2 enrichment ( 200 ppm, 2001–2009) and soil warming ( 4°C since 2007) experiment at treeline in the Swiss Alps (Stillberg, Davos). Summer freezing events caused damage in several abundant subalpine and alpine plant species in four out of six years between 2005 and 2010. Most freezing damage occurred when temperatures dropped below 21.5°C two to three weeks after snow melt. The tree Larix decidua and the dwarf shrubs Vaccinium myrtillus and Empetrum hermaphroditum showed more freezing damage under experimentally elevated CO2 and/or temperatures than under control conditions. Soil warming induced a 50% die-back of E. hermaphroditum during a single freezing event due to melting of the protective snow cover. Although we could not identify a clear mechanism, we relate greater freezing susceptibility to a combination of advanced plant phenology in spring and changes in plant physiology. The climate record since 1975 at the treeline site indicated a summer warming by 0.58°C/decade and a 3.5 days/decade earlier snow melt, but no significant decrease in freezing events during the vegetation period. Therefore, in a warmer climate with higher CO 2 levels but constant likelihood of extreme weather events, subalpine and alpine plants may be more susceptible to freezing events, which may partially offset expected enhanced growth with global change. Hence, freezing damage should be considered when predicting changes in growth of alpine plants or changes in community composition under future atmospheric and climate conditions.

Summary •Rising CO2 concentrations and the associated global warming are expected to have large impacts on high-elevation ecosystems, yet long-term multifactor experiments in these environments are rare. •We investigated how growth of dominant dwarf shrub species (Vaccinium myrtillus, Vaccinium gaultherioides and Empetrum hermaphroditum) and community composition in the understorey of larch and pine trees responded to 9 yr of CO2 enrichment and 3 yr of soil warming at the treeline in the Swiss Alps. •Vaccinium myrtillus was the only species that showed a clear positive effect of CO2 on growth, with no decline over time in the annual shoot growth response. Soil warming stimulated V. myrtillus growth even more than elevated CO2 and was accompanied by increased plant-available soil nitrogen (N) and leaf N concentrations. Growth of Vaccinium gaultherioides and E. hermaphroditum was not influenced by warming. Vascular plant species richness declined in elevated CO2 plots with larch, while the number of moss and lichen species decreased under warming. •Ongoing environmental change could lead to less diverse plant communities and increased dominance of the particularly responsive V. myrtillus in the studied alpine treeline. These changes are the consequence of independent CO2 and soil warming effects, a result that should facilitate predictive modelling approaches.

Many studies indicate increases of shrubs in Arctic and alpine regions in recent decades. This often dramatic increase in the abundance and dominance of shrub species is likely due to recent climate warming and has the potential to alter both the structure and function of tundra ecosystems. To better understand recent shrub dynamics and environmental changes in tundra ecosystems, analyses of shrub growth rings offer a vast variety of methods and approaches. However, classical dendrochronological methods, which were developed for tree species, need to be adapted for shrubs, taking into account their complex morphologies and growth eccentricities. Here, we review current and developing methods to measure radial and axial growth, estimate shrub age, and assess shrub growth dynamics in relation to environmental variables. Recent advances in sampling methods, analysis and applications have improved our ability to investigate growth and recruitment dynamics of shrubs in tundra ecosystems. However, future dendroecological work will require new approaches to better address variation in growth within parts of the plant, among individuals within populations and between species to scale up findings to the landscape or biome level.

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 dominance1,2,3,4. Shrub expansion can modify climate by altering surface albedo, energy and water balance, and permafrost2,5,6,7,8, 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 thawing4 and most of the global permafrost soil carbon pool is stored9. 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.

Responses of alpine tree line ecosystems to increasing atmospheric CO2 concentrations and global warming are poorly understood. We used an experiment at the Swiss tree line to investigate changes in vegetation biomass after 9 years of free air CO2 enrichment (+200 ppm; 2001‐2009) and 6 years of soil warming (+4 °C; 2007‐2012). The study contained two key tree line species, Larix decidua and Pinus uncinata, both approximately 40 years old, growing in heath vegetation dominated by dwarf shrubs. In 2012, we harvested and measured biomass of all trees (including root systems), above-ground understorey vegetation and fine roots. Overall, soil warming had clearer effects on plant biomass than CO2 enrichment, and there were no interactive effects between treatments. Total plant biomass increased in warmed plots containing Pinus but not in those with Larix. This response was driven by changes in tree mass (+50%), which contributed an average of 84% (5.7 kg m ! 2 ) of total plant mass. Pinus coarse root mass was especially enhanced by warming (+100%), yielding an increased root mass fraction. Elevated CO2 led to an increased relative growth rate of Larix stem basal area but no change in the final biomass of either tree species. Total understorey above-ground mass was not altered by soil warming or elevated CO2. However, Vaccinium myrtillus mass increased with both treatments, graminoid mass declined with warming, and forb and nonvascular plant (moss and lichen) mass decreased with both treatments. Fine roots showed a substantial reduction under soil warming (! 40% for all roots

Global warming can have substantial impacts on the phenological and growth patterns of alpine and Arctic species, resulting in shifts in plant community composition and ecosystem dynamics. We evaluated the effects of a six-year experimental soil warming treatment (+4°C, 2007-2012) on the phenology and growth of three co-dominant dwarf shrub species growing in the understory of Larix decidua and Pinus uncinata at treeline in the Swiss Alps. We monitored vegetative and reproductive phenology of Vaccinium myrtillus, Vaccinium gaultherioides and Empetrum hermaphroditum throughout the early growing season of 2012 and, following a major harvest at peak season, we measured the biomass of above-ground ramet fractions. For all six years of soil warming we measured annual shoot growth of the three species and analyzed ramet age and xylem ring width of V. myrtillus. Our results show that phenology of the three species was more influenced by snowmelt timing, and also by plot tree species (Larix or Pinus) in the case of V. myrtillus, than by soil warming. However, the warming treatment led to increased V. myrtillus total above-ground ramet biomass (+36% in 2012), especially new shoot biomass (+63% in 2012), as well as increased new shoot increment length and xylem ring width (+22% and +41%, respectively; average for 2007-2012). These results indicate enhanced overall growth of V. myrtillus under soil warming that was sustained over six years and was not caused by an extended growing period in early summer. In contrast, E. hermaphroditum only showed a positive shoot growth response to warming in 2011 (+21%), and V. gaultherioides showed no significant growth response. Our results indicate that V. myrtillus might have a competitive advantage over the less responsive co-occurring dwarf shrub species under future global warming.

The rapidly warming temperatures in high-latitude and alpine regions have the potential to alter the phenology of Arctic and alpine plants, affecting processes ranging from food webs to ecosystem trace gas fluxes. The International Tundra Experiment (ITEX) was initiated in 1990 to evaluate the effects of expected rapid changes in temperature on tundra plant phenology, growth and community changes using experimental warming. Here, we used the ITEX control data to test the phenological responses to background temperature variation across sites spanning latitudinal and moisture gradients. The dataset overall did not show an advance in phenology; instead, temperature variability during the years sampled and an absence of warming at some sites resulted in mixed responses. Phenological transitions of high Arctic plants clearly occurred at lower heat sum thresholds than those of low Arctic and alpine plants. However, sensitivity to temperature change was similar among plants from the different climate zones. Plants of different communities and growth forms differed for some phenological responses. Heat sums associated with flowering and greening appear to have increased over time. These results point to a complex suite of changes in plant communities and ecosystem function in high latitudes and elevations as the climate warms.

How will carbon source-sink relations of 35-yr-old larch trees (Larix decidua) at the alpine treeline respond to changes in atmospheric CO(2) and climate? We evaluated the effects of previously elevated CO(2) concentrations (9 yr, 580 ppm, ended the previous season) and ongoing soil warming (4 yr, + 4°C). Larch branches were pulse labeled (50 at% (13)CO(2)) in July 2010 to trace fresh assimilates through tissues (buds, needles, bark and wood) and non-structural carbon compounds (NCC; starch, lipids, individual sugars) using compound-specific isotope analysis. Nine years of elevated CO(2) did not lead to increased NCC concentrations, nor did soil warming increase NCC transfer velocities. By contrast, we found slower transfer velocities and higher NCC concentrations than reported in the literature for lowland larch. As a result of low dilution with older carbon, sucrose and glucose showed the highest maximum (13)C labels, whereas labels were lower for starch, lipids and pinitol. Label residence times in needles were shorter for sucrose and starch (c. 2 d) than for glucose (c. 6 d). Although our treatments showed no persistent effect on larch carbon relations, low temperature at high altitudes clearly induced a limitation of sink activities (growth, respiration, root exudation), expressed in slower carbon transfer and higher NCC concentrations.

• Rising CO₂ concentrations and the associated global warming are expected to have large impacts on high-elevation ecosystems, yet long-term multifactor experiments in these environments are rare. • We investigated how growth of dominant dwarf shrub species (Vaccinium myrtillus, Vaccinium gaultherioides and Empetrum hermaphroditum) and community composition in the understorey of larch and pine trees responded to 9 yr of CO₂ enrichment and 3 yr of soil warming at the treeline in the Swiss Alps. • Vaccinium myrtillus was the only species that showed a clear positive effect of CO₂ on growth, with no decline over time in the annual shoot growth response. Soil warming stimulated V. myrtillus growth even more than elevated CO₂ and was accompanied by increased plant-available soil nitrogen (N) and leaf N concentrations. Growth of Vaccinium gaultherioides and E. hermaphroditum was not influenced by warming. Vascular plant species richness declined in elevated CO₂ plots with larch, while the number of moss and lichen species decreased under warming. • Ongoing environmental change could lead to less diverse plant communities and increased dominance of the particularly responsive V. myrtillus in the studied alpine treeline. These changes are the consequence of independent CO₂ and soil warming effects, a result that should facilitate predictive modelling approaches.

1. Using experimental atmospheric CO(2) enrichment, we tested for tree growth stimulation at the high-elevation treeline, where there is overwhelming evidence that low temperature inhibits growth despite an adequate carbon supply. We exposed Larix decidua (European larch) and Pin us mugo ssp. uncinata (mountain pine) to 9 years of free-air CO(2) enrichment (FACE) in an in situ experiment at treeline in the Swiss Alps (2180 m a.s.l.). 2. Accounting for pre-treatment vigour of individual trees, tree ring increments throughout the experimental period were larger in Larix growing under elevated CO(2) but not in Pinus. The magnitude of the CO(2) response in Larix ring width varied over time, with a significant stimulation occurring in treatment years 3-7 (marginal in year 6). 3. After 9 years of treatment, leaf canopy cover, stem basal area and total new shoot production were overall greater in Larix trees growing under elevated CO(2), whereas Pinus showed no such cumulative growth response. The Larix ring width response in years 3-7 could have caused the cumulative CO(2) effect on tree size even if no further stimulation occurred, so it remains unclear if responsiveness was sustained over the longer term. 4. Larix ring width was stimulated more by elevated CO(2) in years with relatively high spring temperatures and an early snowmelt date, suggesting that temperatures were less limiting in these years and greater benefit was gained from extra carbon assimilated under elevated CO(2). The magnitude of CO(2) stimulation was also larger after relatively high temperatures and high solar radiation in the preceding growing season, perhaps reflecting gains due to larger carbon reserves. 5. Synthesis. Contrasting above-ground growth responses of two treeline tree species to elevated CO(2) concentrations suggest that Larix will have a competitive advantage over less responsive species, such as co-occurring Pinus, under future CO(2) concentrations. Stimulation of Larix growt might be especially pronounced in a future warmer climate.