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Treeline ecosystems are of great scientific interest to study the direct and indirect influence of limiting environmental conditions on tree growth. However, tree growth is complex and multidimensional, and its responses to the environment depend on a large number of abiotic and biotic factors and their interactions.In this study, we analyze the growth and xylem anatomy of white spruce trees (Picea glauca [Moench] Voss) from three treelines in Alaska (one warm and drought-limited, and two cold and temperature-limited treelines). We hypothesized (1) no difference between the treelines regarding the relationship between tree DBH and height, yet in general (2) faster growing trees at the warmer site. Additionally, we expected to find differences in xylem anatomical traits with trees from the drought-limited site having adapted to drought conditions by (3) forming smaller lumen diameter due to water deficit but (4) a higher xylem anatomical density due to higher temperatures and a longer vegetation period.Regarding growth in height and diameter, trees at the drought-limited treeline grew relatively (1) taller and (2) faster compared to trees at the temperature-limited treelines. Raw xylem anatomical measurements showed (3) smaller lumen diameters and (4) higher density in trees at the drought-limited treeline. However, using linear mixed-effect models, we found that (i) traits related to water transport like lumen diameter were not significantly correlated with the actual amount of precipitation during the vegetation period but with tree height. We also found that (ii) traits related to mechanical support like density were mainly positively influenced by the mean temperature during the vegetation period.The differences in lumen diameter found in the raw data can be explained by differences in the growth rates of the trees, since lumen diameter at the lower part of the tree stem needs to increase over time with increasing tree height. The greater wood density at the drought-limited treeline is probably caused by the higher temperature that leads to more biomass production, and potentially longer vegetation periods.Our study shows that xylem anatomical traits in white spruce can be directly and indirectly controlled by environmental conditions. While lumen diameter is not directly influenced by environmental conditions but indirectly through tree height, other traits like anatomical density show a direct correlation with environmental conditions. Our results highlight the importance of approaching tree growth in a multidimensional way and considering direct and indirect effects of environmental forcing.

Este artículo contiene 18 páginas, 1 tabla, 5 figuras., Conferences are ideal platforms for studying gender gaps in science because they are important cultural events that reflect barriers to women in academia. Here, we explored women’s participation in ecology conferences by analyzing female representation, behavior, and personal experience at the 1st Meeting of the Iberian Society of Ecology (SIBECOL). The conference had 722 attendees, 576 contributions, and 27 scientific sessions. The gender of attendees and presenters was balanced (48/52% women/men), yet only 29% of the contributions had a woman as last author. Moreover, men presented most of the keynote talks (67%) and convened most of the sessions. Our results also showed that only 32% of the questions were asked by women, yet the number of questions raised by women increased when the speaker or the convener was a woman. Finally, the post-conference survey revealed that attendees had a good experience and did not perceive the event as a threatening context for women. Yet, differences in the responses between genders suggest that women tended to have a worse experience than their male counterparts. Although our results showed clear gender biases, most of the participants of the conference failed to detect it. Overall, we highlight the challenge of increasing women’s scientific leadership, visibility and interaction in scientific conferences and we suggest several recommendations for creating inclusive meetings, thereby promoting equal opportunities for all participants., AL was supported by the Government of Catalonia and the the European Social Fund (ESF) through the program Beatriu de Pino´s (BP-2018- 00082). PR-L was supported by a Margalida Comas postdoctoral contract (PD/031/2018), funded by the Government of the Balearic Islands and the ESF. AA-R was supported by a Humboldt Research Fellowship. MB was supported by the Spanish Government through the project Alkaldia (PID2019-111137GB-C21). SB was supported by a Ramon y Cajal fellowship from the Spanish Government and AEI/FEDER UE (RYC-2017- 22643). AGB was supported by a Marie Sklodowska-Curie (MSCA) Individual Fellowship (H2020-MSCA-IF-2016; project-749645). NC was funded by the European Union’s Horizon 2020 research and innovation programme under the MSCA grant agreement No.839709. MJF was supported by the Portuguese Foundation of Science and Technology (FCT) through MARE strategic project (UIDB/04292/2020) and Norma Transito´ria. AGO was supported by the CESAM and FCT/MCTES (UIDP/50017/2020 + UIDB/50017/ 2020). CG-C was supported by the Spanish Government through a Juan de la Cierva – Incoporacio´n contract (IJC2018-036642-I). FL had a doctoral grant funded by FCT (PD/BD/52598/ 2014). GO was supported by the German Federal Ministry of Education and Research within the Collaborative Project“Bridging in Biodiversity Science – BIBS”(01LC1501A-H). SP was supported by a postdoctoral fellowship MSCA Seal of Excellence of the Research Foundation – Flanders (12ZZS21N)., With the institutional support of the ‘Severo OchoaCentre of Excellence’ accreditation (CEX2019-000928-S).

In the tundra, bryophytes may be the dominant growth form covering the soil surface of shrub communities. They can modulate soil conditions through their capacity to retain moisture and nutrients and their chemical characteristics. The study of the interaction between shrubs and bryophytes is essential to understand the functioning of these shrub communities, which are expanding due to global change. In this study, we collected Betula nana and Empetrum hermaphroditum ramets growing in moss carpets dominated by Hylocomium splendens, Pleurozium schreberi or Sphagnum spp., which differ in growth habit, density of their carpets and water holding capacity, amongst others. We sampled three ramets per site and moss species in eight locations distributed along a precipitation gradient (571-1155 mm/year) in the subarctic alpine tundra near Abisko, Sweden. To investigate structural and functional responses to the dominant moss species and precipitation regime, we prepared microscopic sections of the shrubs stem base and measured growth rings and xylem anatomical parameters (vessel lumen area, vessel density and grouping, and theoretical hydraulic conductivity). We also measured shrub leaf C and N concentration and isotope composition (δ13C, δ15N). To understand moss effects on soil characteristics along the precipitation gradient, we measured soil pH and water and nutrient content (nitrate, ammonium, phosphate, dissolved organic C and dissolved organic N). Preliminary results on shrub leaf physiology and soil characteristics show a significant interaction between moss species and the precipitation gradient, indicating that mosses modulate the effects of climate conditions on shrubs. We discuss the importance of moss species combined with the precipitation regime for the performance of tundra shrubs in the context of a changing climate.

Background and AimsForest peatlands represent 25 % of global peatlands and store large amounts of carbon (C) as peat. Traditionally they have been drained in order to increase forestry yield, which may cause large losses of C from the peat. Rewetting aims to stop these losses and to restore the initial storage function of the peatlands. As roots represent major peat-forming elements in these systems, we sampled roots with diameter MethodsWe cored soil next to Alnus glutinosa stems and sorted root biomass into Key ResultsRoot biomass in the rewetted site was more than double that in the drained site. This difference was mostly driven by very fine roots ConclusionsThe size-dependent opposite patterns between root biomass and their functional characteristics under contrasting water regimes indicate differences between fine and coarse roots in their response to environmental changes. Root age distribution points to similar root turnover rates between the sites, while higher root biomass in the rewetted site clearly indicates larger tree C stocks below ground under rewetting, supporting the C sink function of the ecosystem.

The ecological function of boreal forests is challenged by drastically changing climate conditions. Although an increasing number of studies are investigating how climate change is influencing growth and distribution of boreal tree species, there is a lack of studies examining the potential of these species to genetically adapt or phenotypically adjust. Here, we sampled clonally and non-clonally growing white spruce trees (Picea glauca [Moench] Voss) to investigate spatial and genetic effects on tree ring width and on six xylem anatomical traits representing growth, water transport, mechanical support, and wood density. We compared different methods for estimating broad sense heritability (H2) of each trait and we evaluated the effects of spatial grouping and genetic grouping on the xylem anatomical traits with linear models. We found that the three different methods used to estimate H2 were quite robust, showing overall consistent patterns, while our analyses were unsuccessful at fully separating genetic from spatial effects. By evaluating the effect size, we found a significant effect of genetic grouping in latewood density and earlywood hydraulic diameter. However, evaluating model performances showed that spatial grouping was a better predictor than genetic grouping for variance in earlywood density, earlywood hydraulic diameter and growth. For cell wall thickness neither spatial nor genetic grouping was significant. Our findings imply that (1) the variance in the investigated xylem anatomical traits and growth is mainly influenced by spatial clustering (most probably caused by microhabitat conditions), which (2) makes it rather difficult to estimate the heritability of these traits in naturally grown trees in situ. Yet, (3) latewood density and earlywood hydraulic diameter qualified for further analysis on the genetic background of xylem traits and (4) cell wall thickness seems a useful trait to investigate large-scale climatic effects, decoupled from microclimatic, edaphic and genetic influences.

The majority of variation in six traits critical to the growth, survival and reproduction of plant species is thought to be organised along just two dimensions, corresponding to strategies of plant size and resource acquisition. However, it is unknown whether global plant trait relationships extend to climatic extremes, and if these interspecific relationships are confounded by trait variation within species. We test whether trait relationships extend to the cold extremes of life on Earth using the largest database of tundra plant traits yet compiled. We show that tundra plants demonstrate remarkably similar resource economic traits, but not size traits, compared to global distributions, and exhibit the same two dimensions of trait variation. Three quarters of trait variation occurs among species, mirroring global estimates of interspecific trait variation. Plant trait relationships are thus generalizable to the edge of global trait-space, informing prediction of plant community change in a warming world., It is unclear whether plant trait relationships found at the global scale extend to climatic extremes. Here the authors analyse six major aboveground traits to show that known plant trait relationships extend to the tundra biomes and exhibit the same two dimensions of variation detected at the global scale.

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.

Herbivory is one of the most globally distributed disturbances affecting carbon (C)-cycling in trees, yet our understanding of how it alters tree C-allocation to different functions such as storage, growth or rhizodeposition is still limited. Prioritized C-allocation to storage replenishment vs growth could explain the fast recovery of C-storage pools frequently observed in growth-reduced defoliated trees. We performed continuous 13C-labeling coupled to clipping to quantify the effects of simulated browsing on the growth, leaf morphology and relative allocation of stored vs recently assimilated C to the growth (bulk biomass) and non-structural carbohydrate (NSC) stores (soluble sugars and starch) of the different organs of two tree species: diffuse-porous (Betula pubescens Ehrh.) and ring-porous (Quercus petraea [Matt.] Liebl.). Carbon-transfers from plants to bulk and rhizosphere soil were also evaluated. Clipped birch and oak trees shifted their C-allocation patterns above-ground as a means to recover from defoliation. However, such increased allocation to current-year stems and leaves did not entail reductions in the allocation to the rhizosphere, which remained unchanged between clipped and control trees of both species. Betula pubescens and Q. petraea showed differences in their vulnerability and recovery strategies to clipping, the ring-porous species being less affected in terms of growth and architecture by clipping than the diffuse-porous. These contrasting patterns could be partly explained by differences in their C cycling after clipping. Defoliated oaks showed a faster recovery of their canopy biomass, which was supported by increased allocation of new C, but associated with large decreases in their fine root biomass. Following clipping, both species recovered NSC pools to a larger extent than growth, but the allocation of 13C-labeled photo-assimilates into storage compounds was not increased as compared with controls. Despite their different response to clipping, our results indicate no preventative allocation into storage occurred during the first year after clipping in either of the species.

Carbon (C) allocation is a key determinant of plant functioning and ecosystem processes. Its dynamic responses to environmental conditions and global change are still poorly understood. This holds particularly true for dwarf shrubs, which are an important component of high mountain vegetation. We studied the short-term C allocation dynamics in two dominant dwarf shrub species, Vaccinium myrtillus and Vaccinium uliginosum, at two subalpine sites, in the Pyrenees and in the Alps, and additionally performed an experimental rain exclusion to simulate summer drought at the site in the Alps. We compared net assimilation rates ( A max ), stomatal conductance ( g s ) and shoot water potentials ( Ψ S ) between the two species and carried out a 13 C pulse labelling experiment to trace their C allocation dynamics, both under natural conditions and in the drought experiment. Under ambient conditions, V. uliginosum showed higher A max and g s and used recent assimilates as a respiratory substrate more slowly than V. myrtillus . C allocation dynamics in current-year shoots differed significantly between the species, showing a progressive decrease after the pulse labelling in V. uliginosum shoots but a progressive increase in V. myrtillus shoots. In the drought experiment, a significant reduction of soil moisture caused predawn Ψ S below −1.2 MPa in both species, which is considered close to the turgor loss point. However, the stomatal conductance and the speed of transfer of newly assimilated C to belowground organs decreased in V. uliginosum only. We found a strong coupling in C allocation dynamics between xylem and phloem tissues of the rhizome, which was maintained under drought. Although both species were largely tolerant to the drought imposed, our results indicate a higher drought sensitivity of V. uliginosum compared to V. myrtillus , with potential implications for community composition and ecosystem C cycling in a future climate.

Species occurrence is subjected to the particular ecological constraints of each site where species that can become dominant exert strong influence on community structure and functioning. Given the harsh environmental conditions in alpine areas and the low number of woody species able to support them, we wanted to study the effect of the main abiotic (environmental conditions related to topographical situation and bedrock type) and biotic (functional type of the dominant species) factors on functional structure and processes of alpine subshrub communities. In 24 study sites, we collected aboveground biomass and raw litter. Biomass was sorted into functional groups (shrubs, graminoids, forbs, bryophytes, lichens), which, in turn, were separated into photosynthetic and structural compartments. We characterized community structure in terms of biomass allocation of functional groups, estimated leaf primary production and litter persistence, and performed analyses of variance for each of the factors considered. Our results showed that community structure is driven mainly by the functional type of the dominant species, although abiotic factors are also relevant. Production depends on abiotic conditions. It is constrained by low temperatures in northern exposures and by low water availability in permeable bedrock types. Litter decomposition depends on environmental conditions and the dominant species characteristics. It is limited by dry conditions in southern slopes and by the thick and N-poor leaves of evergreen subshrubs. In summary, the few alpine woody species able to build specific communities in the Pyrenees grow in particular topographical situations, where they become crucial drivers of ecosystem structure and functioning.

Motivation: The Tundra Trait Team (TTT) database includes field-based measurements of key traits related to plant form and function at multiple sites across the tundra biome. This dataset can be used to address theoretical questions about plant strategy and trade-offs, trait environment relationships and environmental filtering, and trait variation across spatial scales, to validate satellite data, and to inform Earth system model parameters. Main types of variable contained: The database contains 91,970 measurements of 18 plant traits. The most frequently measured traits (>1,000 observations each) include plant height, leaf area, specific leaf area, leaf fresh and dry mass, leaf dry matter content, leaf nitrogen, carbon and phosphorus content, leaf C:N and N:P, seed mass, and stem specific density. Spatial location and grain: Measurements were collected in tundra habitats in both the Northern and Southern Hemispheres, including Arctic sites in Alaska, Canada, Greenland, Fennoscandia and Siberia, alpine sites in the European Alps, Colorado Rockies, Caucasus, Ural Mountains, Pyrenees, Australian Alps, and Central Otago Mountains (New Zealand), and sub-Antarctic Marion Island. More than 99% of observations are georeferenced. Time period and grain: All data were collected between 1964 and 2018. A small number of sites have repeated trait measurements at two or more time periods. Major taxa and level of measurement: Trait measurements were made on 978 terrestrial vascular plant species growing in tundra habitats. Most observations are on individuals (86%), while the remainder represent plot or site means or maximums per species. Software format: csv file and GitHub repository with data cleaning scripts in R; contribution to TRY plant trait database (www.try-db.org) to be included in the next version release. 2Ecoinformatics and Biodiversity, Department of Bioscience, Aarhus University, Aarhus, Denmark 3Senckenberg Gesellschaft fD?r Naturforschung, Biodiversity and Climate Research Centre (BiK?F), Frankfurt, Germany 4Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, Colorado 5National Ecological Observatory Network, Boulder, Colorado 6Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado 7Arctic Research Center, Department of Bioscience, Aarhus University, Aarhus, Denmark 8Center for Biodiversity Dynamics in a Changing World (BIOCHANGE), Department of Bioscience, Aarhus University, Aarhus, Denmark 9Department of Biological and Environmental Sciences, Qatar University, Doha, Qatar 10Department of Forestry, Forest and Wildlife Research Center, Mississippi State University, Mississippi 11Department of Evolutionary Biology, Ecology and Environmental Sciences, University of Barcelona, Barcelona, Spain 12Biodiversity Research Institute, University of Barcelona, Barcelona, Spain 13Institute of Botany and Landscape Ecology, Greifswald University, Greifswald, Germany 14Center for Integrative Conservation, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Xishuangbanna, China 15Department of Evolutionary Biology and Environmental Studies, University of Zurich, Zurich, Switzerland 16Department of Ecology and Environmental Science, Ume� University, Ume�, Sweden 17Environmental Sciences, Copernicus Institute of Sustainable Development, Utrecht University, Utrecht, The Netherlands 18School of Informatics, Computing, and Cyber Systems, Northern Arizona University, Flagstaff, Arizona 19Department of Earth Sciences, University of Gothenburg, Gothenburg, Sweden 20Gothenburg Global Biodiversity Centre, GD?teborg, Sweden 21Department of Physical Geography and Ecosystem Science, Lund University, Lund, Sweden 22Martin Luther University Halle?Wittenberg, Institute of Biology / Geobotany and Botanical Garden, Halle (Saale), Germany 23German Centre for Integrative Biodiversity Research (iDiv) Halle?Jena?Leipzig, Leipzig, Germany 24Adam Mickiewicz University, Institute of Geoecology and Geoinformation, Poznan, Poland 25University of Alaska Anchorage, Department of Biological Sciences, Anchorage, Alaska 26Technische Universit�t MD?nchen, Freising, Germany 27Department of Chemistry, Life Sciences and Environmental Sustainability, University of Parma, Parma, Italy 28The Alaska Department of Fish and Game, Anchorage, Alaska 29Department of Biology, Memorial University, St. John�s, Newfoundland and Labrador, Canada 30Department of Arctic and Marine Biology, Faculty of Biosciences, Fisheries and Economics, UiT? The Arctic University of Norway, Troms�, Norway 31Systems Ecology, Department of Ecological Science, Vrije Universiteit, Amsterdam, The Netherlands 32Department of Botany, University of Otago, Dunedin, New Zealand 33Department of Botany and Biodiversity Research, University of Vienna, Vienna, Austria 34Center for Permafrost (CENPERM), Department of Geosciences and Natural Resource Management, University of Copenhagen, Copenhagen, Denmark 35Department of Physiological Diversity, Helmholtz Centre for Environmental Research ? UFZ, Leipzig, Germany 36Department of Ecology and Genetics, University of Oulu, Oulu, Finland 37Arctic Centre, University of Lapland, Rovaniemi, Finland 38Swiss Federal Research Institute WSL, Birmensdorf, Switzerland 39Department of Geography, University of British Columbia, Vancouver, British Columbia, Canada 40Faculty of Science and Technology, Federation University, Ballarat, Victoria, Australia 41Global Ecology Unit, CREAF?CSIC?UAB, Bellaterra, Catalonia, Spain 42CREAF, Bellaterra, Cerdanyola del Vall�s, Catalonia, Spain 43Department of Ecology, Environment and Evolution, La Trobe University, Bundoora, Australia 44Department of Plant and Soil Sciences, University of Pretoria, Pretoria, South Africa 45Department of Biology, Queen�s University, Kingston, Ontario, Canada Scopus

Plant growth and performance in snowbed communities are strongly influenced by the length of the growing period remaining after the snowmelt date. Under the scenario of climate change, studying the phenological responses of snowbed species to variations in snowmelt date might be crucial to understand their ability to adapt and survive under changing conditions. We studied 13 plant species from contrasting biological and ecological groups (i.e. ‘snowbed specialists’, ‘preferential species’ and ‘generalist species’) growing in four snowbed plots with siliceous substrates in the Central Pyrenees. These species were monitored for two consecutive years and in two different microsites, namely the central part and the peripheral part of each snowbed plot. We characterised their phenological cycle and length and their success in seed setting, measured as the number of individuals succeeding in dispersing their seeds. Phenology and cycle length did not differ greatly between locations for any of the species, but their success in setting seeds was variable. We did not find substantial differences in species’ cycle length under varying snowmelt regimes (neither between years nor between microsites); this lack of differences was more evident for snowbed specialists. Moreover, this group showed shorter cycles and less ability to modify their phenology with varying snowmelt regimes than generalist species and some of the preferential species. These two latter groups showed a trend towards reduced success in setting seeds with retarded snowmelt, but the plant community structure exerted a complementary effect on seed setting which in some cases was even greater than snowmelt date. The ability to produce and disperse seeds seems to be a less conservative feature than cycle length in snowbed plants. Thus, their adaptation to future climate changes might take place as a result of variations in their reproductive performance rather than in the modification of their phenological cycle. In addition, shifts in plant community composition as a result of environmental changes might be of special importance in the behaviour of these species.

Plant–plant interactions are key drivers of treeline dynamics. At the Pyrenean treeline, the dwarf shrub Vaccinium myrtillus grows in pure stands or in mixed stands with Vaccinium uliginosum or Rhododendron ferrugineum. They form sparse shrub patches that colonize subalpine grasslands, having dramatic impacts on their structure and functioning. We investigated the role of the two co-occurring shrubs as possible modulators of the structure and performance of V. myrtillus in the Central Pyrenees. We analysed biomass, growth, functional parameters, age distribution, N and C concentrations and isotope compositions (δ15N and δ 13C) of V. myrtillus ramets in pure stands, and stands mixed with V. uliginosum or R. ferrugineum. Volume, above-ground biomass and age of the ramets did not differ between stand types. We found lower δ13C values, indicative of lower water-use efficiency (WUE), in leaves and shoots of V. myrtillus in stands with R. ferrugineum than in pure stands. The N content and δ15N of V. myrtillus leaves and shoots in pure stands were higher than in mixed stands, pointing to a competition for N in mixed stands. Our results indicate that V. myrtillus competes for nutrients with its neighbours, but neither this competition nor the lower WUE affect its above-ground performance. Therefore, the interaction with co-occurring shrubs does not have a major effect on V. myrtillus structure and functioning at treeline and, consequently, should not be considered as a key driver on the dynamics of this species in the encroachment of subalpine grasslands.

We thank Clara Borrull, Noelia Seguer, Estela Illa, Victoria Lafuente, Elena Lahoz and Santiago Pérez for their field and laboratory assistance. We are grateful to CCiT of the University of Barcelona for the use of their facilities and their technical assistance. This work was supported by Conselh Generau d'Aran and the project ARBALMONT/786-2012 (Organismo Autónomo Parques Nacionales, Ministerio de Agricultura, Alimentación y Medio Ambiente, Spain). AAR was funded by an FPU grant (Ministerio de Educación, Cultura y Deporte, Spain) and SP was funded by a Ramón y Cajal fellowship (RYC-2013-14164, Ministerio de Economía y Competitividad, Spain). Climate warming can lead to changes in alpine plant species interactions through modifications in environmental conditions, which may ultimately cause drastic changes in plant communities. We explored the effects of 4 years of experimental warming with open-top chambers (OTC) on Vaccinium myrtillus performance and its interaction with neighbouring shrubs at the Pyrenean treeline ecotone. We examined the effects of warming on height, above-ground (AG) and below-ground (BG) biomass and the C and N concentration and isotope composition of V. myrtillus growing in pure stands or in stands mixed with Vaccinium uliginosum or Rhododendron ferrugineum. We also analysed variations in soil N concentrations, rhizosphere C/N ratios and the functional diversity of the microbial community, and evaluated whether warming altered the biomass, C and N concentration and isotope composition of V. uliginosum in mixed plots. Our results showed that warming induced positive changes in the AG growth of V. myrtillus but not BG, while V. uliginosum did not respond to warming. Vaccinium myrtillus performance did not differ between stand types under increased temperatures, suggesting that warming did not induce shifts in the interaction between V. myrtillus and its neighbouring species. These findings contrast with previous studies in which species interactions changed when temperature was modified. Our results show that species interactions can be less responsive to warming in natural plant communities than in removal experiments, highlighting the need for studies involving the natural assembly of plant species and communities when exploring the effect of environmental changes on plant-plant interactions.

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.