* Winter is a period of dormancy for plants of cold environments. However, winter climate is changing, leading to an increasing frequency of stochastic warm periods (winter warming events) and concomitant reductions in snow cover. These conditions can break dormancy for some plants and expose them to freeze-and-thaw stress. Mosses are a major component of high-latitude ecosystems, yet the longer-term impacts of such winter warming events on mosses remain unknown. * In order to determine the longer-term legacy effects of winter warming events on mosses, we undertook a simulation of these events over three consecutive winters in a sub-Arctic dwarf shrub-dominated open woodland. The mat-forming feather moss, Hylocomium splendens (the most abundant cryptogam in this system), is one of the most widespread Arctic and boreal mosses and plays a key functional role in ecosystems. We studied the ecophysiological performance of this moss during the summers of the experimental period (2007–2009) and in the following years (2010–2013). * We show that the previously reported warming-induced reduction in segment growth and photosynthesis during the experimental years was persistent. Four years after the last event, photosynthesis and segment growth were still 30 and 36% lower than control levels, which was only a slight improvement from 44 and 43% 4 years earlier. Winter warming did not affect segment symmetry. During the years after the last simulated event, in both warmed and control plots, chlorophyll fluorescence and segment growth, but not net photosynthesis, increased slightly. The increases were probably driven by increased summer rainfall over the study years, highlighting the sensitivity of this moss to rainfall change. * Overall, the legacy effects shown here demonstrate that this widespread and important moss is likely to be significantly disadvantaged in a future sub-Arctic climate where frequent winter warming events may become the norm. Given the key importance of mosses for soil insulation, shelter and carbon sequestration in high-latitude regions, such persistent impacts may ultimately affect important ecosystem functions. A Lay Summary is available for this article.

1.Earlier snowmelt at high latitudes advances aboveground plant phenology, thereby affecting water, nutrient and carbon cycles. Despite the key role of fine roots in these ecosystem processes, phenological responses to earlier snowmelt have never been assessed belowground.2.We experimentally advanced snowmelt in two contrasting plant community types (heath and meadow) in northern Sweden and measured above- and belowground phenology (leaf-out, flowering and fine root growth). We expected earlier snowmelt to advance both above- and belowground phenology, and shrub-dominated heath to be more responsive than meadow.3.Snow melted on average nine days earlier in the manipulated plots than in controls, and soil temperatures were on average 0.9 °C higher during the snowmelt period of three weeks. This resulted in small advances in aboveground phenology, but contrary to our expectations, root phenology was unresponsive, with root growth generally starting before leaf-out. These responses to the snowmelt treatment were similar in both plant community types, despite strong differences in dominating plant functional types and root properties, such as root length and turnover.4.The lack of a response in root phenology, despite warmer soil temperatures and aboveground phenological advances, adds evidence that aboveground plant responses might not be directly translated to belowground plant responses, and that our understanding of factors driving belowground phenology is still limited, although of major importance for water, nutrient and carbon cycling.This article is protected by copyright. All rights reserved.

Functional overlap among species (redundancy) is considered important in shaping competitive and mutualistic interactions that determine how communities respond to environmental change. Most studies view functional redundancy as static, yet traits within species?which ultimately shape functional redundancy?can vary over seasonal or spatial gradients. We therefore have limited understanding of how trait turnover within and between species could lead to changes in functional redundancy or how loss of traits could differentially impact mutualistic interactions depending on where and when the interactions occur in space and time. Using an Arctic bumblebee community as a case study, and 1277 individual measures from 14 species over three annual seasons, we quantified how inter- and intraspecific body-size turnover compared to species turnover with elevation and over the season. Coupling every individual and their trait with a plant visitation, we investigated how grouping individuals by a morphological trait or by species identity altered our assessment of network structure and how this differed in space and time. Finally, we tested how the sensitivity of the network in space and time differed when simulating extinction of nodes representing either morphological trait similarity or traditional species groups. This allowed us to explore the degree to which trait-based groups increase or decrease interaction redundancy relative to species-based nodes. We found that (i) groups of taxonomically and morphologically similar bees turn over in space and time independently from each other, with trait turnover being larger over the season; (ii) networks composed of nodes representing species versus morphologically similar bees were structured differently; and (iii) simulated loss of bee trait groups caused faster coextinction of bumblebee species and flowering plants than when bee taxonomic groups were lost. Crucially, the magnitude of these effects varied in space and time, highlighting the importance of considering spatiotemporal context when studying the relative importance of taxonomic and trait contributions to interaction network architecture. Our finding that functional redundancy varies spatiotemporally demonstrates how considering the traits of individuals within networks is needed to understand the impacts of environmental variation and extinction on ecosystem functioning and resilience. 

1. Herbivory and litter decomposition are key controllers of ecosystem carbon and nutrient cycling. We hypothesized that foliar defences of plant species against vertebrate herbivores would reduce leaf digestibility and would subsequently, through ‘afterlife effects’, reduce litter decomposability. 2. We tested this hypothesis by screening 32 subarctic plant species, belonging to eight types in terms of life form and nutrient economy strategy, for (1) leaf digestibility in cow rumen juice; (2) biochemical and structural traits that might explain variation in digestibility; and (3) litter mass loss during simultaneous incubation in an outdoor subarctic litter bed. 3. Interspecific variation in green-leaf digestibility corresponded significantly with that in litter decomposability; this relationship was strongly driven by overall variation among the eight plant types (r = 0.92). The same relationship was not detectable within plant types in taxonomic relatedness tests. 4. Several biochemical and structural parameters (phenol-to-N ratio, lignin-to-N ratio) explained a significant part of the variation in leaf digestibility, but again only between and not within plant types. 5. Our results provide further support for the role played by foliar defence in the link between plant and soil via the decomposition pathway. They are also a new example of the potential control of plant functional types over carbon and nutrient dynamics in ecosystems.

Plant chemical and structural defence compounds are well known to impact upon herbivory of fresh leaves and influence decomposition rates after leaf senescence. A number of theories predict that alleviating nutrient limitation and reducing other environmental stressors will result in decreased production of plant chemical defences. In this study, we measured plant defence properties [total polyphenols (TP), condensed tannins (CT) and lignin concentrations, and protein complexation capacity (PCC)] in both fresh and senesced plant leaves in a fully factorial N and P fertilization experiment set-up at each of three elevations along an elevational gradient in Swedish subarctic tundra heath vegetation. Further, we performed a decomposition of variance analysis on community-weighted averages (CWAs) of plant defence properties to determine the relative contributions of interspecific and intraspecific variation to the total variation observed in response to elevation and nutrient addition. We hypothesized that N fertilization would reduce plant defence properties and that this reduction would be greater at higher elevations, while the effects of P fertilization would have no effect at any elevation. At the community level, N addition reduced CT and PCC in both fresh and senesced leaves and TP in senesced leaves, while P addition had few effects, broadly in line with our hypothesis. The effects of N addition frequently varied with elevation, but in contrast to our hypothesis, the said effects were strongest at the lowest elevations. The effects of N addition and the interactive effect of N with elevation were primarily driven by intraspecific, rather than interspecific, variation. Our findings suggest that as temperatures warm and N availability increases due to global climate change, secondary metabolites in subarctic heath vegetation will decline particularly within species. Our results highlight the need to consider the effects of both nutrient availability and temperature, and their interaction, in driving subarctic plant defence.

1. Vascular plant growth forms in northern peatlands differ in their strategies to cope with the harsh climate, low nutrient availability and progressively increasing height of the Sphagnum carpet in which they grow. Climate change may therefore affect growth forms differentially, both directly and through changes in the length growth of Sphagnum mosses. However, the role of mosses as modifiers of climate-change effects on vascular plants has been largely overlooked so far. We investigated the direct and Sphagnum-mediated effects of experimental changes in summer, winter and spring climate on four species of contrasting growth forms (evergreen and deciduous shrubs, graminoid, forb) in a subarctic bog, by studying their biomass and nitrogen losses through leaf litter, and the length growth of the two shrubs. 2. Direct and indirect effects of summer warming differed among the growth forms. Enhanced Sphagnum overgrowth of leaves due to summer warming initially stimulated leaf litter losses of the evergreen shrub Empetrum nigrum. However, changes in its shoot morphology, related to an apparent small increase in its length growth, prevented further effects. A stronger increase in stem growth of the deciduous shrub Betula nana in response to summer warming directly reduced its leaf litter mass, N concentration and N losses. The changed allocation prevented indirect, Sphagnum-mediated effects on its leaf and N dynamics through overgrowth of buds. In contrast, leaf litter mass, N concentrations or N losses of the forb Rubus chamaemorus and the graminoid Calamagrostis lapponica were not affected by summer warming or enhanced Sphagnum growth. 3. Increases in winter snow cover, with or without spring warming, did not affect shrub growth, nor the total shoot leaf litter mass or N dynamics of any of the growth forms. 4. Altogether, summer warming is likely to enhance Sphagnum overgrowth of small shrubs with a limited growth response such as Empetrum. Moreover, increased vertical growth may allow Sphagnum to keep pace with inclined growing, responsive shrubs such as Betula. This might prevent net longer-term positive effects of summer warming on the vascular plant canopy height. However, leaf litter and N losses are more likely to be affected by direct warming effects on shoot morphology and allocation than by Sphagnum growth. The different responses of the growth forms to summer warming suggest that both direct and Sphagnum-mediated climate effects have the potential to change the vascular plant community and N dynamics in peatlands.

Summary 1.Recent evidence indicates tight control of plant resource economics over interspecific trait variation amongst species, both within and across organs, referred to as ‘plant economics spectrum’ (PES). Whether and how these coordinated whole-plant economics strategies can influence the decomposition system and thereby impact on ecosystem carbon and nutrient cycling are yet an open question. More specifically, it is yet unknown whether plant functional traits have consistent afterlife effects across different plant organs. 2.To answer those questions, we conducted a common-garden decomposition experiment bringing together leaves, fine stems, coarse stems, fine roots and reproductive parts from a wide range of subarctic plant types, clades and environments. We measured all plant parts for the same (green and litter) plant economics traits and identified a whole-plant axis of carbon and nutrient economics. 3.€‚We demonstrated that our local ‘PES’ has important afterlife effects on carbon turnover by driving coordinated decomposition rates of different organs across species. All organ decomposabilities were consistently controlled by the same structure-related traits (lignin, C and dry matter content) whilst nutrient-related traits (N, P, pH, phenols) had more variable influence, likely due to their contrasting functions across organs. Nevertheless, consistent shifts in elevation of parallel trait–decomposition relationships between organs indicate that other variables, potentially related to organ dimensions, configuration or chemical contents, codetermine litter decomposition rates. 4.Whilst the coordinated litter decomposabilities across species organs imply a coordinated impact of plant above-ground and below-ground litters on plant–soil feedbacks, the contrasting decomposabilities between plant parts suggest a major role for the relative inputs of organ litter as driver of soil properties and ecosystem biogeochemistry. These relationships, underpinning the afterlife effects of the PES on whole-plant litter decomposability, will provide comprehensive input of vegetation composition feedback to soil carbon turnover.

1. The pattern of responses of adult trees of Mountain Birch (Betula pubescens ssp. czerepanovii) to an outbreak of a folivorous insect (Epirrita autumnata) causing approximate to90% defoliation was studied over 8 years in a subarctic area in northern Sweden. 2. Tree recovery was monitored in terms of the numbers, area, mass and nitrogen content of leaves, the numbers of leaf-carrying long and short shoots, and the widths of annual tree rings in stems. 3. The most prominent characteristic of defoliated trees was a threefold increase in the proportion of long shoots 1 year after defoliation. These shoots subsequently produced many new leaf-carrying short shoots, resulting in shoot populations approximate to50% larger than those of control trees. Thereafter the most strongly defoliated trees showed decreasing leaf area. 4. Defoliated trees produced more female catkins but fewer male catkins than control trees. 5. Released apical dominance did not result in any overcompensation in terms of growth or leaf area production. However, it presumably increased the rate of recovery from herbivory. For the most strongly defoliated trees, the benefits from increased production of long shoots was not sustainable.

1. Although direct contributions of soil invertebrates to carbon turnover are modest, they have a disproportionally large indirect impact through their control over the activity of microbial decomposers. Shifts in soil invertebrate species distribution might have a substantial effect on the decomposition process because their functional role depends on the species’ vertical position in soils. Gradients in microclimate and substrate quality and structure largely determine the vertical position of soil invertebrates. Because of the possible impact of climate change on soil invertebrate distribution, and consequently on decomposition, it is important to know the relative contributions of microclimate and substrate quality to the vertical distribution patterns of soil invertebrates.

2. We studied this for springtails (Collembola) as a keystone group in cool and cold biomes, by turning peat cores in a subarctic blanket bog upside down, thereby reversing the substrate quality gradient and leaving temperature and moisture gradients intact.

3. Two opposing groups of springtail species could be distinguished with respect to their abundance responses along the vertical gradient: (i) species that remain associated with the stratum they were originally found in (‘stayers’) and (ii) species that re‐establish the original stratification pattern, by remigration either to the top or deeper layers, irrespective of any substrate quality change (‘movers’). Within the ‘mover’ response pattern, the direction of their migration in response to microclimate changes seemed to coincide with their ecomorphological traits.

4. Our results not only demonstrate that springtail species differ in their responses to changes in climate or substrate quality; they also suggest that interspecific faunal trait variation may provide a useful tool to predict animal responses to climatic changes.

Alpine and Arctic treeline expansion depends on establishment of tree seedlings beyond the current treeline, which is expected to occur with climate warming. However, treelines often fail to respond to higher temperatures, and it is therefore likely that other environmental factors are important for seedling establishment. We aimed to analyse our current understanding of how temperature and a range of other environmental drivers affect tree seedling establishment at the alpine and Arctic treelines world-wide and to assess the relative importance of temperature compared with other factors and how they interact. We collected 366 observations from 76 experimental and observational papers for a qualitative analysis of the role of a wide range of environmental factors on tree seed germination, tree seedling growth, survival and natural occurrence. For a subset of these studies, where the experimental design allowed, we conducted formal meta-analyses to reveal if there were global drivers for different seedling life traits. The analyses showed that a wide range of abiotic and biotic factors affected tree seedling establishment besides from temperature, including water, snow, nutrients, light and surrounding vegetation. The meta-analyses showed that different seedling life stages do not respond similarly to environmental factors. For example, temperature had positive effects on growth, while tree seedling survival and germination showed mixed responses to warming. Further, warming was as often as not the strongest factor controlling tree seedling establishment, when compared to with one of five other environmental factors. Moreover, warming effects often depended on other factors such as moisture or the presence of surrounding vegetation. Our results suggest that population dynamics of trees at the alpine and Arctic treeline is responsive to environmental changes and show that there is a clear need for multifactorial studies if we want to fully understand and predict the interplay between warming and other environmental factors and their effect on tree seedling establishment across current treelines. A plain language summary is available for this article.

The on-going climate warming is promoting shrub abundance in high latitudes, but the effect of this phenomenon on ecosystem functioning is expected to depend on whether deciduous or evergreen species increase in response to warming. To explore effects of long-term warming on shrubs and further on ecosystem functioning, we analysed vegetation and ecosystem CO₂ exchange after 20 years of warming in the forest–tundra ecotone in subarctic Sweden. A previous study conducted 9 years earlier had found increased evergreen Empetrum nigrum ssp. hermaphroditum in the forest and increased deciduous Betula nana in the tundra. Following current understanding, we expected continued increase in shrub abundance that would be stronger in tundra than in forest. We expected warming to increase ecosystem respiration (R_e) and gross primary productivity (GPP), with a greater increase in R_e in tundra due to increased deciduous shrub abundance, leading to a less negative net ecosystem exchange and reduced ecosystem C sink strength. As predicted, vascular plant abundances were higher in the warmed plots with a stronger response in tundra than in forest. However, whereas B. nana had increased in abundance since the last survey, E. hermaphroditum abundance had declined due to several moth and rodent outbreaks during the past decade. In contrast to predictions, R_e was significantly lower in the warmed plots irrespective of habitat, and GPP increased marginally only in the forest. The lower R_e and a higher GPP under warming in the forest together led to increased net C sink. R_e was negatively associated with the total vascular plant abundance. Our results highlight the importance of disturbance regimes for vegetation responses to warming. Climate warming may promote species with both a high capacity to grow under warmer conditions and a resilience towards herbivore outbreaks. Negative correlation between R_e and total vascular plant abundance further indicate that the indirect impacts of increased plants on soil microclimate may become increasingly important for ecosystem CO₂ exchange in the long run, which adds to the different mechanisms that link warming and CO₂ fluxes in northern ecosystems. Read the free Plain Language Summary for this article on the Journal blog.

1. We tested the hypotheses that (i) reproductive ramets of Linnaea borealis L. (Caprifoliaceae) compensate for reproductive investments by enhanced resorption of nitrogen (N) and phosphorus (P) from leaves before abscission, and that (ii) resorption is decreased by increased soil nutrient availability. We also investigated (iii) the significance of resorption for ramet growth and nutrient pool sizes. 2. Leaves from reproductive, post-reproductive and non-reproductive ramets were used to investigate compensatory resorption of N and P. To study resorption responses to increased nutrient availability, slow-release fertilizer was applied and differences between defoliated and shaded ramets were noted. 3. No compensation for reproductive investments via resorption from senescing leaves was detected. Resorption efficiency increased with increased nutrient availability for N, but showed no response for P. No effect of experimentally denied resorption was seen. 4. We propose that extensive physiological integration within clonal fragments of L. borealis buffers resource alterations in ramets associated with reproduction, fertilization and prevented resorption. 5. Our results show that the performance of a ramet of a physiologically well integrated plant is not governed by local conditions - ramets can exceed patch nutrient availability without reduced growth and nutrient status.

Rising temperatures can influence ecosystem processes both directly and indirectly, through effects on plant species and communities. An improved understanding of direct versus indirect effects of warming on ecosystem processes is needed for robust predictions of the impacts of climate change on terrestrial ecosystem carbon (C) dynamics.To explore potential direct and indirect effects of warming on C dynamics in arctic tundra heath, we established a warming (open top chambers) and dominant plant species (Empetrum hermaphroditum Hagerup) removal experiment at a high and low elevation site. We measured the individual and interactive effects of warming, dominant species removal and elevation on plant species cover, the normalized difference vegetation index (NDVI), leaf area index (LAI), temperature, soil moisture and instantaneous net ecosystem CO2 exchange.We hypothesized that ecosystems would be stronger CO2 sinks at the low elevation site, and that warming and species removal would weaken the CO2 sink because warming should increase ecosystem respiration (ER) and species removal should reduce gross primary productivity (GPP). Furthermore, we hypothesized that warming and species removal would have the greatest impact on processes at the high elevation where site temperature should be most limiting and dominant species may buffer the overall community to environmental stress more compared to the low elevation site where plants are more likely to compete with the dominant species.The instantaneous CO2 flux, which reflected a weak CO2 sink, was similar at both elevations. Neither experimental warming nor dominant species removal significantly changed GPP or instantaneous net ecosystem CO2 exchange even though species removal significantly reduced ER, NDVI and LAI.Our results show that even the loss of dominant plant species may not result in significant landscape‐scale responses of net ecosystem CO2 exchange to warming. They also show that NDVI and LAI may be limited in their ability to predict changes in GPP in these tundra heaths systems. Our study highlights the need for more detailed vegetation analyses and ground‐truthed measurements in order to accurately predict direct and indirect impacts of climatic change on ecosystem C dynamics.

* Spatial heterogeneity affects plant performance and is influenced by plants, but the scale at which fine roots react to or generate spatial heterogeneity has received little attention. Fine roots might be expected to respond to heterogeneity at a scale comparable to their diameter (mm), but studies to date have been conducted at much coarser resolutions (cm – m). Here we quantify root heterogeneity in contrasting habitats with special attention to the influence of resolution. * We measured fine root length heterogeneity at resolutions ranging from 1 to 300 mm2, at four elevations along an arctic alpine gradient from 500 m a.s.l. (forest) to 1100 m (tundra). We calculated the magnitude of heterogeneity as the coefficient of variation of root length, and the scale of heterogeneity using semivariance analysis. * The magnitude of heterogeneity was about twofold greater at fine than coarse resolution. Further, the magnitude of heterogeneity was generally greatest at the highest elevation, suggesting that soil at 1100 m was less evenly occupied by plant roots than soils at lower elevations. The exception to this was at the 1 mm2 resolution, for which the magnitude of heterogeneity did not vary with elevation, possibly because heterogeneity at this scale is related to ecophysiological processes common to all vegetation types. * The scale of root length heterogeneity increased significantly with resolution coarseness, suggesting that roots respond to or generate patchiness at small scales that have not previously been examined. In contrast, the scale of heterogeneity did not vary significantly with elevation and the accompanying turnover in growth form. * Our results suggest that roots in four vegetation types respond to or generate very fine scales of spatial heterogeneity, including scales much smaller than those that have previously been examined. Both the magnitude and scale of heterogeneity varied with sampling resolution, suggesting resolutions as small as a few millimetres are relevant to studies of spatial root interactions and below-ground processes.

* The ‘home-field advantage’ (HFA) hypothesis predicts that plant litter is decomposed faster than expected underneath the plant from which it originates (‘home’) than underneath other plants (‘away’), because decomposer communities are specialized to break down litter from the plants they associate with. However, empirical evidence shows that the occurrence of HFA is highly variable, and the reasons for this are little understood. * In our study, we progress our understanding by investigating whether HFA is stronger for more recalcitrant litter types and under colder conditions and how soil properties and plant functional traits affect the magnitude and direction of HFA. * In subarctic tundra in northern Sweden, we set up a reciprocal transplant litter decomposition experiment along an elevational gradient where three highly contrasting vegetation types (heath, meadow and Salix) occur at all elevations, and where temperature decreases strongly with elevation. In this study, we used a litter bag approach where litters from each elevation × vegetation type combination were decomposed in all combinations of elevation × vegetation type. We also measured community-level plant functional traits, such as leaf and litter nutrient content. We determined soil biotic and abiotic properties, such as microbial biomass and soil nutrient content, in soil cores collected for each elevation × vegetation type combination. * We found that mass loss increased with plant and litter nutrient content and with soil temperature. In contrast, the occurrence of HFA was limited in our study system, and its magnitude and direction could not be explained by vegetation type, elevation, plant traits or soil properties, despite these factors serving as powerful drivers of litter mass loss in our study. * We conclude that although vegetation type and climate are major drivers of litter mass loss, they do not emerge as important determinants of HFA. Therefore, while rapid shifts in plant community composition or temperature due to global change are likely to influence litter mass loss directly by altering environmental conditions, plant trait spectra and litter quality, indirect effects of global change resulting from decoupling of specialist interactions between litter and decomposer communities appear to be of less importance.