Salix Pulchra Research Articles

Deciduous Tundra Shrubs Shift Toward More Acquisitive Light Absorption Strategy Under Climate Change Treatments

AbstractThe effects of climate change on plants are particularly pronounced in the Arctic region. Warming relaxes the temperature and nutrients boundaries that limit tundra plant growth. Increased resource availability under future climate conditions may induce a shift from a conservative economic strategy to an acquisitive one. Following the leaf economics spectrum that hypothesizes a strategy gradient between survival, plant size and costs for the photosynthetic leaf area, light absorption of tundra plants may increase. We investigated climate change effects on light absorptance and the relationship between light absorptance (fraction of absorbed photosynthetically active radiation, FAPAR) and structural and nutritional leaf traits, performing a soil warming and surface soil fertilization experiment on two deciduous tundra shrub species. Our results show that fertilization and warming combined increase light absorptance in Arctic shrubs and that FAPAR is correlated with leaf nutrients but not with structural leaf traits. This indicates an economic strategy shift of shrubs from conservative to acquisitive induced by warming and fertilization combined. We found species‐specific differences: FAPAR was influenced by warming alone in Betula nana but not in Salix pulchra, and FAPAR was correlated with leaf phosphorus in B. nana but not in S. pulchra. We attribute this to water limitation of B. nana that generally grows in drier areas within the study site compared to S. pulchra. We conclude that FAPAR is a measure that opens up more possibilities to estimate nutritional leaf traits and nutrient cycles, plant economic strategies, and ecological feedbacks of the tundra ecosystem on broader scales.

Forage plants for the northern pika (<i>Ochotona hyperborea</i> Pallas) in the Chuvansky Range System (South Chukotka)

The northern pika (Ochotona hyperborea Pallas) is a small herbivorous animal from the Ochtonidae family, widely distributed in the mountains of Northeast Asia. Despite its small size, the northern pika plays a noticeable role in the transformation of organic substances of biocenoses. An essential feature of these mammals is the procurement of plant food for the winter period. The food ration of the animals is very diverse and depends on the structure of the surrounding vegetation. It includes aerial parts of shrubs, sub‑shrubs, herbaceous plants, ferns, Equisetum species, rarely trees, bushy lichens, fungi and very rarely mosses. According to our analysis of the literature data, more than 220 species of vascular plants are included in the diet of the northern pika in Northeast Asia. During a survey of the northern spurs of the Chuvansky Range in the basin of the Ozernoe Gorlo river (Anadyr river basin), 14 species of vascular plants, 8 species of bushy lichens and 1 species of moss were identified in winter stocks of the northern pika on 2 terrain areas differing in landscape and vegetation structure. Salix pulchra, S. berberifolia subsp. tschuktschorum, Chamaenerion latifolium, Vaccinium uliginosum subsp. microphyllum, Empetrum nigrum, Ledum palustre subsp. Decumbens are of great importance as the preferred food of northern pika in this territory. In addition, the Dryopteris fragrans fern and more rarely Betula nana, are harvested quite sustainably in one of the sections of a stone scree (stone run) surrounded by a Pinus pumila community. The role of bushy lichens and mosses in the nutrition of the northern pika needs to be clarified.

NDVI changes in the Arctic: Functional significance in the moist acidic tundra of Northern Alaska.

The Normalized Difference Vegetation Index (NDVI), derived from reflected visible and infrared radiation, has been critical to understanding change across the Arctic, but relatively few ground truthing efforts have directly linked NDVI to structural and functional properties of Arctic tundra ecosystems. To improve the interpretation of changing NDVI within moist acidic tundra (MAT), a common Arctic ecosystem, we coupled measurements of NDVI, vegetation structure, and CO2 flux in seventy MAT plots, chosen to represent the full range of typical MAT vegetation conditions, over two growing seasons. Light-saturated photosynthesis, ecosystem respiration, and net ecosystem CO2 exchange were well predicted by NDVI, but not by vertically-projected leaf area, our nondestructive proxy for leaf area index (LAI). Further, our data indicate that NDVI in this ecosystem is driven primarily by the biochemical properties of the canopy leaves of the dominant plant functional types, rather than purely the amount of leaf area; NDVI was more strongly correlated with top cover and repeated cover of deciduous shrubs than other plant functional types, a finding supported by our data from separate "monotypic" plots. In these pure stands of a plant functional type, deciduous shrubs exhibited higher NDVI than any other plant functional type. Likewise, leaves from the two most common deciduous shrubs, Betula nana and Salix pulchra, exhibited higher leaf-level NDVI than those from the codominant graminoid, Eriophorum vaginatum. Our findings suggest that recent increases in NDVI in MAT in the North American Arctic are largely driven by expanding deciduous shrub canopies, with substantial implications for MAT ecosystem function, especially net carbon uptake.

Retrogressive thaw slumps in the Alaskan Low Arctic may influence tundra shrub growth more strongly than climate

AbstractThermokarst disturbance in permafrost landscapes is likely to increase across the tundra biome with climate warming, resulting in changes to topography, vegetation, and biogeochemical cycling. Tundra shrubs grow on permafrost, but shrub–thermokarst relationships are rarely studied in detail. Since the 1980s, Alaska's North Slope has experienced increased thermokarst activity, including retrogressive thaw slumps (RTSs) on hillslopes. Within decades, RTSs near Toolik Lake, Alaska, were colonized by tall (≥0.5 m) deciduous shrubs. We used dendrochronology methods on 66 shrubs (182 stem cross sections) representing dominant deciduous species: willows (Salix pulchra and S. glauca) and dwarf birch (Betula nana) at two RTS chronosequences on Alaska's North Slope comprising seven sites, to quantify thermokarst and climate effects (25 years of temperature and precipitation records) on shrub secondary growth (i.e., annual rings) in RTS‐disturbed and undisturbed moist acidic tussock (MAT) tundra. Across species, average growth ring widths were two times wider for shrubs in RTSs than in MAT, and ring widths decreased with RTS age. A 1°C June temperature increase was associated with 2% wider rings across species and sites, but shrubs showed marginal growth in warmer summers, supporting tundra‐wide shrub climate sensitivity studies. A 4.5% average ring width increase per 1 mm of previous year's September precipitation was seen in shrubs in mid‐successional RTSs, suggesting protective effects of early snowfall in RTSs versus open tundra. Retrogressive thaw slump age category explained 47% and 30% of average ring width variance of willows and dwarf birch, respectively, in linear mixed‐effects models. Climate variables explained 2% average ring width variance across species. Our results suggest that RTS exerts strong successional effects on tundra shrub growth. Climate effects appear to show weaker synoptic patterns across the study area. Retrogressive thaw slumps will likely contribute to tundra greening where RTS activity is increasing.

Plant trait response of tundra shrubs to permafrost thaw and nutrient addition

Abstract. Plant traits reflect growth strategies and trade-offs in response to environmental conditions. Because of climate warming, plant traits might change, altering ecosystem functions and vegetation–climate interactions. Despite important feedbacks of plant trait changes in tundra ecosystems with regional climate, with a key role for shrubs, information on responses of shrub functional traits is limited. Here, we investigate the effects of experimentally increased permafrost thaw depth and (possibly thaw-associated) soil nutrient availability on plant functional traits and strategies of Arctic shrubs in northeastern Siberia. We hypothesize that shrubs will generally shift their strategy from efficient conservation to faster acquisition of resources through adaptation of leaf and stem traits in a coordinated whole-plant fashion. To test this hypothesis, we ran a 4 year permafrost thaw and nutrient fertilization experiment with a fully factorial block design and six treatment combinations – permafrost thaw (control, unheated cable, heated cable) × fertilization (no nutrient addition, nutrient addition). We measured 10 leaf and stem traits related to growth, defence and the resource economics spectrum in four shrub species (Betula nana, Salix pulchra, Ledum palustre and Vaccinium vitis-idaea), which were sampled in the experimental plots. The plant trait data were statistically analysed using linear mixed-effect models and principal component analysis (PCA). The response to increased permafrost thaw was not significant for most shrub traits. However, all shrubs responded to the fertilization treatment, despite decreased thaw depth and soil temperature in fertilized plots. Shrubs tended to grow taller but did not increase their stem density or bark thickness. We found a similar coordinated trait response for all four species at leaf and plant level; i.e. they shifted from a conservative towards a more acquisitive resource economy strategy upon fertilization. In accordance, results point towards a lower investment into defence mechanisms, and hence increased shrub vulnerability to herbivory and climate extremes. Compared to biomass and height only, detailed data involving individual plant organ traits such as leaf area and nutrient contents or stem water content can contribute to a better mechanistic understanding of feedbacks between shrub growth strategies, permafrost thaw and carbon and energy fluxes. In combination with observational data, these experimental tundra trait data allow for a more realistic representation of tundra shrubs in dynamic vegetation models and robust prediction of ecosystem functions and related climate–vegetation–permafrost feedbacks.

Vegetation cover of the North of the Koryak Region (Kamchatsky Krai) and its geobotanical subdivision

«Вестник Санкт-Петербургского университета. Науки о Земле» — научно-теоретический рецензируемый журнал, обобщающий результаты исследований в области геологии, гидрогеологии и геохимии, петрологии и экологии, ландшафтного планирования и геоморфологии, экономической и социальной географии.

Recruitment dynamics and population structure of willows in tundra disturbed by retrogressive thaw slump thermokarst on Alaska’s North slope

Retrogressive thaw slumps (RTS) are large (> 1 ha) depressions of exposed mineral soil on hillslopes caused by the thaw and displacement of ice-rich permafrost soils in high-latitude regions; since the 1980s the number of RTS observed on Alaska’s North Slope has increased by two-thirds. Some RTS in the Toolik Lake area are filled with tall (≥ 0.5 m) willow thickets, likely within decades after disturbance. Tall shrub thickets are different in structure and function from mixed dwarf tundra communities and may have different long-term impacts on ecosystems and wildlife. Currently it is unknown to what degree seedlings versus clonal recruitment contribute to shrub thickets. We assessed size and distribution of clones (modular stems of individuals) using eight microsatellite (SSR) markers to genotype leaf tissue of 223 willow ramets (stems) at two sites: an RTS aged 11–30 years since disturbance, and nearby undisturbed moist acidic tussock (MAT) tundra. Genotypes of known clones from excavated ramets were used to determine the mutation rate of clones. Spatial arrangement of ramets within clones was assessed in 18 × 18 m sampling grids nested at 2 m (“far clones”), 1 m (intermediate), and 0.25 m (“near clones”) between ramets. We identified 121 genotypes including 10 clonal genotypes in the RTS, and 63 genotypes including 11 clonal genotypes in the undisturbed MAT. Percent distinguishable was greater than 76% at both sites. Mean spatial distance between clonal ramets was not different at either site but among far clones, ramets were separated 7–16 m downslope in the RTS. Salix pulchra was the dominant willow at both sites; a third of willow genotypes in the RTS were identified as S. glauca, a disturbance colonizer. Rarer and hybrid species comprised 4–20% of all genotypes but were more abundant in the RTS. At both sites, within-species expected heterozygosity (HT) ranged from 0.49 - 0.85. Our results suggest: 1) sexual recruitment and low clonal expansion likely explain genetic diversity of willows in disturbed and late-successional sites; and 2) downslope separation of far clones in the RTS suggests disturbance effects; and 3) species richness was higher in the disturbed site.

The importance of secondary growth to plant responses to snow in the arctic

Abstract In arctic environments, drifting snow around large shrub patches during winter may enhance growth by insulating the soil and facilitating overwinter nutrient turnover, leading to increased summer plant growth and potentially widespread increases in shrub cover. To determine whether snow enhances growth of arctic plants, we examined the effect of 6 years of added snow on plant biomass allocation and growth of nine common vascular plant species collected in 2010 and 2011 on either side of snowfences established in 2005 across a gradient of shrub biomass and productivity near Toolik Lake, Alaska. The deciduous shrub Salix pulchra responded most positively to added snow showing an 88% increase in total ramet biomass, because increased secondary growth allowed plants to support more branches and leaves. Some graminoid species also showed growth increases, especially where they were more abundant and larger than nearby shrub species. Species sharing the same growth strategy (deciduous shrubs, evergreen shrubs and graminoids) responded differently to increased snow, with some showing increased growth, others showing decreased growth, and some showing no effects. We found that relative biomass per ramet in combination with growth strategy is a better predictor of species’ growth responses to added snow than growth strategy alone. Our research shows that a primary mechanism for enhanced growth of shrubs in response to deeper snow is increased secondary growth. Improved growth may be an important way in which arctic plants spread, since they are often long‐lived and clonal, and this may help explain the increase in shrub cover observed across northern latitudes. As shrub patches continue to expand and increase surrounding snow depths during winter, secondary growth of fast‐growing deciduous shrub species may play an important role in defining future landscape community composition and ecosystem processes. A plain language summary is available for this article.

Elevated rates of heterotrophic respiration in shrub-conditioned arctic tundra soils

The response of arctic ecosystems to global change will be critical for future climate, due to their vast stores of carbon. Climate warming appears to be linked to the expansion of shrubs across tundra, but it is unclear how shrubs affect the activity of soil microbes. We investigated three potential mechanisms by which shrubs may stimulate soil microbial activity, by 1) increasing the rate of litter inputs, 2) promoting soil microbial adaptation to litter and 3) reducing nutrient limitation. We created microcosms of root-free soils collected from shrub (Alnus fruticosa, Betula nana, and Salix pulchra) and non-shrub plots in arctic Alaska and conducted two experiments. We quantified heterotrophic soil respiration rates in response to litter inputs (experiment 1), or nutrient inputs (nitrogen, phosphorus, or both nutrients together, experiment 2). We found that shrub-conditioned soils maintained higher rates of soil respiration in both experiments. Shrub litter increased respiration in both soil types, but the relative response was greater in non-shrub soils. We found no evidence that shrubs reduce nutrient limitation to heterotrophic respiration, although we observed a short-term increase in respiration after phosphorus addition in both soil types. Collectively, our results suggest that higher rates of respiration in shrub-conditioned soils may be the result of higher litter input rates, but other factors such as organic matter quality or microbial community structure may also contribute to our observed differences in respiration.

Arctic plant ecophysiology and water source utilization in response to altered snow: isotopic (δ18O and δ2H) evidence for meltwater subsidies to deciduous shrubs

Warming-linked woody shrub expansion in the Arctic has critical consequences for ecosystem processes and climate feedbacks. The snow-shrub interaction model has been widely implicated in observed Arctic shrub increases, yet equivocal experimental results regarding nutrient-related components of this model have highlighted the need for a consideration of the increased meltwater predicted in expanding shrub stands. We used a 22-year snow manipulation experiment to simultaneously address the unexplored role of snow meltwater in arctic plant ecophysiology and nutrient-related components of the snow-shrub hypothesis. We coupled measurements of leaf-level gas exchange and leaf tissue chemistry (%N and δ13C) with an analysis of stable isotopes (δ18O and δ2H) in soil water, precipitation, and stem water. In deeper snow areas photosynthesis, conductance, and leaf N increased and δ13C values decreased in the deciduous shrubs, Betula nana and Salix pulchra, and the graminoid, Eriophorum vaginatum, with the strongest treatment effects observed in deciduous shrubs, consistent with predictions of the snow-shrub hypothesis. We also found that deciduous shrubs, especially S. pulchra, obtained much of their water from snow melt early in the growing season (40-50%), more than either E. vaginatum or the evergreen shrub, Rhododendron tomentosum (Ledum palustre). This result provides the basis for adding a meltwater-focused feedback loop to the snow-shrub interaction model of shrub expansion in the Arctic. Our results highlight the critical role of winter snow in the ecophysiology of Arctic plants, particularly deciduous shrubs, and underline the importance of understanding how global warming will affect the Arctic winter snowpack.

Contrasting shrub species respond to early summer temperatures leading to correspondence of shrub growth patterns

The Arctic-alpine biome is warming rapidly, resulting in a gradual replacement of low statured species by taller woody species in many tundra ecosystems. In northwest North America, the remotely sensed normalized difference vegetation index (NDVI), suggests an increase in productivity of the Arctic and alpine tundra and a decrease in productivity of boreal forests. However, the responses of contrasting shrub species growing at the same sites to climate drivers remain largely unexplored.Here, we test growth, climate, and NDVI relationships of two contrasting species: the expanding tall deciduous shrub Salix pulchra and the circumarctic evergreen dwarf shrub Cassiope tetragona from an alpine tundra site in the Pika valley in the Kluane Region, southwest Yukon Territories, Canada.We found that annual growth variability of both species at this site is strongly driven by early summer temperatures, despite their contrasting traits and habitats. Shrub growth chronologies for both species were correlated with the regional climate signal and showed spatial correspondence with interannual variation in NDVI in surrounding alpine and Arctic regions. Our results suggest that early summer warming represents a common driver of vegetation change for contrasting shrub species growing in different habitats in the same alpine environments.

Xanthophyll Cycle Activity in Two Prominent Arctic Shrub Species

ABSTRACTWhen the capacity for photosynthesis is constrained by unfavorable growing conditions, excess absorbed light is safely lost from leaves via thermal energy dissipation—a photoprotective mechanism ubiquitous among higher plants. The relatively low irradiance conditions yet stressful growing environment of the arctic tundra suggest contrasting hypotheses regarding the necessity for plant investment in photoprotection. To examine these hypotheses, the photoprotective pigments of the xanthophyll cycle were investigated in conjunction with non-photochemical quenching (NPQ) of chlorophyll fluorescence emission in two dominant arctic shrub species, Salix pulchra and Betula nana. The xanthophyll cycle pool sizes of S. pulchra leaves were substantially higher than those reported in most other higher plant species, whereas B. nana leaves maintain modestly high xanthophyll cycle pool sizes. In addition, high retention of de-epoxidized xanthophyll cycle pigments in both species and saturation of xanthophyll cycle conversion at low-light intensities were observed and associated with high levels of NPQ. The xanthophyll cycle leaf pigment pools reported are among the first published for arctic plants and support the hypothesis that foliar xanthophyll cycle activity is greater in environments prone to harsher growing conditions.

Arctic shrub growth trajectories differ across soil moisture levels.

The circumpolar expansion of woody deciduous shrubs in arctic tundra alters key ecosystem properties including carbon balance and hydrology. However, landscape-scale patterns and drivers of shrub expansion remain poorly understood, inhibiting accurate incorporation of shrub effects into climate models. Here, we use dendroecology to elucidate the role of soil moisture in modifying the relationship between climate and growth for a dominant deciduous shrub, Salix pulchra, on the North Slope of Alaska, USA. We improve upon previous modeling approaches by using ecological theory to guide model selection for the relationship between climate and shrub growth. Finally, we present novel dendroecology-based estimates of shrub biomass change under a future climate regime, made possible by recently developed shrub allometry models. We find that S.pulchra growth has responded positively to mean June temperature over the past 2.5 decades at both a dry upland tundra site and an adjacent mesic riparian site. For the upland site, including a negative second-order term in the climate-growth model significantly improved explanatory power, matching theoretical predictions of diminishing growth returns to increasing temperature. A first-order linear model fit best at the riparian site, indicating consistent growth increases in response to sustained warming, possibly due to lack of temperature-induced moisture limitation in mesic habitats. These contrasting results indicate that S.pulchra in mesic habitats may respond positively to a wider range of temperature increase than S.pulchra in dry habitats. Lastly, we estimate that a 2°C increase in current mean June temperature will yield a 19% increase in aboveground S.pulchra biomass at the upland site and a 36% increase at the riparian site. Our method of biomass estimation provides an important link toward incorporating dendroecology data into coupled vegetation and climate models.

Растительность акватории и побережий озера Таловское и его окрестностей (Корякский округ, Камчатский край)

Растительность акватории и побережий озера Таловское и его окрестностей (Корякский округ, Камчатский край)

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

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

LiDAR canopy radiation model reveals patterns of photosynthetic partitioning in an Arctic shrub

Abstract Characterizing the wide range of light availability and photosynthetic properties throughout a plant canopy is important for modeling the exchanges of carbon, water, and nutrients between the biosphere and the atmosphere. Such characterization could be especially important in one of the world's most rapidly changing biomes – the Arctic tundra – where further warming-induced increases in the size, abundance and complexity of small arctic shrubs are projected. An improved understanding of their canopy organization could provide insights into associated climate feedbacks since multifaceted interactions between 3D canopy structure, environmental conditions, leaf physiology, and light availability affect the potentials and limitations of vegetation carbon assimilation and storage. The aim of this study was to explore new methods to elucidate evidence for photosynthetic partitioning according to light availability within a short canopy ( Salix pulchra exposed to near continuous sunlight at low solar angles in the Arctic tundra. Instantaneous photosynthetic photon flux density (PPFD) and daily integrated quantum flux density ( Q int ) were modeled from a ray-tracing algorithm for voxels (edge-length .01 m) within the canopy that were each assigned a physically based directional gap fraction (DGF) and extinction coefficient ( k ) from which each voxel's effective leaf area index (LAI e ) was calculated. Voxel parameters for the ray-tracing model were derived from the x , y , and z locations of high spatial resolution ( Q int and path-length (determined as the accumulated photon travel distance from the canopy-edge) were compared with two variables derived from traditional light quantification techniques – ceptometer derived leaf-area index (LAI), and manually measured vertical canopy depth. Insignificant relationships were observed between traditional measurements of light environment (LAI and vertical canopy depth) and TLS derived measurements ( Q int and path length), suggesting wide variability among these methods. When each of four light quantification variables were compared against leaf-level variables classically associated with photosynthetic partitioning (percent nitrogen (N%), chlorophyll a to b ratio (Chl a / b ), and photosynthetic capacity ( A max )), patterns suggesting photosynthetic partitioning emerged only when the LiDAR-derived 3D locations of the leaf samples were considered ( Q int and path length). Statistically significant ( p Q int ( r = 0.31, 0.46, 0.49 for N%, Chl a / b , and A max , respectively), while 2 of 3 showed a statistically significant response to LiDAR derived path length. Only 1 of 3 photosynthetic variables showed a statistically significant response to manually measured canopy depth or ceptometer derived LAI. Results from this study suggest that LiDAR-based techniques for quantifying the 3D light environment of small shrubs exposed to low solar angles reveal patterns of photosynthetic partitioning that may otherwise be overlooked using more traditional techniques.

Coupled long-term summer warming and deeper snow alters species composition and stimulates gross primary productivity in tussock tundra.

Climate change is expected to increase summer temperature and winter precipitation throughout the Arctic. The long-term implications of these changes for plant species composition, plant function, and ecosystem processes are difficult to predict. We report on the influence of enhanced snow depth and warmer summer temperature following 20 years of an ITEX experimental manipulation at Toolik Lake, Alaska. Winter snow depth was increased using snow fences and warming was accomplished during summer using passive open-top chambers. One of the most important consequences of these experimental treatments was an increase in active layer depth and rate of thaw, which has led to deeper drainage and lower soil moisture content. Vegetation concomitantly shifted from a relatively wet system with high cover of the sedge Eriophorum vaginatum to a drier system, dominated by deciduous shrubs including Betula nana and Salix pulchra. At the individual plant level, we observed higher leaf nitrogen concentration associated with warmer temperatures and increased snow in S. pulchra and B. nana, but high leaf nitrogen concentration did not lead to higher rates of net photosynthesis. At the ecosystem level, we observed higher GPP and NEE in response to summer warming. Our results suggest that deeper snow has a cascading set of biophysical consequences that include a deeper active layer that leads to altered species composition, greater leaf nitrogen concentration, and higher ecosystem-level carbon uptake.

Vegetation composition and shrub extent on the Yukon coast, Canada, are strongly linked to ice-wedge polygon degradation

Changing environmental and geomorphological conditions are resulting in vegetation change in ice-wedge polygons in Arctic tundra. However, we do not yet know how microscale vegetation patterns relate to individual environmental and geomorphological parameters. This work aims at examining these relations in polygonal terrain. We analysed composition and cover of vascular plant taxa and surface height, active layer depth, soil temperature, carbon and nitrogen content, pH and electrical conductivity in four polygon mires located on the Yukon coast. We found that vascular plant species composition and cover correlates best with relative surface height. Ridges of low-centred polygons and raised centres of high-centred polygons support the growth of mesic and wetland species (e.g., Betula glandulosa , Salix pulchra , S. reticulata , Rubus chamaemorus , various ericaceous dwarf shrubs, Eriophorum vaginatum , Poa arctica ). Wetland and aquatic plant species (e.g., E. angustifolium , Carex aquatilis , C. chordorrhiza , Pedicularis sudetica ) grow in low-lying centres of polygons and in troughs between polygons. We also found a relationship between vascular plant species composition and substrate characteristics such as pH, electrical conductivity and total organic carbon, although the individual influence of these parameters could not be determined because of their correlation with relative surface height. Our findings stress the regulatory role of microtopography and substrate in vegetation dynamics of polygonal terrain. Ongoing warming in this region will lead to changes to polygonal terrain through permafrost degradation and subsequent conversion of low-centred into high-centred polygons. Our results indicate that shrubs, particularly Betula glandulosa and heath species, have the potential to expand most. Keywords: Ice-wedge polygon mires; western Canadian Arctic; modern vegetation; microtopography; permafrost. (Published: 17 June 2016) To access the supplementary material for this article, please see the supplementary files in the column to the right (under Article Tools). Citation: Polar Research 2016, 35 , 27489, http://dx.doi.org/10.3402/polar.v35.27489

Plant phenological responses to a long-term experimental extension of growing season and soil warming in the tussock tundra of Alaska.

Climate warming is strongly altering the timing of season initiation and season length in the Arctic. Phenological activities are among the most sensitive plant responses to climate change and have important effects at all levels within the ecosystem. We tested the effects of two experimental treatments, extended growing season via snow removal and extended growing season combined with soil warming, on plant phenology in tussock tundra in Alaska from 1995 through 2003. We specifically monitored the responses of eight species, representing four growth forms: (i) graminoids (Carex bigellowii and Eriophorum vaginatum); (ii) evergreen shrubs (Ledum palustre, Cassiope tetragona, and Vaccinium vitis-idaea); (iii) deciduous shrubs (Betula nana and Salix pulchra); and (iv) forbs (Polygonum bistorta). Our study answered three questions: (i) Do experimental treatments affect the timing of leaf bud break, flowering, and leaf senescence? (ii) Are responses to treatments species-specific and growth form-specific? and (iii) Which environmental factors best predict timing of phenophases? Treatment significantly affected the timing of all three phenophases, although the two experimental treatments did not differ from each other. While phenological events began earlier in the experimental plots relative to the controls, duration of phenophases did not increase. The evergreen shrub, Cassiope tetragona, did not respond to either experimental treatment. While the other species did respond to experimental treatments, the total active period for these species did not increase relative to the control. Air temperature was consistently the best predictor of phenology. Our results imply that some evergreen shrubs (i.e., C.tetragona) will not capitalize on earlier favorable growing conditions, putting them at a competitive disadvantage relative to phenotypically plastic deciduous shrubs. Our findings also suggest that an early onset of the growing season as a result of decreased snow cover will not necessarily result in greater tundra productivity.

Tall Deciduous Shrubs Offset Delayed Start of Growing Season Through Rapid Leaf Development in the Alaskan Arctic Tundra

Increasing temperatures in arctic regions are causing earlier spring snowmelt, leading to earlier plant emergence, which could lengthen the period of carbon uptake. Warming is also leading to a shift from graminoid to deciduous shrub-dominated tundra, and in many areas deciduous shrubs are becoming taller. As taller shrubs become increasingly dominant, arctic landscapes may retain more snow, which could lengthen spring snow cover duration and offset advances in the start of the growing season that are expected as a result of earlier spring snowmelt. As a consequence, deeper snow and later snowmelt in taller shrub tundra could delay plant emergence and shorten the period of carbon uptake. This study tracked leaf development of two abundant deciduous shrubs, Betula nana and Salix pulchra, and compared individuals along a natural shrub height gradient on the North Slope of Alaska. We measured spring snow depth and snow cover duration, bud and developing leaf nitrogen content, as well as the timing of budburst and leaf expansion. Taller deciduous shrubs in shrub-dominated communities had deeper snow surrounding them, and became snow-free 1 to 6 days later, delaying budburst by 2 to 12 days relative to shorter deciduous shrubs in graminoid-dominated communities. However, leaf development of tall shrubs caught up to that of short shrubs; occasionally, tall shrubs reached full leaf expansion 1 to 4 days before short shrubs, indicating more rapid leaf development. This convergence in the timing of later leaf development stages is potentially enabled by approximately 16% to 25% greater nitrogen in buds and developing leaves of taller shrubs compared with shorter shrubs. Our findings suggest that delayed snowmelt in areas dominated by taller shrubs may have a short-lived impact on the timing of leaf development, likely resulting in no difference in duration of peak photosynthetic period between tall and short stature shrubs.