Tundra Vegetation Research Articles

Landscape-scale characterization of Arctic tundra vegetation composition, structure, and function with a multi-sensor unoccupied aerial system

The Arctic is experiencing some of the most rapid climate change on Earth, with strong impacts on tundra ecosystems that are characterized by high land-surface and vegetation heterogeneity. Previous studies have explored this complexity using satellite remote sensing, however these typically coarse spatial resolution data have generally missed sub-pixel heterogeneity, leaving critical gaps in our understanding of tundra vegetation dynamics from the community to landscape scales. To address these gaps, we collected very high-resolution (1–5 cm) optical, structural, and thermal data at three low-Arctic tundra sites on the Seward Peninsula, Alaska, using a multi-sensor unoccupied aerial system (UAS). We examined the application of these data to studying tundra vegetation dynamics, by quantifying (a) canopy height and thermoregulation (leaf–air temperature) of representative plant functional types (PFTs), (b) fine-scale patterns of vegetation composition across landscapes, and (c) impacts of fine-scale vegetation composition on landscape-scale variation of canopy height and thermoregulation. Our results show that deciduous tall shrubs (those that can potentially grow >2 m) had a strong cooling effect, with canopy temperatures significantly lower than local air temperatures and other PFTs. Increased cover of tall shrubs also had the potential to reduce the cover of low-stature PFTs across the landscape, potentially associated with their closed canopy (i.e. increased light competition) and strong thermoregulation. To understand the connections between fine-scale vegetation composition and large-scale ecosystem processes, we produced a random forest model which showed that fine-scale PFT composition accounted for 86.8% and 74.2% of the landscape-scale variation in canopy height and thermoregulation, respectively. These findings highlight the importance of spatially detailed characterization of tundra PFTs to improve our ecological understanding and model representation of tundra vegetation, also transcend our study to show the need for continued collection of similar datasets to better understand the impacts of surface heterogeneity on the mapping and modeling of tundra ecosystem dynamics, as well as assist with conservation management and biodiversity monitoring strategies.

Non-growing season plant nutrient uptake controls Arctic tundra vegetation composition under future climate

Plant growth and distribution in high-latitude tundra ecosystems is strongly limited by nutrient availability and is critical for quantifying centennial-scale carbon-climate interactions. However, land model representations of plant–nutrient interactions are uncertain, leading to poor comparisons with high-latitude observations. Although it has been recognized for decades in the observational community that plants continue to acquire nutrients well past when aboveground activity has ceased, most large-scale land models ignore this process. Here we address the role tundra plant nutrient acquisition during the non-growing season (NGS) has on centennial-scale vegetation growth and dynamics, with a focus on shrub expansion. We apply a well-tested mechanistic model of coupled plant, microbial, hydrological, and thermal dynamics that explicitly represents nutrient acquisition based on plant and microbial traits, thereby allowing a prognostic assessment of NGS nutrient uptake. We first show that the model accurately represents observed seasonality of NGS plant nutrient uptake in a northern Alaskan tundra site. Applying the model across the North America tundra indicates that NGS nutrient uptake is consistent with observations and ranges between ∼5% and 50% of annual uptake, with large spatial variability and dependence on plant functional type. We show that NGS plant nutrient acquisition strongly enhances modeled 21st century tundra shrub growth and expansion rates. Our results suggest that without NGS nutrient uptake, total shrub aboveground dominance would be ∼50% lower, limited primarily by their inability to grow tall enough to maximize their inherent capacity for light competition. Evergreen shrubs would be more strongly affected because of their relatively lower capacity for nutrient remobilization and acquisition compared to deciduous shrubs. Our results highlight the importance of NGS plant and soil processes on high-latitude biogeochemistry and vegetation dynamics and motivate new observations and model structures to represent these dynamics.

Predicting Soil Respiration from Plant Productivity (NDVI) in a Sub-Arctic Tundra Ecosystem

Soils represent the largest store of carbon in the biosphere with soils at high latitudes containing twice as much carbon (C) than the atmosphere. High latitude tundra vegetation communities show increases in the relative abundance and cover of deciduous shrubs which may influence net ecosystem exchange of CO2 from this C-rich ecosystem. Monitoring soil respiration (Rs) as a crucial component of the ecosystem carbon balance at regional scales is difficult given the remoteness of these ecosystems and the intensiveness of measurements that is required. Here we use direct measurements of Rs from contrasting tundra plant communities combined with direct measurements of aboveground plant productivity via Normalised Difference Vegetation Index (NDVI) to predict soil respiration across four key vegetation communities in a tundra ecosystem. Soil respiration exhibited a nonlinear relationship with NDVI (y = 0.202e3.508 x, p < 0.001). Our results further suggest that NDVI and soil temperature can help predict Rs if vegetation type is taken into consideration. We observed, however, that NDVI is not a relevant explanatory variable in the estimation of SOC in a single-study analysis.

Seasonal cryogenic processes control supra-permafrost pore water chemistry in two contrasting Cryosols

Over the last decades, Arctic landscapes have experienced intense warming leading to permafrost degradation and rapid ecosystem changes. Active layer thickening, widespread melting of ground ice and thermo-erosion have affected the mobilization of organic and mineral elements. While the carbon and nitrogen cycles are intensively studied, the soil weathering has been less documented. In the present study, we monitored the chemistry of soil capillary and gravitational pore waters, rainfall and stream waters daily during the growing season in two experimental sites under tussock tundra vegetation in the low-Arctic region, in Salluit (Nunavik, Canada). We aimed to investigate the seasonal thaw controls on the evolution of concentrations of major organic and inorganic elements in the active layer (i.e., seasonally thawed surface layers) of two permafrost soils (Cryosols) differing in parental materials: an ombrotrophic bog (i.e., Histic Cryosol) and post-glacial marine sediments continuously waterlogged (i.e., Turbic Cryosol). In the Histic Cryosol, the electrical conductivity was <100 µS cm−1 and Cl− and Na+ were the dominant soluble ions originating from atmospheric depositions. In the Turbic Cryosol, decarbonated in the first 40 cm, Ca2+ and Mg2+ were the dominant soluble ions in the capillary water reflecting the dissolution of soil minerals, while Cl− and SO42− dominated in gravitational water, illustrating inputs from uphill. In the two soils, Ca2+ and Mg2+ concentrations as well as Mg/Na and Ca/Na increased with depth. Along summer, the soil pore water chemistry evolved with thaw front and water table depths in the two sites. Particularly in the Histic Cryosol, electrical conductivity, solute concentrations, Mg/Na and Ca/Na ratios increased with the thaw front deepening. Our observations suggest that the active layer thickening and increasing supra-permafrost flow contribution, expected to increase with Arctic warming, could lead to a shift in chemistry of pore waters in organic and mineral permafrost soils, differently depending on permafrost landform settings.

Growing season leaf carbon:nitrogen dynamics in Arctic tundra vegetation from ground and Sentinel-2 observations reveal reallocation timing and upscaling potential

Plant nitrogen (N) use is an essential component of the N cycle in Arctic terrestrial ecosystems, and important processes include plant N uptake and reallocation during the growing season. While the availability of N to deciduous tundra plants in part relies on their internal reallocation of N from leaves to stems and roots during autumn senescence, the species-specific importance of reallocation timing and its community-wide implications on landscape- and regional-scales remains not well known. Here, we quantified leaf N contents and C:N ratios of four widespread shrub species in West Greenland from June through October and compared plot observations to landscape scale based on a new Sentinel-2-derived index. Our Sentinel-2 index captures overall N reallocation trends well across time and space at the plot level (R2 = 0.81, p < 0.001). Using this novel approach is therefore relevant for upscaling of current and future changes in plant C:N dynamics. Using satellite data, we estimate the leaf N mass reallocated during senescence equaled about 0.8 g N m−2 from leaves to stems and roots. We conclude that (1) in-situ data from the entire growing season is critical to quantify timing and contrasting strategies of N allocation at species level, (2) key species such as Salix glauca are capable of halving their leaf N content during senescence, (3) Sentinel-2 (S-2) satellite data is a strong candidate for quantifying these plant functional dynamics in space and time. Our study has implications for research on competition among Arctic plants, litter decomposition and consequently carbon accumulation in tundra soils from a climate change perspective. Further, it captures plant functional dynamics during critical parts of the growing season, including autumn senescence which is generally more complex than capturing spring greenup. Lastly, our new index has important implications for remotely mapping temporal and spatial variation in substrate quality for both wild and domesticated herbivores, as it is a first step towards a tool to assess the tundra vegetation and fodder quality for cold region animal husbandry.

Long-term warming manipulations reveal complex decomposition responses across different tundra vegetation types

In a rapidly warming tundra, ecosystems will undergo major environmental changes that are predicted to significantly alter belowground processes such as decomposition of plant litter. Making use of International Tundra Experiment sites (ITEX), which were established approximately two decades ago, we examined the long-term impacts of warming on decomposition. We used the Tea Bag Index (TBI) methodology to measure the annual mass loss (%) of two tea types as a proxy for potential decomposition rates, across five tundra vegetation types. Direct effects of warming were assessed by comparing mass loss within and outside warming manipulations. Indirect effects of warming, such as those caused by warming-induced changes in plant community composition, were assessed through the relationship between mass loss of tea and local biotic and abiotic conditions. We found positive effects of warming on decomposition, although the responses varied between vegetation and tea types. Interestingly, we found support for the indirect influence of long-term warming on decomposition through warming-induced changes in the composition of plant communities. Our findings demonstrate the complexity in decomposition responses to warming across different vegetation types and highlight the importance of long-term legacies of warming in decomposition responses across the Arctic.

Climate drivers of Arctic tundra variability and change using an indicators framework

This study applies an indicators framework to investigate climate drivers of tundra vegetation trends and variability over the 1982–2019 period. Previously known indicators relevant for tundra productivity (summer warmth index (SWI), coastal spring sea-ice (SI) area, coastal summer open-water (OW)) and three additional indicators (continentality, summer precipitation, and the Arctic Dipole (AD): second mode of sea level pressure variability) are analyzed with maximum annual Normalized Difference Vegetation Index (MaxNDVI) and the sum of summer bi-weekly (time-integrated) NDVI (TI-NDVI) from the Advanced Very High Resolution Radiometer time-series. Climatological mean, trends, and correlations between variables are presented. Changes in SI continue to drive variations in the other indicators. As spring SI has decreased, summer OW, summer warmth, MaxNDVI, and TI-NDVI have increased. However, the initial very strong upward trends in previous studies for MaxNDVI and TI-NDVI are weakening and becoming spatially and temporally more variable as the ice retreats from the coastal areas. TI-NDVI has declined over the last decade particularly over High Arctic regions and southwest Alaska. The continentality index (CI) (maximum minus minimum monthly temperatures) is decreasing across the tundra, more so over North America than Eurasia. The relationship has weakened between SI and SWI and TI-NDVI, as the maritime influence of OW has increased along with total precipitation. The winter AD is correlated in Eurasia with spring SI, summer OW, MaxNDVI, TI-NDVI, the CI and total summer precipitation. This winter connection to tundra emphasizes the role of SI in driving the summer indicators. The winter (DJF) AD drives SI variations which in turn shape summer OW, the atmospheric SWI and NDVI anomalies. The winter and spring indicators represent potential predictors of tundra vegetation productivity a season or two in advance of the growing season.

Изменение кормовых запасов оленьих пастбищ в южных субарктических тундрах Ямала

Abstract. The relevance of the research is related to the need to monitor the resource potential of the vegetation in the southern subarctic tundra. The Yamal tundra has been experiencing a high grazing pressure since 1990. As a result of the increase in the number of private farms and the number of domesticated reindeer in Yamal, pasture degradation occurs (trampling of forage mass, shredding vegetation, disappearance of lichen cover, increased soil deflation). Purpose of the research is compare the forage stocks of reindeer pastures in the southern subarctic tundra of Yamal in 2017–2018 and in the 1990s. In the course of the study, estimates of the amount and structure of forage stocks in plant communities of the southern subarctic tundra of the Yamal Peninsula was assessed in the vicinity of the Payutayakha river. The stock of aboveground phytomass was determined by the mowing method. In 2017, degraded areas of the tundra were studied, and in 2018 undisturbed and intact areas were investigated. Scientific novelty. The data obtained are compared with the estimates published for the southern subarctic tundra in the 1990s. Results. Forage stocks in degraded areas, compared to stocks in the southern subarctic tundra in the 1990s, decreased 14 times; while in on undisturbed areas they decreased 3 times. In the structure of forage stocks in 2017–2018, compared to the situation at the end of the 20th century, the proportions of shrubs, lichens and grasses has decreased; the proportion of dwarf shrubs in degraded areas increased 4 times, while in undisturbed areas it increased 6 times. The study confirms the existence of overgrazing and a shortage of forage resources on pastures in the zone of the southern subarctic tundra of Yamal.

Dietary paleoecology of bison and horses on the mammoth steppe of eastern Beringia based on dental microwear and mesowear analyses

In contrast to the modern Arctic, high-latitude ecosystems of the Late Pleistocene supported a diverse range of large mammalian herbivores, including abundant bison (Bison priscus) and horses (Equus sp.). This ‘mammoth steppe’ biome has no extant analog and modern tundra vegetation is likely incapable of supporting such a high density of large mammals. Compared to modern Arctic ecosystems, higher diversity and biomass of Late Pleistocene large mammal populations may have been sustained by more nutritious forage and/or dietary niche partitioning. We used dental microwear texture analysis and dental mesowear analysis of bison and horses (Bison priscus, Equus sp.) to characterize diet and assess the degree to which dietary differences supported co-existence of these dominant and likely competing herbivores on the mammoth steppe. Additionally, we compared microwear and mesowear of Late Pleistocene specimens to modern Alaska bison (reintroduced Bison bison athabascae and introduced Bison bison bison) and published microwear and mesowear data for extant bovids and equids. Our results demonstrate that Late Pleistocene bison and horses had less abrasive diets than modern obligate grazers, suggesting that these “grazers” of the mammoth steppe likely incorporated more forbs in their diets than modern grazers. Furthermore, Late Pleistocene bison and horses ate foods with similar textures, indicating that dietary niche partitioning alone cannot explain their co-occurrence. However, taphonomic differences between bison and horse specimens indicate potential spatial or temporal niche partitioning during the Late Pleistocene.

Phenological stage of tundra vegetation controls bidirectional exchange of BVOCs in a climate change experiment on a subarctic heath.

Traditionally, biogenic volatile organic compound (BVOC) emissions are often considered a unidirectional flux, from the ecosystem to the atmosphere, but recent studies clearly show the potential for bidirectional exchange. Here we aimed to investigate how warming and leaf litter addition affect the bidirectional exchange (flux) of BVOCs in a long‐term field experiment in the Subarctic. We also assessed changes in net BVOC fluxes in relation to the time of day and the influence of different plant phenological stages. The study was conducted in a full factorial experiment with open top chamber warming and annual litter addition treatments in a tundra heath in Abisko, Northern Sweden. After 18 years of treatments, ecosystem‐level net BVOC fluxes were measured in the experimental plots using proton‐transfer‐reaction time‐of‐flight mass spectrometry (PTR–ToF–MS). The warming treatment increased monoterpene and isoprene emissions by ≈50%. Increasing temperature, due to diurnal variations, can both increase BVOC emission and simultaneously, increase ecosystem uptake. For any given treatment, monoterpene, isoprene, and acetone emissions also increased with increasing ambient air temperatures caused by diurnal variability. Acetaldehyde, methanol, and sesquiterpenes decreased likely due to a deposition flux. For litter addition, only a significant indirect effect on isoprene and monoterpene fluxes (decrease by ~50%–75%) was observed. Litter addition may change soil moisture conditions, leading to changes in plant species composition and biomass, which could subsequently result in changes to BVOC emission compositions. Phenological stages significantly affected fluxes of methanol, isoprene and monoterpenes. We suggest that plant phenological stages differ in impacts on BVOC net emissions, but ambient air temperature and photosynthetically active radiation (PAR) also interact and influence BVOC net emissions differently. Our results may also suggest that BVOC fluxes are not only a response to changes in temperature and light intensity, as the circadian clock also affects emission rates.

Increasing shrub damage by invertebrate herbivores in the warming and drying tundra of West Greenland.

Rapid warming is predicted to increase insect herbivory across the tundra biome, yet how this will impact the community and ecosystem dynamics remains poorly understood. Increasing background invertebrate herbivory could impede Arctic greening, by serving as a top-down control on tundra vegetation. Many tundra ecosystems are also susceptible to severe insect herbivory outbreaks which can have lasting effects on vegetation communities. To explore how tundra-insect herbivore systems respond to warming, we measured shrub traits and foliar herbivory damage at 16 sites along a landscape gradient in western Greenland. Here we show that shrub foliar insect herbivory damage on two dominant deciduous shrubs, Salix glauca and Betula nana, was positively correlated with increasing temperatures throughout the first half of the 2017 growing season. We found that the majority of insect herbivory damage occurred in July, which was outside the period of rapid leaf expansion that occurred throughout most of June. Defoliators caused the most foliar damage in both shrub species. Additionally, insect herbivores removed a larger proportion of B. nana leaf biomass in warmer sites, which is due to a combination of increased foliar herbivory with a coinciding decline in foliar biomass. These results suggest that the effects of rising temperatures on both insect herbivores and host species are important to consider when predicting the trajectory of Arctic tundra shrub expansion.

Communities of Festuca komarovii – endemic of cryo-arid Pleistocene vegetation of the Eastern Sayan

The identity of the highland vegetation of the eastern part of the ridge East Sayan is due to the history of the formation of flora and vegetation in the late Pleistocene, as well as to the features of underlying rocks, mainly represented by limestones. As a result, endemic plant species and communities formed on these ancient alignment surfaces. One of these species is Festuca komarovii Krivot. (Komarov fescue). The relictness of Festuca komarovii is confirmed by the systematic position of the species in the generic complex; it belongs to the distinctive section Leocopoa (Griseb.) Krivot. The section combines only three species – Festuca sibirica, F. hubsugulica, F. komarovii, of which the last two are endemic of the Hangai-Sayan center. The Komarov fescue in the modern vegetation of the East Sayan shows itself as a coenogen and indicator of paleocryogenic landscapes. The research results showed a high coenotic role of this fescue in the vegetation of the studied territory, in particular the syntaxa of the cryophyte-steppe florocenotype (unions Kobresion myosuroidis Mirkin et al. 1983 em. Hilbig 2000 and Potentillo niveae–Caricion pediformisТеlyatnikov 2011). We also described the new association Ptilagrosto mongholicae–Festucetum komarovii Namzalov ass. hoc loco, floristically and coenotically related to the associations of the union Dryadion oxyodontae Zhitlukhina et Onishchenko ex Chytrý et al. 1993, which belongs to the class Carici rupestris–Kobresietea bellardii Ohba 1974. However, one important feature should be noted that the Komarov fescue in communities of associations of tundra and subalpine forest vegetation is noted in a small abundance as an accompanying species. To some extent, this confirms the initial coenogenetic proximity of Festuca komarovii to grass ecosystems – cryophytic stepoids of the periglacial Pleistocene complex.

Biophysical controls of increased tundra productivity in the western Canadian Arctic

Rapid climate warming has widely been considered as the main driver of recent increases in Arctic tundra productivity. Field observations and remote sensing both show that tundra “greening” has been widespread, but heterogeneity in regional and landscape-scale trends suggest that additional controls are mediating the response of tundra vegetation to warming. In this study, we examined the relationship between changes in vegetation productivity in the western Canadian Arctic and biophysical variables by analyzing trends in the Enhanced Vegetation Index (EVI) obtained from nonparametric regression of annual Landsat surface reflectance composites. We used Random Forests classification and regression tree modelling to predict the trajectory and magnitude of greening from 1984 to 2016 and identify biophysical controls. More than two-thirds of our study area showed statistically significant increases in vegetation productivity, but observed changes were heterogeneous, occurring most rapidly within areas of the Southern Arctic that were: (1) dominated by dwarf and upright shrub cover types, (2) moderately sloping, and (3) located at lower elevation. These findings suggest that the response of tundra vegetation to warming is mediated by regional- and landscape-scale variation in microclimate, topography and soil moisture, and physiological differences among plant functional groups. Our work highlights the potential of the joint analysis of annual remotely sensed vegetation indices and broad-scale biophysical data to understand spatial variation in tundra vegetation change.

Clast rind-paleosol record of the Antarctic early Alpine glaciation

The early Alpine glaciation is known from few sources detailing initial growth of ice either during the late Oligocene or early Miocene, localities limited to high summits of East Antarctica. The Ant-831 paleosol, developed in moraine emplaced during the Alpine event, has been studied with respect to its pedogenesis and to rinds recovered from its pebble pavement. This pedostratigraphic section is assigned a late Oligocene/early Miocene age on the basis of its fossil Coleoptera fauna and tundra vegetation, Fe/Al relative-age-dating parameters, and extensive pebble clast weathering. SEM, XRD and XRF analyses of rind specimens from the pebble pavement yield new information on the weathering history of the Beacon sandstone and its link to a juxtaposed paleosol. The paleosol provides a mirror image to weathering zones in pavement clasts, the salt-rich zones interpreted to have formed during the cold-dry paleoclimate of the post-Middle Miocene Climatic Optimum (MMCO ~15 Ma) following a stronger oxidizing paleoclimate of the earlier cold-wet phase of the Alpine glaciation. Essentially this database provides a 30-Myr window into the weathering progression from inception of the Alpine glaciation, through the warmer MMCO dated to ~15 Ma, to the onset of cold-dry conditions that brought on the later development of the Inland Ice Sheet.

An Object-Based Approach for Mapping Tundra Ice-Wedge Polygon Troughs from Very High Spatial Resolution Optical Satellite Imagery

Very high spatial resolution commercial satellite imagery can inform observation, mapping, and documentation of micro-topographic transitions across large tundra regions. The bridging of fine-scale field studies with pan-Arctic system assessments has until now been constrained by a lack of overlap in spatial resolution and geographical coverage. This likely introduced biases in climate impacts on, and feedback from the Arctic region to the global climate system. The central objective of this exploratory study is to develop an object-based image analysis workflow to automatically extract ice-wedge polygon troughs from very high spatial resolution commercial satellite imagery. We employed a systematic experiment to understand the degree of interoperability of knowledge-based workflows across distinct tundra vegetation units—sedge tundra and tussock tundra—focusing on the same semantic class. In our multi-scale trough modelling workflow, we coupled mathematical morphological filtering with a segmentation process to enhance the quality of image object candidates and classification accuracies. Employment of the master ruleset on sedge tundra reported classification accuracies of correctness of 0.99, completeness of 0.87, and F1 score of 0.92. When the master ruleset was applied to tussock tundra without any adaptations, classification accuracies remained promising while reporting correctness of 0.87, completeness of 0.77, and an F1 score of 0.81. Overall, results suggest that the object-based image analysis-based trough modelling workflow exhibits substantial interoperability across the terrain while producing promising classification accuracies. From an Arctic earth science perspective, the mapped troughs combined with the ArcticDEM can allow hydrological assessments of lateral connectivity of the rapidly changing Arctic tundra landscape, and repeated mapping can allow us to track fine-scale changes across large regions and that has potentially major implications on larger riverine systems.

The relationships of present vegetation, bacteria, and soil properties with soil organic matter characteristics in moist acidic tundra in Alaska

Soil organic matter (SOM) is related to vegetation, soil bacteria, and soil properties; however, not many studies link all these parameters simultaneously, particularly in tundra ecosystems vulnerable to climate change. Our aim was to describe the relationships between vegetation, bacteria, soil properties, and SOM composition in moist acidic tundra by integrating physical, chemical, and molecular methods. A total of 70 soil samples were collected at two different depths from 36 spots systematically arranged over an area of about 300 m × 50 m. Pyrolysis-gas chromatography/mass spectrometry and pyrosequencing of the 16S rRNA gene were used to identify the molecular compositions of the SOM and bacterial community, respectively. Vegetation and soil physicochemical properties were also measured. The sampling sites were grouped into three, based on their SOM compositions: Sphagnum moss-derived SOM, lipid-rich materials, and aromatic-rich materials. Our results show that SOM composition is spatially structured and linked to microtopography; however, the vegetation, soil properties, and bacterial community composition did not show overall spatial structuring. Simultaneously, soil properties and bacterial community composition were the main factors explaining SOM compositional variation, while vegetation had a residual effect. Verrucomicrobia and Acidobacteria were related to polysaccharides, and Chloroflexi was linked to aromatic compounds. These relationships were consistent across different hierarchical levels. Our results suggest that SOM composition at a local scale is closely linked with soil factors and the bacterial community. Comprehensive observation of ecosystem components is recommended to understand the in-situ function of bacteria and the fate of SOM in the moist acidic tundra.

Winter snow and spring temperature have differential effects on vegetation phenology and productivity across Arctic plant communities.

Tundra dominates two-thirds of the unglaciated, terrestrial Arctic. Although this region has experienced rapid and widespread changes in vegetation phenology and productivity over the last several decades, the specific climatic drivers responsible for this change remain poorly understood. Here we quantified the effect of winter snowpack and early spring temperature conditions on growing season vegetation phenology (timing of the start, peak, and end of the growing season) and productivity of the dominant tundra vegetation communities of Arctic Alaska. We used daily remotely sensed normalized difference vegetation index (NDVI), and daily snowpack and temperature variables produced by SnowModel and MicroMet, coupled physically based snow and meteorological modeling tools, to (1) determine the most important snowpack and thermal controls on tundra vegetation phenology and productivity and (2) describe the direction of these relationships within each vegetation community. Our results show that soil temperature under the snowpack, snowmelt timing, and air temperature following snowmelt are the most important drivers of growing season timing and productivity among Arctic vegetation communities. Air temperature after snowmelt was the most important control on timing of season start and end, with warmer conditions contributing to earlier phenology in all vegetation communities. In contrast, the controls on the timing of peak season and productivity also included snowmelt timing and soil temperature under the snowpack, dictated in part by the snow insulating capacity. The results of this novel analysis suggest that while future warming effects on phenology may be consistent across communities of the tundra biome, warming may result in divergent, community-specific productivity responses if coupled with reduced snow insulating capacity lowers winter soil temperature and potential nutrient cycling in the soil.

Does fire always accelerate shrub expansion in Arctic tundra? Examining a novel grass-dominated successional trajectory on the Seward Peninsula

ABSTRACT Over the last century in the circumpolar north, notable terrestrial ecosystem changes include shrub expansion and an intensifying wildfire regime. Shrub invasion into tundra may be further accelerated by wildfire disturbance, which creates opportunities for establishment where recruitment is otherwise rare. The Seward Peninsula currently experiences more frequent and larger fires than other tundra regions in Alaska. There are areas of overlapping burn scars dating back to the 1950s. Using a chronosequence approach, we examined vegetation and ecosystem dynamics in tussock tundra. Increasing burn severity and fire frequency corresponded with an increase in grass cover and a decrease in shrub basal area. We used multivariate ordination analysis to create a single integrator variable of fire effect that accounted for time after fire, burn severity, and number of times burned. This fire effect was significantly associated with decreases in soil organic layer thickness and overall plant biomass. Unlike previous studies in Arctic Alaska tundra, we found that increases in fire frequency and severity did not increase shrub cover and biomass. Instead, intensifying fire disturbance, and particularly repeat fires, led to grass dominance. Our findings support the hypothesis that intensifying tundra fire regimes initiate alternative post-fire trajectories that are not shrub dominated and that are structurally and functionally quite different from sedge or shrub-dominated tundra.

Растительность класса Scheuchzerio–Caricetea fuscae Tx. 1937 в районе горного массива Янганапэ (восточный макросклон Полярного Урала)

Растительность класса Scheuchzerio–Caricetea fuscae Tx. 1937 в районе горного массива Янганапэ (восточный макросклон Полярного Урала)

Vegetation science during hectic times

Vegetation science during hectic times