Tundra Sites Research Articles

Evaluation of Spaceborne L-Band Radiometer Measurements for Terrestrial Freeze/Thaw Retrievals in Canada

The landscape freeze/thaw (F/T) state has an important impact on the surface energy balance, carbon fluxes, and hydrologic processes; the timing of spring melt is linked to active layer dynamics in permafrost areas. L-band (1.4 GHz) microwave emission could allow the monitoring of surface state dynamics due to its sensitivity to the pronounced permittivity difference between frozen and thawed soil. The aim of this paper is to evaluate the performance of both Aquarius and soil moisture and ocean salinity (SMOS) L-band passive microwave measurements using a polarization ratio (PR)-based algorithm for landscape F/T monitoring. Weekly L-band satellite observations are compared with a large set of reference data at 48 sites across Canada spanning three environments: 1) tundra; 2) boreal forest; and 3) prairies. The reference data include in situ measurements of soil temperature ( ${\mathbf {T}}_{\mathbf{soil}}$ ) and air temperature ( ${\mathbf {T}}_{\mathbf{air}}$ ), and moderate resolution imaging spectroradiometer (MODIS) land surface temperature (LST) and snow cover area (SCA) products. Results show generally good agreement between L-band F/T detection and the surface state estimated from four reference datasets. The best apparent accuracies for all seasons are obtained using ${\mathbf{T}}_{\mathbf{air}}$ as the reference. Aquarius radiometer 2 (incidence angle of 39.6°) data give the best accuracies (90.8%), while for SMOS, using the Aquarius temporal characteristics, the best results (87.8% of accuracy) are obtained at higher incidence angles ( ${\mathbf{55}}^\circ\!\!-\!\!{\mathbf{60}}^\circ $ ). The F/T algorithm identifies both freeze onset and end with a delay of about 1 week in tundra and 2 weeks in forest and prairies, when compared to ${\mathbf{T}}_{\mathbf{air}}$ . The analysis shows a stronger F/T signal at tundra sites due to the typically clean transitions between consistently frozen and thawed conditions (and vice versa) and the absence of surface vegetation. Results in the prairies were poorer because of the influence of vegetation growth in summer (which decreases the PR) and the high frequency of ephemeral thaw events during winter. Freeze onset and end maps created from the same algorithm applied to SMOS and Aquarius measurements characterize similar F/T patterns over Canada. This study shows the potential of using L-band spaceborne observations for F/T monitoring, but underlines some limitations due to ice crusts in the snowpack, liquid water content in snow cover during the spring freeze to thaw transition, and vegetation growth.

The Polar Vegetation Photosynthesis and Respiration Model: a parsimonious, satellite-data-driven model of high-latitude CO<sub>2</sub> exchange

Abstract. We introduce the Polar Vegetation Photosynthesis and Respiration Model (PolarVPRM), a remote-sensing-based approach for generating accurate, high-resolution (≥ 1 km2, 3 hourly) estimates of net ecosystem CO2 exchange (NEE). PolarVPRM simulates NEE using polar-specific vegetation classes, and by representing high-latitude influences on NEE, such as the influence of soil temperature on subnivean respiration. We present a description, validation and error analysis (first-order Taylor expansion) of PolarVPRM, followed by an examination of per-pixel trends (2001–2012) in model output for the North American terrestrial region north of 55° N. PolarVPRM was validated against eddy covariance (EC) observations from nine North American sites, of which three were used in model calibration. Comparisons of EC NEE to NEE from three models indicated that PolarVPRM displayed similar or better statistical agreement with eddy covariance observations than existing models showed. Trend analysis (2001–2012) indicated that warming air temperatures and drought stress in forests increased growing season rates of respiration, and decreased rates of net carbon uptake by vegetation when air temperatures exceeded optimal temperatures for photosynthesis. Concurrent increases in growing season length at Arctic tundra sites allowed for increases in photosynthetic uptake over time by tundra vegetation. PolarVPRM estimated that the North American high-latitude region changed from a carbon source (2001–2004) to a carbon sink (2005–2010) to again a source (2011–2012) in response to changing environmental conditions.

Characterisation of bacterial populations in Arctic permafrost soils using bacteriohopanepolyols

Bacteriohopanepolyols (BHPs) are biomarkers providing taxonomically and environmentally diagnostic information. BHPs may help to unravel the composition of bacterial communities residing in recent as well as ancient permafrost soils and sediments and also provide information on associated environmental conditions. However, detailed data on their distribution in the heterogeneous Arctic environment are scarce. The distribution and structural diversity of BHPs were studied in the annually thawing (active) layer of three different sites in the polygonal tundra of the Lena Delta in the Siberian Arctic. Variations between permafrost structures and soil horizons caused by differences in the physical and chemical soil properties were observed. C and N content is significantly correlated with the BHP composition so that the highest BHP concentrations and greatest structural diversity occur in the uppermost organic soil horizons, which consist mainly of fresh or little degraded plant material. Furthermore, statistical analyses reveal that higher abundances of adenosylhopane-type soil marker BHPs are linked to higher soil pH values. Small scale environmental controls on BHP distributions are reflected by amine-functionalised BHPs from methanotrophic bacteria only occurring in the water-saturated, oxygen-depleted polygon centres and by soil marker BHPs, which are significantly more abundant in the well aerated polygon rims than in the centres. In contrast, C-2 methylated BHPs, putative indicators of plant-bacterial interactions, are present in all soil horizons and permafrost structures and their relative distribution is not systematically linked to soil properties. Overall, lipid-based results agree with published 16S rRNA based community structure assessments highlighting the usefulness of BHPs to represent bacterial populations in recent and ancient permafrost soils.

Direct current (DC) resistivity and induced polarization (IP) monitoring of active layer dynamics at high temporal resolution

With permafrost thawing and changes in active layer dynamics induced by climate change, interactions between biogeochemical and thermal processes in the ground are of great importance. Here, active layer dynamics have been monitored using direct current (DC) resistivity and induced polarization (IP) measurements at high temporal resolution and at a relatively large scale at a heath tundra site on Disko Island on the west coast of Greenland (69°N). At the field site, the active layer is disconnected from the deeper permafrost, due to isothermal springs in the region. Borehole sediment characteristics and subsurface temperatures supplemented the DC-IP measurements. A time-lapse DC-IP monitoring system has been acquiring at least six datasets per day on a 42-electrode profile with 0.5m electrode spacing since July 2013. Remote control of the data acquisition system enables interactive adaptation of the measurement schedule, which is critically important to acquire data in the winter months, where extremely high contact resistances increase the demands on the resistivity meter. Data acquired during the freezing period of October 2013 to February 2014 clearly image the soil freezing as a strong increase in resistivity. While the freezing horizon generally moves deeper with time, some variations in the freezing depth are observed along the profile. Comparison with depth-specific soil temperature indicates an exponential relationship between resistivity and below-freezing temperature. Time-lapse inversions of the full-decay IP data indicate a decrease of normalized chargeability with freezing of the ground, which is the result of a decrease in the total unfrozen water and of the higher ion concentration in the pore-water. We conclude that DC-IP time-lapse measurements can non-intrusively and reliably image freezing patterns and their lateral variation on a 10–100m scale that is difficult to sample by point measurements. In combination with laboratory experiments, the different patterns in resistivity and chargeability changes will enable the disentanglement of processes (e.g., fluid migration and freezing, advective and diffusive heat transport) occurring during freezing of the ground. The technology can be expanded to three dimensions and also to larger scale.

A Comparison between Simulated and Observed Surface Energy Balance at the Svalbard Archipelago

AbstractThe surface energy balance at the Svalbard Archipelago has been simulated at high resolution with the Weather Research and Forecasting Model and compared with measurements of the individual energy fluxes from a tundra site near Ny-Ålesund (located north of Norway), as well as other near-surface measurements across the region. For surface air temperature, a good agreement between model and observations was found at all locations. High correlations were also found for daily averaged surface energy fluxes within the different seasons at the main site. The four radiation components showed correlations above 0.5 in all seasons (mostly above 0.9), whereas correlations between 0.3 and 0.8 were found for the sensible and latent heat fluxes. Underestimation of cloud cover and cloud optical thickness led to seasonal biases in incoming shortwave and longwave radiation of up to 30%. During summer, this was mainly a result of distinct days on which the model erroneously simulated cloud-free conditions, whereas the incoming radiation biases appeared to be more related to underestimation of cloud optical thickness during winter. The model overestimated both sensible and latent heat fluxes in most seasons. The model also initially overestimated the average Bowen ratio during summer by a factor of 6, but this bias was greatly reduced with two physically based model modifications that are related to frozen-ground hydrology. The seasonally averaged ground/snow heat flux was mostly in agreement with observations but showed too little short-time variability in the presence of thick snow. Overall, the model reproduced average temperatures well but overestimated diurnal cycles and showed considerable biases in the individual energy fluxes on seasonal and shorter time scales.

Uncertainties of Temperature Measurements on Snow-Covered Land and Sea Ice from In Situ and MODIS Data during BROMEX

AbstractThe Bromine, Ozone, and Mercury Experiment (BROMEX) was conducted in March and April of 2012 near Barrow, Alaska, to investigate impacts of Arctic sea ice reduction on chemical processes. During BROMEX, multiple sensors were deployed to measure air and surface temperature. The uncertainties in temperature measurement on snow-covered land and sea ice surfaces were examined using in situ data and temperature measurements that were derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) and are part of the Terra and Aqua ice-surface temperature and land-surface temperature (LST) standard data products. Following an ~24-h cross-calibration study, two Thermochrons (small temperature-sensing devices) were deployed at each of three field sites: a sea ice site in the Chukchi Sea, a mixed-cover site, and a homogeneous tundra site. At each site, one Thermochron was shielded from direct sunlight and one was left unshielded, and they were placed on top of the snow or ice. The best agreement between the Thermochron- and MODIS-derived temperatures was found between the shielded Thermochrons and the Aqua MODIS LSTs, with an average agreement of 0.6° ± 2.0°C (sample size of 84) at the homogeneous tundra site. The results highlight some uncertainties associated with obtaining consistent air and surface temperature measurements in the harsh Arctic environment, using both in situ and satellite sensors. It is important to minimize uncertainties that could introduce biases in long-term temperature trends.

Deepened winter snow increases stem growth and alters stem δ13C and δ15N in evergreen dwarf shrub Cassiope tetragona in high-arctic Svalbard tundra

Deeper winter snow is hypothesized to favor shrub growth and may partly explain the shrub expansion observed in many parts of the arctic during the last decades, potentially triggering biophysical feedbacks including regional warming and permafrost thawing. We experimentally tested the effects of winter snow depth on shrub growth and ecophysiology by measuring stem length and stem hydrogen (δ2H), carbon (δ13C), nitrogen (δ15N) and oxygen (δ18O) isotopic composition of the circumarctic evergreen dwarf shrub Cassiope tetragona growing in high-arctic Svalbard, Norway. Measurements were carried out on C. tetragona individuals sampled from three tundra sites, each representing a distinct moisture regime (dry heath, meadow, moist meadow). Individuals were sampled along gradients of experimentally manipulated winter snow depths in a six-year old snow fence experiment: in ambient (c. 20 cm), medium (c. 100 cm), and deep snow (c. 150 cm) plots. The deep-snow treatment consistently and significantly increased C. tetragona growth during the 2008–2011 manipulation period compared to growth in ambient-snow plots. Stem δ15N and stem N concentration values were significantly higher in deep-snow individuals compared to individuals growing in ambient-snow plots during the course of the experiment, suggesting that soil N-availability was increased in deep-snow plots as a result of increased soil winter N mineralization. Although inter-annual growing season-precipitation δ2H and stem δ2H records closely matched, snow depth did not change stem δ2H or δ18O, suggesting that water source usage by C. tetragona was unaltered. Instead, the deep insulating snowpack may have protected C. tetragona shrubs against frost damage, potentially compensating the detrimental effects of a shortened growing season and associated phenological delay on growth. Our findings suggest that an increase in winter precipitation in the High Arctic, as predicted by climate models, has the potential to alter the growth and ecophysiology of evergreen shrub C. tetragona through changes in plant mineral nutrition and frost damage protection.

‘Warm’ Tundra: Atmospheric and Near‐Surface Ground Temperature Inversions Across an Alpine Treeline in Continuous Permafrost, Western Arctic, Canada

AbstractPermafrost conditions were examined between 2010 and 2014 at four sites across an alpine treeline on Peel Plateau, Northwest Territories, Canada. Ground and air temperature sensors were installed in forest and tundra between 30 and 500 m asl. Annual mean air temperatures increased and the number of freezing degree days declined with elevation, due to persistent winter air temperature inversions. The annual mean temperature at the permafrost surface (Tps) in mineral soils increased with elevation from about −2.5 °C in lowland forest to about −1.5 °C in dwarf shrub tundra. The increase in Tps coincided with higher air temperatures and earlier snow accumulation at tundra sites. The higher Tps in alpine tundra compared to lowland forest in Peel Plain contrasts with the northward decrease in Tps across latitudinal treeline elsewhere in the western Arctic. An increase in Tps with elevation may be common in Arctic mountain environments due to the prevalence of atmospheric temperature inversions in winter. In such contexts, although vegetation characteristics are governed by summer climate, permafrost conditions are critically influenced by the winter regime. The tundra permafrost on Peel Plateau is considerably warmer and, hence, more sensitive to disturbance than perennially frozen ground north of treeline in other parts of the western Arctic. Copyright © 2015 John Wiley & Sons, Ltd.

Recovery of Tundra Vegetation Three Decades after Hydrocarbon Drilling with and without Seeding of Non-Native Grasses + Supplementary Appendices (See Article Tools)

Oil and gas exploration conducted in the 1970s left a legacy of abandoned test wells with sumps, containing drilling waste, in the Mackenzie Delta region of Canada’s Northwest Territories. One to two years after the wells were decommissioned, a set of sites were seeded with non-native grasses and fertilized to test whether these treatments could accelerate vegetation recovery and prevent erosion. We sampled seeded and unseeded sumps and adjacent tundra vegetation in the Mackenzie Delta region three decades later to examine the impact of post-disturbance seeding treatments on site recovery. Plant species composition and environmental data were collected at 12 sump sites (6 seeded and fertilized and 6 unseeded and unfertilized) in lowland and upland tundra. Multivariate analyses using NMDS and perMANOVA indicated that in the lowlands, seeding and fertilization treatments had small but significant effects on plant species composition that differentiated seeded from unseeded sump caps. Plant communities on sump caps for all treatment types were significantly different from surrounding undisturbed tundra, even after more than 30 years of recovery. Seeded non-native grasses were found on both seeded and unseeded sumps, but not in the surrounding undisturbed tundra. Undisturbed tundra appears resistant to the spread of introduced agronomic grasses, but disturbed areas, such as sumps, provide areas of suitable habitat where non-native plants can persist.

Abrupt changes in invertebrate herbivory on woody plants at the forest–tundra ecotone

Invertebrate herbivores, insects in particular, play important functional roles in terrestrial ecosystems. At background (non-outbreak) densities, they consume 5–15 % of woody plant foliage in forests across the planet. At the same time, almost nothing is known about the levels of insect herbivory in Arctic tundra. To test the hypothesis that the amount of plant biomass lost to insects in tundra is substantially less than in subarctic forests, in 2013, we explored foliar herbivory in woody plants at three sites in the Arctic tundra and four sites in the subarctic forests of European Russia. A vast majority of foliar damage was imposed by externally feeding defoliators. In forests, defoliators damaged three times more leaves and consumed eight times more leaf area than in the tundra. No miners were found in the tundra, and gallers affected five times less leaf area in the tundra compared with forests. An abrupt decrease in loss of woody plant foliage to insects between subarctic forests and tundra (from 4.34 to 0.56 %) supports the existence of a latitudinal gradient in herbivory in terrestrial ecosystems. More studies are needed to predict how tundra plants, which have been historically exposed to low levels of insect herbivory, will cope with the increased levels of damage that are expected to occur due to climate-driven range expansion and increased abundances of plant-feeding insects.

Spatio-temporal influence of tundra snow properties on Ku-band (17.2 GHz) backscatter

AbstractDuring the 2010/11 boreal winter, a distributed set of backscatter measurements was collected using a ground-based Ku-band (17.2 GHz) scatterometer system at 26 open tundra sites. A standard snow-sampling procedure was completed after each scan to evaluate local variability in snow layering, depth, density and water equivalent (SWE) within the scatterometer field of view. The shallow depths and large basal depth hoar encountered presented an opportunity to evaluate backscatter under a set of previously untested conditions. Strong Ku-band response was found with increasing snow depth and snow water equivalent (SWE). In particular, co-polarized vertical backscatter increased by 0.82 dB for every 1 cm increase in SWE (R2 = 0.62). While the result indicated strong potential for Ku-band retrieval of shallow snow properties, it did not characterize the influence of sub-scan variability. An enhanced snow-sampling procedure was introduced to generate detailed characterizations of stratigraphy within the scatterometer field of view using near-infrared photography along the length of a 5 m trench. Changes in snow properties along the trench were used to discuss variations in the collocated backscatter response. A pair of contrasting observation sites was used to highlight uncertainties in backscatter response related to short length scale spatial variability in the observed tundra environment.

Effect of ablation rings and soil temperature on 3-year spring CO<sub>2</sub> efflux along the Dalton Highway, Alaska

Abstract. Winter and spring soil CO2 efflux measurements represent a significant component in the assessment of annual carbon budgets of tundra and boreal forest ecosystems, reflecting responses to climate change in the Arctic. This study was conducted in order to quantify CO2 efflux, using a portable chamber system at representative sites along the Dalton Highway. Study sites included three tundra, two white spruce, and three black spruce forest locations during the winter and spring seasons of 2010–2012; the study of these sites promised better understanding of winter and spring carbon contributions to the annual carbon budget, as well as the respective ablation-ring effects during spring. Three-year spring CO2 efflux depends on soil temperature at 5 cm depth on a regional scale. At their highest, Q10 values were 4.2 × 106, within the exposed tussock tundra of the upland tundra site, which tundra soils warmed from −0.9 to 0.5 °C, involving soil microbial activity. From the forest census (400 m2) of the two white spruce forest sites, CO2 emissions were estimated as 0.09–0.36 gC m−2 day−1 in winter and 0.14–4.95 gC m−2 day−1 in spring, corresponding to 1–3% and 1–27% of annual carbon, respectively. Contributions from spring CO2 emissions are likely to increase as exposed soils widen in average length (major axis) from the east-, west-, south-, and north-side lengths (minor axis). Considering the periods of winter and spring seasons across tundra and boreal forests, average winter- and spring-seasonal CO2 contributions to annual carbon budgets correspond roughly to 14–22% for tundra and 9–24% for boreal forest sites during 2011 and 2012. Spring carbon contributions, such as growing season CO2 emissions, are sensitive to subtle changes at the onset of spring and during the snow-covered period in northern high latitudes, in response to recent Arctic climate change.

Permafrost collapse after shrub removal shifts tundra ecosystem to a methane source

Arctic tundra ecosystems are warming almost twice as fast as the global average. Permafrost thaw and the resulting release of greenhouse gases from decomposing soil organic carbon have the potential to accelerate climate warming. In recent decades, Arctic tundra ecosystems have changed rapidly, including expansion of woody vegetation, in response to changing climate conditions. How such vegetation changes contribute to stabilization or destabilization of the permafrost is unknown. Here we present six years of field observations in a shrub removal experiment at a Siberian tundra site. Removing the shrub part of the vegetation initiated thawing of ice-rich permafrost, resulting in collapse of the originally elevated shrub patches into waterlogged depressions within five years. This thaw pond development shifted the plots from a methane sink into a methane source. The results of our field experiment demonstrate the importance of the vegetation cover for protection of the massive carbon reservoirs stored in the permafrost and illustrate the strong vulnerability of these tundra ecosystems to perturbations. If permafrost thawing can more frequently trigger such local permafrost collapse, methane-emitting wet depressions could become more abundant in the lowland tundra landscape, at the cost of permafrost-stabilizing low shrub vegetation.

Polygonal tundra geomorphological change in response to warming alters future CO2 and CH4 flux on the Barrow Peninsula.

The landscape of the Barrow Peninsula in northern Alaska is thought to have formed over centuries to millennia, and is now dominated by ice-wedge polygonal tundra that spans drained thaw-lake basins and interstitial tundra. In nearby tundra regions, studies have identified a rapid increase in thermokarst formation (i.e., pits) over recent decades in response to climate warming, facilitating changes in polygonal tundra geomorphology. We assessed the future impact of 100 years of tundra geomorphic change on peak growing season carbon exchange in response to: (i) landscape succession associated with the thaw-lake cycle; and (ii) low, moderate, and extreme scenarios of thermokarst pit formation (10%, 30%, and 50%) reported for Alaskan arctic tundra sites. We developed a 30 × 30 m resolution tundra geomorphology map (overall accuracy:75%; Kappa:0.69) for our ~1800 km² study area composed of ten classes; drained slope, high center polygon, flat-center polygon, low center polygon, coalescent low center polygon, polygon trough, meadow, ponds, rivers, and lakes, to determine their spatial distribution across the Barrow Peninsula. Land-atmosphere CO2 and CH4 flux data were collected for the summers of 2006-2010 at eighty-two sites near Barrow, across the mapped classes. The developed geomorphic map was used for the regional assessment of carbon flux. Results indicate (i) at present during peak growing season on the Barrow Peninsula, CO2 uptake occurs at -902.3 10(6) gC-CO2 day(-1) (uncertainty using 95% CI is between -438.3 and -1366 10(6) gC-CO2 day(-1)) and CH4 flux at 28.9 10(6) gC-CH4 day(-1) (uncertainty using 95% CI is between 12.9 and 44.9 10(6) gC-CH4 day(-1)), (ii) one century of future landscape change associated with the thaw-lake cycle only slightly alter CO2 and CH4 exchange, while (iii) moderate increases in thermokarst pits would strengthen both CO2 uptake (-166.9 10(6) gC-CO2 day(-1)) and CH4 flux (2.8 10(6) gC-CH4 day(-1)) with geomorphic change from low to high center polygons, cumulatively resulting in an estimated negative feedback to warming during peak growing season.

Penguins significantly increased phosphine formation and phosphorus contribution in maritime Antarctic soils.

Most studies on phosphorus cycle in the natural environment focused on phosphates, with limited data available for the reduced phosphine (PH3). In this paper, matrix-bound phosphine (MBP), gaseous phosphine fluxes and phosphorus fractions in the soils were investigated from a penguin colony, a seal colony and the adjacent animal-lacking tundra and background sites. The MBP levels (mean 200.3 ng kg−1) in penguin colony soils were much higher than those in seal colony soils, animal-lacking tundra soils and the background soils. Field PH3 flux observation and laboratory incubation experiments confirmed that penguin colony soils produced much higher PH3 emissions than seal colony soils and animal-lacking tundra soils. Overall high MBP levels and PH3 emissions were modulated by soil biogeochemical processes associated with penguin activities: sufficient supply of the nutrients phosphorus, nitrogen, and organic carbon from penguin guano, high soil bacterial abundance and phosphatase activity. It was proposed that organic or inorganic phosphorus compounds from penguin guano or seal excreta could be reduced to PH3 in the Antarctic soils through the bacterial activity. Our results indicated that penguin activity significantly increased soil phosphine formation and phosphorus contribution, thus played an important role in phosphorus cycle in terrestrial ecosystems of maritime Antarctica.

Cumulative Impacts and Feedbacks of a Gravel Road on Shrub Tundra Ecosystems in the Peel Plateau, Northwest Territories, Canada

Gravel highways in the continuous permafrost zone provide critical transportation links that are increasingly vulnerable to the impacts of climate warming and permafrost thaw. To examine if the physical effects associated with the construction, maintenance, and use of gravel roads alter vegetation and permafrost conditions, we measured vegetation, soils, and near-surface ground temperatures at tall and dwarf shrub tundra sites adjacent to and distant from the Dempster Highway in the Northwest Territories of Canada. We found that alder growth and recruitment were significantly enhanced adjacent to the highway. Where alder shrubs had formed closed canopies, we observed dramatic alterations to plant community composition, soil properties, and ground temperatures. Tall shrub sites adjacent to the road exhibited less understory vegetation, greater litter and organic layer thickness, higher nutrient availability, and thicker snowpack than all other site types. Our results show that in shrub tundra ecosystems the conditions generated by the maintenance and use of a gravel road can drive ecological feedbacks that magnify changes to vegetation communities and soils. We found that where the road facilitated shrub dominance, feedbacks were initiated that enhanced snow accumulation and altered ground temperatures and soil chemistry. In turn, these changes likely promoted enhanced shrub recruitment and growth. Shrub proliferation adjacent to highways is an important consideration for the planning and maintenance of this form of infrastructure. To improve our understanding of the spatial heterogeneity of shrub proliferation, research exploring the relationships between biophysical landscape features and shrub development is also needed.

A reduced-order modeling approach to represent subgrid-scale hydrological dynamics for land-surface simulations: application in a polygonal tundra landscape

Abstract. Existing land surface models (LSMs) describe physical and biological processes that occur over a wide range of spatial and temporal scales. For example, biogeochemical and hydrological processes responsible for carbon (CO2, CH4) exchanges with the atmosphere range from the molecular scale (pore-scale O2 consumption) to tens of kilometers (vegetation distribution, river networks). Additionally, many processes within LSMs are nonlinearly coupled (e.g., methane production and soil moisture dynamics), and therefore simple linear upscaling techniques can result in large prediction error. In this paper we applied a reduced-order modeling (ROM) technique known as "proper orthogonal decomposition mapping method" that reconstructs temporally resolved fine-resolution solutions based on coarse-resolution solutions. We developed four different methods and applied them to four study sites in a polygonal tundra landscape near Barrow, Alaska. Coupled surface–subsurface isothermal simulations were performed for summer months (June–September) at fine (0.25 m) and coarse (8 m) horizontal resolutions. We used simulation results from three summer seasons (1998–2000) to build ROMs of the 4-D soil moisture field for the study sites individually (single-site) and aggregated (multi-site). The results indicate that the ROM produced a significant computational speedup (> 103) with very small relative approximation error (< 0.1%) for 2 validation years not used in training the ROM. We also demonstrate that our approach: (1) efficiently corrects for coarse-resolution model bias and (2) can be used for polygonal tundra sites not included in the training data set with relatively good accuracy (< 1.7% relative error), thereby allowing for the possibility of applying these ROMs across a much larger landscape. By coupling the ROMs constructed at different scales together hierarchically, this method has the potential to efficiently increase the resolution of land models for coupled climate simulations to spatial scales consistent with mechanistic physical process representation.

Assessing the spatial variability in peak season CO<sub>2</sub> exchange characteristics across the Arctic tundra using a light response curve parameterization

Abstract. This paper aims to assess the spatial variability in the response of CO2 exchange to irradiance across the Arctic tundra during peak season using light response curve (LRC) parameters. This investigation allows us to better understand the future response of Arctic tundra under climatic change. Peak season data were collected during different years (between 1998 and 2010) using the micrometeorological eddy covariance technique from 12 circumpolar Arctic tundra sites, in the range of 64–74° N. The LRCs were generated for 14 days with peak net ecosystem exchange (NEE) using an NEE–irradiance model. Parameters from LRCs represent site-specific traits and characteristics describing the following: (a) NEE at light saturation (Fcsat), (b) dark respiration (Rd), (c) light use efficiency (α), (d) NEE when light is at 1000 μmol m−2 s−1 (Fc1000), (e) potential photosynthesis at light saturation (Psat) and (f) the light compensation point (LCP). Parameterization of LRCs was successful in predicting CO2 flux dynamics across the Arctic tundra. We did not find any trends in LRC parameters across the whole Arctic tundra but there were indications for temperature and latitudinal differences within sub-regions like Russia and Greenland. Together, leaf area index (LAI) and July temperature had a high explanatory power of the variance in assimilation parameters (Fcsat, Fc1000 and Psat, thus illustrating the potential for upscaling CO2 exchange for the whole Arctic tundra. Dark respiration was more variable and less correlated to environmental drivers than were assimilation parameters. This indicates the inherent need to include other parameters such as nutrient availability, substrate quantity and quality in flux monitoring activities.

Distribution and activity of ice wedges across the forest‐tundra transition, western Arctic Canada

AbstractRemote sensing, regional ground temperature and ground ice observations, and numerical simulation were used to investigate the size, distribution, and activity of ice wedges in fine‐grained mineral and organic soils across the forest‐tundra transition in uplands east of the Mackenzie Delta. In the northernmost dwarf‐shrub tundra, ice wedge polygons cover up to 40% of the ground surface, with the wedges commonly exceeding 3 m in width. The largest ice wedges are in peatlands where thermal contraction cracking occurs more frequently than in nearby hummocky terrain with fine‐grained soils. There are fewer ice wedges, rarely exceeding 2 m in width, in uplands to the south and none have been found in mineral soils of the tall‐shrub tundra, although active ice wedges are found there throughout peatlands. In the spruce forest zone, small, relict ice wedges are restricted to peatlands. At tundra sites, winter temperatures at the top of permafrost are lower in organic than mineral soils because of the shallow permafrost table, occurrence of phase change at 0°C, and the relatively high thermal conductivity of icy peat. Due to these factors and the high coefficient of thermal contraction of frozen saturated peat, ice wedge cracking and growth is more common in peatlands than in mineral soil. However, the high latent heat content of saturated organic active layer soils may inhibit freezeback, particularly where thick snow accumulates, making the permafrost and the ice wedges in spruce forest polygonal peatlands susceptible to degradation following alteration of drainage or climate warming.

Summertime CO2 fluxes and ecosystem respiration from marine animal colony tundra in maritime Antarctica

Net ecosystem CO2 exchange (NEE) and ecosystem respiration (ER) were investigated at penguin, seal and skua colony tundra and the adjacent animal-lacking tundra sites in maritime Antarctica. Net CO2 fluxes showed a large difference between marine animal colonies and animal-lacking tundra sites. The mean NEE from penguin, seal and skua colony tundra sites ranged from −37.2 to 5.2 mg CO2 m−2 h−1, whereas animal-lacking tundra sites experienced a larger net gain of CO2 with the mean flux range from −85.6 to −23.9 mg CO2 m−2 h−1. Ecosystem respiration rates at penguin colony tundra sites (mean 201.3 ± 31.4 mg CO2 m−2 h−1) were significantly higher (P < 0.01) than those at penguin-lacking tundra sites (64.0–87.1 mg CO2 m−2 h−1). The gross photosynthesis (Pg) showed a consistent trend to ER with the highest mean Pg (219.7 ± 34.5 mg CO2 m−2 h−1) at penguin colony tundra sites. When all the data were combined from different types of tundra ecosystems, summertime tundra NEE showed a weak or strong positive correlation with air temperature, 0–10 cm soil temperature or precipitation. The NEE from marine animal colony and animal-lacking tundra was significantly positively correlated (P < 0.001) with soil organic carbon (SOC), total nitrogen (TN) contents and C:N ratios. The ER showed a significant exponential correlation (P < 0.01) with mean 0–15 cm soil temperature, and much higher Q10 value (9.97) was obtained compared with other terrestrial ecosystems, indicating greater temperature sensitivity of tundra ecosystem respiration. Our results indicate that marine animals and the deposition of their excreta might have an important effect on tundra CO2 exchanges and ecosystem respiration, and current climate warming will further decrease tundra CO2 sink in maritime Antarctica.