Arctic Transect Research Articles

Spatial patterns of arctic tundra vegetation properties on different soils along the Eurasia Arctic Transect, and insights for a changing Arctic

Vegetation properties of arctic tundra vary dramatically across its full latitudinal extent, yet few studies have quantified tundra ecosystem properties across latitudinal gradients with field-based observations that can be related to remotely sensed proxies. Here we present data from field sampling of six locations along the Eurasia Arctic Transect in northwestern Siberia. We collected data on the aboveground vegetation biomass, the normalized difference vegetation index (NDVI), and the leaf area index (LAI) for both sandy and loamy soil types, and analyzed their spatial patterns. Aboveground biomass, NDVI, and LAI all increased with increasing summer warmth index (SWI—sum of monthly mean temperatures > 0 °C), although functions differed, as did sandy vs. loamy sites. Shrub biomass increased non-linearly with SWI, although shrub type biomass diverged with soil texture in the southernmost locations, with greater evergreen shrub biomass on sandy sites, and greater deciduous shrub biomass on loamy sites. Moss biomass peaked in the center of the gradient, whereas lichen biomass generally increased with SWI. Total aboveground biomass varied by two orders of magnitude, and shrubs increased from 0 g m−2 at the northernmost sites to >500 g m−2 at the forest-tundra ecotone. Current observations and estimates of increases in total aboveground and shrub biomass with climate warming in the Arctic fall short of what would represent a ‘subzonal shift’ based on our spatial data. Non-vascular (moss and lichen) biomass is a dominant component (>90% of the photosynthetic biomass) of the vegetation across the full extent of arctic tundra, and should continue to be recognized as crucial for Earth system modeling. This study is one of only a few that present data on tundra vegetation across the temperature extent of the biome, providing (a) key links to satellite-based vegetation indices, (b) baseline field-data for ecosystem change studies, and (c) context for the ongoing changes in arctic tundra vegetation.

Variability in 210Pb and 210Po partition coefficients (Kd) along the US GEOTRACES Arctic transect

The naturally occurring 210Po (t1/2 = 138.3d) and 210Pb (t1/2 = 22.3 y) radionuclide pair has been extensively used to study particle scavenging, cycling and transport in the ocean. However, the role of particle composition in the scavenging, sorption and fractionation of 210Po210Pb needs to be better understood to successfully utilize these radionuclides as oceanographic tracers. Here, we measure dissolved and particulate 210Pb and 210Po activities for both small (1-51 μm) and large particles (> 51 μm) with abundance of major phases of suspended particulate matter (SPM) from the US GEOTRACES Arctic transect to examine the role of the concentration and composition of SPM in scavenging and fractionation of these radionuclides along the coastal, Canada Basin and interior stations. 210Po210Pb disequilibria were observed throughout the water column for all stations. The Pacific end member had the highest activity of 210Po and 210Pb in dissolved samples. The calculated partition coefficients (Kd) of 210Po and 210Pb varied between 3.2 × 105 to 9.9 × 108 L kg−1 and 7.9 × 105 to 6.9 × 108 L kg−1 respectively and followed a log-linear trend with SPM for particle size fractions. Principal Component Analysis (PCA) and correlation analysis for six major particle phases indicated a significant positive correlation between 210Pb and the lithogenic phase. Both 210Po and 210Pb had significant negative correlations with opal. Our results suggest that particle composition alone cannot explain the observed variability in Kd across the different oceanic regime along the transect. Estimated Kd values from an end member mixing model were found to be within the range of observed Kd values and suggest that higher particulate Mn concentration in the Arctic may be partly responsible for the higher Kd observed in the basin compared to Atlantic and Pacific.

Vegetation on mesic loamy and sandy soils along a 1700-km maritime Eurasia Arctic Transect.

QuestionsHow do plant communities on zonal loamy vs. sandy soils vary across the full maritime Arctic bioclimate gradient? How are plant communities of these areas related to existing vegetation units of the European Vegetation Classification? What are the main environmental factors controlling transitions of vegetation along the bioclimate gradient?Location1700‐km Eurasia Arctic Transect (EAT), Yamal Peninsula and Franz Josef Land (FJL), Russia.MethodsThe Braun‐Blanquet approach was used to sample mesic loamy and sandy plots on 14 total study sites at six locations, one in each of the five Arctic bioclimate subzones and the forest–tundra transition. Trends in soil factors, cover of plant growth forms (PGFs) and species diversity were examined along the summer warmth index (SWI) gradient and on loamy and sandy soils. Classification and ordination were used to group the plots and to test relationships between vegetation and environmental factors.ResultsClear, mostly non‐linear, trends occurred for soil factors, vegetation structure and species diversity along the climate gradient. Cluster analysis revealed seven groups with clear relationships to subzone and soil texture. Clusters at the ends of the bioclimate gradient (forest–tundra and polar desert) had many highly diagnostic taxa, whereas clusters from the Yamal Peninsula had only a few. Axis 1 of a DCA was strongly correlated with latitude and summer warmth; Axis 2 was strongly correlated with soil moisture, percentage sand and landscape age.ConclusionsSummer temperature and soil texture have clear effects on tundra canopy structure and species composition, with consequences for ecosystem properties. Each layer of the plant canopy has a distinct region of peak abundance along the bioclimate gradient. The major vegetation types are weakly aligned with described classes of the European Vegetation Checklist, indicating a continuous floristic gradient rather than distinct subzone regions. The study provides ground‐based vegetation data for satellite‐based interpretations of the western maritime Eurasian Arctic, and the first vegetation data from Hayes Island, Franz Josef Land, which is strongly separated geographically and floristically from the rest of the gradient and most susceptible to on‐going climate change.

Permafrost and lakes control river isotope composition across a boreal Arctic transect in the Western Siberian lowlands

The Western Siberian Lowlands (WSL) store large quantities of organic carbon that will be exposed and mobilized by the thawing of permafrost. The fate of mobilized carbon, however, is not well understood, partly because of inadequate knowledge of hydrological controls in the region which has a vast low-relief surface area, extensive lake and wetland coverage and gradually increasing permafrost influence. We used stable water isotopes to improve our understanding of dominant landscape controls on the hydrology of the WSL. We sampled rivers along a 1700 km South–North transect from permafrost-free to continuous permafrost repeatedly over three years, and derived isotope proxies for catchment hydrological responsiveness and connectivity. We found correlations between the isotope proxies and catchment characteristics, suggesting that lakes and wetlands are intimately connected to rivers, and that permafrost increases the responsiveness of the catchment to rainfall and snowmelt events, reducing catchment mean transit times. Our work provides rare isotope-based field evidence that permafrost and lakes/wetlands influence hydrological pathways across a wide range of spatial scales (10–105 km2) and permafrost coverage (0%–70%). This has important implications, because both permafrost extent and lake/wetland coverage are affected by permafrost thaw in the changing climate. Changes in these hydrological landscape controls are likely to alter carbon export and emission via inland waters, which may be of global significance.

Distributions of bacterial and archaeal membrane lipids in surface sediments reflect differences in input and loss of terrestrial organic carbon along a cross-shelf Arctic transect

Enhanced climate warming affecting the Arctic region could have a dramatic impact on the terrigenous organic carbon (terrOC) stored in the Eurasian permafrost and could increase the amount of OC remobilized to the Arctic shelves. An improved understanding of the fate of this remobilized soil OC is essential for better understanding of the consequences for the Arctic and global carbon cycle. In this study, glycerol dialkyl glycerol tetraethers (GDGTs) and bacteriohopanepolyols (BHPs) in surface sediments along a 500km cross shelf transect from the mouth of the Kolyma River to the middle of the vast East Siberian Sea were analysed to assess their potential and that of the associated branched and isoprenoid tetraether (BIT) and R′soil indices for tracing terrOC in Arctic systems. Both BHP and GDGT contributions indicated the greatest contribution of terrOC close to the river mouth, while the associated indices showed declining trends in an offshore direction, supporting an increasing marine OC input and/or a decrease in terrOC. However, while the BHPs indicated a dominance of terrOC at the start of the transect, the GDGTs suggested a much larger, almost 50%, marine OC input at this point. In addition, the BIT index displayed an exponential decline, controlled mainly by a substantial contribution of marine GDGTs, while R′soil revealed a linear trend governed primarily by the removal of soil marker BHPs.These field results suggest that both biomarker approaches could be used to trace terrigenous derived OC in the Arctic environment. However, using a single proxy approach is not recommended and may lead to an under or over estimation of the relative importance of terrOC. Using a multi-proxy approach is valuable for fully understanding the fate of terrigenous derived OC along Arctic land-ocean transects.

Rich and cold: diversity, distribution and drivers of fungal communities in patterned‐ground ecosystems of the North American Arctic

Fungi are abundant and functionally important in the Arctic, yet comprehensive studies of their diversity in relation to geography and environment are not available. We sampled soils in paired plots along the North American Arctic Transect (NAAT), which spans all five bioclimatic subzones of the Arctic. Each pair of plots contrasted relatively bare, cryoturbated patterned-ground features (PGFs) and adjacent vegetated between patterned-ground features (bPGFs). Fungal communities were analysed via sequencing of 7834 ITS-LSU clones. We recorded 1834 OTUs - nearly half the fungal richness previously reported for the entire Arctic. These OTUs spanned eight phyla, 24 classes, 75 orders and 120 families, but were dominated by Ascomycota, with one-fifth belonging to lichens. Species richness did not decline with increasing latitude, although there was a decline in mycorrhizal taxa that was offset by an increase in lichen taxa. The dominant OTUs were widespread even beyond the Arctic, demonstrating no dispersal limitation. Yet fungal communities were distinct in each subzone and were correlated with soil pH, climate and vegetation. Communities in subzone E were distinct from the other subzones, but similar to those of the boreal forest. Fungal communities on disturbed PGFs differed significantly from those of paired stable areas in bPGFs. Indicator species for PGFs included lichens and saprotrophic fungi, while bPGFs were characterized by ectomycorrhizal and pathogenic fungi. Our results suggest that the Arctic does not host a unique mycoflora, while Arctic fungi are highly sensitive to climate and vegetation, with potential to migrate rapidly as global change unfolds.

On seasonal changes of the carbonate system in the Barents Sea: observations and modeling

The role of seasonality of organic matter (OM) production and decay in the seasonal changes of the carbonate system was studied on the basis of data received for a transect from Tromsø to Spitsbergen with a Ferrybox-equipped vessel. A 2D simplified vertical model was used for parameterization of the hydrophysical processes for a Coast–Open Arctic Transect. The biogeochemical processes were parameterized using OxyDep, a simplified biogeochemical model, that considered inorganic nutrient (NUT), dissolved organic matter (DOM), particular organic matter (POM) and biota (BIO). The carbonate system equilibrium was considered as a fast process and calculated at each time step using an iteration procedure for pH. According to the model estimates, OM production and decay play leading roles in the carbonate system seasonal dynamics. The modelled seasonal variations of pH (~0.2) are close to those observed, i.e. 7.94–7.99 in February and 8.04–8.16 in August (total scale). The surface layer pCO2 varies from 280 ppm during the OM production period to about 390 ppm in the centre of the sea and 430–460 ppm in the coastal regions in winter. The summer CO2 invasion is replaced by winter evasion. These estimates can be helpful for the planning of expedition studies and analysing the archived field data, as well as for elaborating the interannual and multidecade dynamics models.

Ground-Based Hyperspectral Characterization of Alaska Tundra Vegetation along Environmental Gradients

Remote sensing has become a valuable tool in monitoring arctic environments. The aim of this paper is ground-based hyperspectral characterization of Low Arctic Alaskan tundra communities along four environmental gradients (regional climate, soil pH, toposequence, and soil moisture) that all vary in ground cover, biomass, and dominating plant communities. Field spectroscopy in connection with vegetation analysis was carried out in summer 2012, along the North American Arctic Transect (NAAT). Spectral metrics were extracted, including the averaged reflectance and absorption-related metrics such as absorption depths and area of continuum removal. The spectral metrics were investigated with respect to “greenness”, biomass, vegetation height, and soil moisture regimes. The results show that the surface reflectances of all sites are similar in shape with a reduced near-infrared (NIR) reflectance that is specific for low-growing biomes. The main spectro-radiometric findings are: (i) Southern sites along the climate gradient have taller shrubs and greater overall vegetation biomass, which leads to higher reflectance in the NIR. (ii) Vegetation height and surface wetness are two antagonists that balance each other out with respect to the NIR reflectance along the toposequence and soil moisture gradients. (iii) Moist acidic tundra (MAT) sites have “greener” species, more leaf biomass, and green-colored moss species that lead to higher pigment absorption compared to moist non-acidic tundra (MNT) sites. (iv) MAT and MNT plant community separation via narrowband Normalized Difference Vegetation Index (NDVI) shows the potential of hyperspectral remote sensing applications in the tundra.

Soil Nitrogen Transformations Associated with Small Patterned‐Ground Features along a North American Arctic Transect

ABSTRACTSmall patterned‐ground features (PGFs) in the Arctic have unique soil properties that vary with latitude and may greatly affect tundra biogeochemistry. Because nitrogen availability can strongly limit arctic vegetation growth, we examined how soil nitrogen transformations differ between PGFs and the surrounding inter‐PGF tundra along an arctic latitudinal gradient. We collected soils at eight sites from the Alaskan Low Arctic to the Canadian High Arctic. The soils were incubated for 21 days at 9 °C and 15 °C and analysed for changes in total inorganic nitrogen, nitrate and extractable organic nitrogen (EON). We found greater nitrogen immobilisation in the surrounding inter‐PGF soils than in the PGF soils. Along the latitudinal gradient, differences in net nitrogen mineralisation and EON cycling between PGF and inter‐PGF soils were strongly influenced by the presence of a pH boundary within the Low Arctic and the transition between the High and Low Arctic, with greater immobilisation in the nonacidic and Low Arctic sites, respectively. Incubation temperature affected EON flux but did not affect net nitrogen mineralisation or nitrification. These results show that spatial heterogeneity at several scales can influence soil nitrogen dynamics, and is therefore an important influence on arctic ecosystem function. Copyright © 2012 John Wiley & Sons, Ltd.

Environment, vegetation and greenness (NDVI) along the North America and Eurasia Arctic transects

Satellite-based measurements of the normalized difference vegetation index (NDVI; an index of vegetation greenness and photosynthetic capacity) indicate that tundra environments are generally greening and becoming more productive as climates warm in the Arctic. The greening, however, varies and is even negative in some parts of the Arctic. To help interpret the space-based observations, the International Polar Year (IPY) Greening of the Arctic project conducted ground-based surveys along two >1500 km transects that span all five Arctic bioclimate subzones. Here we summarize the climate, soil, vegetation, biomass, and spectral information collected from the North America Arctic transect (NAAT), which has a more continental climate, and the Eurasia Arctic transect (EAT), which has a more oceanic climate. The transects have broadly similar summer temperature regimes and overall vegetation physiognomy, but strong differences in precipitation, especially winter precipitation, soil texture and pH, disturbance regimes, and plant species composition and structure. The results indicate that summer warmth and NDVI increased more strongly along the more continental transect.

Imminent ocean acidification in the Arctic projected with the NCAR global coupled carbon cycle-climate model

Abstract. Ocean acidification from the uptake of anthropogenic carbon is simulated for the industrial period and IPCC SRES emission scenarios A2 and B1 with a global coupled carbon cycle-climate model. Earlier studies identified seawater saturation state with respect to aragonite, a mineral phase of calcium carbonate, as a key variable governing impacts on corals and other shell-forming organisms. Globally in the A2 scenario, water saturated by more than 300%, considered suitable for coral growth, vanishes by 2070 AD (CO2≈630 ppm), and the ocean volume fraction occupied by saturated water decreases from 42% to 25% over this century. The largest simulated pH changes worldwide occur in Arctic surface waters, where hydrogen ion concentration increases by up to 185% (ΔpH=−0.45). Projected climate change amplifies the decrease in Arctic surface mean saturation and pH by more than 20%, mainly due to freshening and increased carbon uptake in response to sea ice retreat. Modeled saturation compares well with observation-based estimates along an Arctic transect and simulated changes have been corrected for remaining model-data differences in this region. Aragonite undersaturation in Arctic surface waters is projected to occur locally within a decade and to become more widespread as atmospheric CO2 continues to grow. The results imply that surface waters in the Arctic Ocean will become corrosive to aragonite, with potentially large implications for the marine ecosystem, if anthropogenic carbon emissions are not reduced and atmospheric CO2 not kept below 450 ppm.

Soils and frost boil ecosystems across the North American Arctic Transect

We studied soil properties of frost boils at nine zonal‐vegetation locations across the North American Arctic Transect (NAAT) in order to better understand arctic soils and their interaction with other biogeophysical components of frost boil ecosystems. Soil genetic horizons were analyzed for particle size, pH, electrical conductivity, total organic carbon (OC) and nitrogen (N), bulk density and volumetric water content. Surface soils (0–5 cm) across frost boil patterns were analyzed for of pH, OC, water content, extractable N and P, and exchangeable K, Ca and Na. Our results revealed that soil texture, pH, EC, P, Na and Ca contents are strongly influenced by local parent materials. Soil pH was acidic in the north going to alkaline in the midtransect and then again back to acidic in the south. Simple correlations between soil analytical data and observed frost boil properties across the NAAT support and are consistent with the laboratory and theoretical‐conceptual models of pattern ground dynamics that have been developed by others. Soil water related well to texture. Soil horizon %OC and profile OC stocks under the pattern corresponded well to biomass and frost heave, respectively. Also soil water was closely related to biomass and heave. Nutrients in surface soils at sites corresponded to OC stocks. An interaction between soil water and segregation of Na and Ca between the pattern and interpattern was found for locations with high cation availability. At these sites, chemical as well as physical disruption of the pattern area could affect plant establishment. Overall there was a good linkage between properties relating to frost boil pattern dynamics and soil biogeochemical properties. Our study offers insight into the important process of cryoturbation for carbon sequestration in Gelisols across the Arctic.

Arctic patterned‐ground ecosystems: A synthesis of field studies and models along a North American Arctic Transect

Arctic landscapes have visually striking patterns of small polygons, circles, and hummocks. The linkages between the geophysical and biological components of these systems and their responses to climate changes are not well understood. The “Biocomplexity of Patterned Ground Ecosystems” project examined patterned‐ground features (PGFs) in all five Arctic bioclimate subzones along an 1800‐km trans‐Arctic temperature gradient in northern Alaska and northwestern Canada. This paper provides an overview of the transect to illustrate the trends in climate, PGFs, vegetation, n‐factors, soils, active‐layer depth, and frost heave along the climate gradient. We emphasize the thermal effects of the vegetation and snow on the heat and water fluxes within patterned‐ground systems. Four new modeling approaches build on the theme that vegetation controls microscale soil temperature differences between the centers and margins of the PGFs, and these in turn drive the movement of water, affect the formation of aggradation ice, promote differential soil heave, and regulate a host of system properties that affect the ability of plants to colonize the centers of these features. We conclude with an examination of the possible effects of a climate warming on patterned‐ground ecosystems.

A map analysis of patterned‐ground along a North American Arctic Transect

Arctic patterned‐ground features have been described individually, but never examined as parts of integrated landscape/ecosystems that vary along the Arctic climate gradient. Here we examine the complex interrelationships between patterned ground, climate, vegetation and soil along a north‐south transect through all five bioclimate subzones of the North American Arctic. We mapped the vegetation, biomass, end‐of‐summer thaw depths, and snow cover on twenty 10 × 10‐m grids. The vegetation maps illustrate the transition of vegetation types and patterns from north to south. Biomass maps showed lower biomass in the centers of patterned‐ground features than in areas between features, and increasing biomass from north to south. Thaw‐depth maps showed deeper thaw in the centers of features than between features, and shallow thaw on the north and south ends of the transect. Snow depth maps showed less snow on patterned‐ground features subject to differential frost heave compared to areas between features which did not heave, and a north‐south gradient of increasing snow depth. The maps also documented the change from small nonsorted polygons to larger nonsorted circles from north to south, and increasing pattern size with moisture. Principal components analysis revealed underlying relationships between patterned‐ground landscapes and measured vegetation and environmental variables. Climate in combination with the vegetation was the most important factor affecting patterned ground on zonal sites, but soil moisture, texture and chemistry were also important.

Differential frost heave model for patterned ground formation: Corroboration with observations along a North American arctic transect

Frost boils in the Arctic are a manifestation of patterned ground in the form of nonsorted circles. Active frost boils involve convection of water through the soil that can bring basic salts from depth to the surface. As such, active frost boils can mitigate acidification and thereby strongly influence the type of vegetation supported by Arctic soils. The presence or absence of active frost boils is thought to play a pivotal role in establishing the sharp demarcation between moist nonacidic tundra (MNT) and moist acidic tundra (MAT) in the Arctic. The focus of this paper is to corroborate the predictions of a mathematical model that relates observable patterned ground features to ecosystem parameters with observations at the field sites along the North American Arctic Transect (NAAT) established by the Biocomplexity of Patterned‐Ground Ecosystems Project. Model predictions indicate that recurrent one‐dimensional frost heave can become unstable and evolve into multidimensional differential frost heave (DFH). A laboratory frost heave simulation produced a 28‐cm pattern in an active layer of 10 cm, which agrees with linear stability theory predictions. A finite element solution predicts three‐dimensional patterns with approximately 3‐m spacing develop in a 1.0‐m active layer with a surface n factor of 0.35, which agrees well with field observations from the NAAT. The lack of significant frost boil activity in the MAT is a result of suppression of DFH owing to denser surface vegetation characterized by low n factors. Prominent active frost boils are observed in the MNT at higher latitudes with more sparse vegetation characterized by higher n factors that promote DFH. However, at the northernmost field sites frost boils cannot be generated even though the n factors are relatively high owing to very rapid freezing conditions that mitigate DFH.

Combining Research and Education: Bioclimatic Zonation along a Canadian Arctic Transect

Scientists and students from five countries combined research and education in an investigation of bioclimatic zonation along a Canadian Arctic transect, from Amund Ringnes Island and Ellesmere Island in the north to the Daring Lake research camp at the southern edge of the tundra in Nunavut. We addressed three important needs in Arctic science: 1) to integrate education and research, 2) to provide field experiences for undergraduates, and 3) to foster international collaboration. We describe five subzones within the Arctic tundra zone. Subzones are defined by the vegetation typical of mesic environments at low elevations and the dominant growth forms of vegetation in these environments. Subzonal boundaries coincide with the northern limits of several species of woody plants with distinct upright or prostrate growth forms, and ultimately with the northern limit of woody plant species. The five subzones, A-E, from north to south, are characterized by dominant growth form: (A) cushion forb, (B) prostrate dwarf shrub, (C) hemiprostrate dwarf shrub, (D) erect dwarf shrub, and (E) low shrub.

Fungal and bacterial biomass in tundra soils along an arctic transect from Taimyr Peninsula, central Siberia

Microscopic analyses of tundra soils from northern central Siberia, Taimyr Peninsula (74.5°N, 98.5°E) were performed in order to investigate spatial variation of fungal and bacterial biomass. Biomass figures of fungi and bacteria (µg C g–1 dry wt.) were measured from 11 permafrost soil pits. Fungal biovolume of up to 3.5 mm3 g–1 dry wt. (median 0.19 mm3 g–1 dry wt.) and a maximum hyphal length of 393 m g–1 dry wt. (median 21 m g–1 dry wt.) were determined. Fungal biomass was found up to 455 µg C g–1 dry wt. (median 24 µg C g–1 dry wt.). The amounts generally decreased with depth but increased within organic horizons. Little fungal biomass was found in the unvegetated soils or deep horizons above the permafrost table. Bacterial counts ranged from 0.16 to 7.38*109 g–1 dry wt. and bacterial biomass ranged from 0.68 to 20.38 µg C g–1 dry wt. (median 6.19 µg C g–1 dry wt.) because of small cell volume (median 0.04 µm3). Microbial biomass was generally dominated by fungi as shown by the ratio of fungal to bacterial biomass, which was between 0 and 174.1 (median 4.5). Plant cover and soil organic matter content were found to be the important keys in understanding microbial ecology in arctic tundra soils.

Acoustic thermometry along an Arctic Ocean path

Acoustic thermometry has been shown to be a very effective technique for monitoring average heat content and average temperature in the Arctic Ocean and in particular in the Arctic Intermediate Water (AIW) layer. As part of the U.S./Russian Arctic Climate Observations using Underwater Sound (ACOUS) program a 14-month time series of acoustic transmissions were analyzed along a 1250 km propagation path that extended from the Franz Victoria Strait to the Lincoln Sea from Oct. 1998 through Dec. 1999. The receive array mooring in the Lincoln Sea was recovered in April 2001. Modal travel times were estimated after pulse compression processing and mode filtering of the vertical line array. The interarrival time between mode 1 and modes 2 and 3 show net cooling during the first several months followed by a dramatic warming of the AIW along the propagation path. This warming is consistent with direct CTD measurements made along a central Arctic transect performed by the USS Hawkbill during the Scientific Ice Exercise (SCICEX) 2000. [Work supported by ONR, NSF, the Civilian Research and Development Foundation, and the Ministry of Industry, Science and Technology of the Russian Federation.]

The controls on net ecosystem productivity along an Arctic transect: a model comparison with flux measurements.

Assessments of carbon (C) fluxes in the Arctic require detailed data on both how and why these fluxes vary across the landscape. Such assessments are complicated because tundra vegetation has diverse structure and function at both local and regional scales. To investigate this diversity, the Arctic Flux Study has used the eddy covariance technique to generate ecosystem CO2 -exchange data along a transect in northern Alaska. We use an extant process-based model of the soil-plant-atmosphere continuum to make independent predictions of gross photosynthesis and foliar respiration at 9 of the sites along the transect, using data on local canopy structure and meteorology. We make two key assumptions: (i) soil respiration is constant throughout the flux measurement period, so that the diurnal cycle in CO2 exchange is driven by canopy processes only (except at two sites where a soil respiration-temperature relationship was indicated in the data); and (ii) mosses and lichens play an insignificant role in ecosystem C exchange, even though in some locations their live biomass exceeds 300 g m-2 . We found that even with these assumptions the model could explain much of the dynamics of net ecosystem production (NEP) at sites with widely differing vegetation structure and moss/lichen cover. Errors were mostly associated with the predictions of maximum NEP; the likely cause of such discrepancies was (i) a mismatch between vegetation sampled for characterizing the canopy structure and that contained within the footprint of the eddy covariance flux measurements, or (ii) an increase in daytime soil and root respiration. Thus the model results tended to falsify our first assumption but not our second. We also note evidence for an actual reduction in NEP caused by water stress on warm, dry days at some sites. The model-flux comparison also suggests that photosynthesis may be less sensitive to low temperatures than leaf-level gas-exchange measurements have indicated.

Vegetation characteristics and primary productivity along an arctic transect: implications for scaling‐up

Summary 1 Arctic terrestrial ecosystems, which are important components of the global carbon (C) cycle, are expected to undergo considerable future climate change. However, predicting arctic C budgets is complex because the landscape is highly diverse and plant biomass very variable. 2 We investigated the diversity of vegetation at 14 sites along an arctic transect in northern Alaska, including sites on wet coastal tundra, tussock tundra, heath and shrub tundra. We applied these data to a model of the soil–plant–atmosphere (SPA) continuum, to investigate how variations in vegetation structure might affect landscape patterns of gross primary productivity (GPP). 3 Although the dominant species varied, there was a highly significant relationship for vascular plants between the leaf area index (LAI) and total foliar nitrogen (N; g m–2 ground area) at each site. 4 A modelled response surface of GPP to changes in both N and LAI indicated that for tundra plants there is an almost constant ratio between the sensitivity of productivity to the two variables. 5 The conservative nature of canopy LAI–N ratios simplifies the task of generating regional predictions of C assimilation by vascular plants (for which LAI and N are key requirements) because measurements of LAI can be used to infer N. 6 Spatial and temporal patterns of LAI and N must be determined if the photosynthetic component of regional C budgets is to be modelled accurately for diverse arctic ecosystems. Patterns of solar irradiance, together with air temperature and soil moisture constraints, are also influential but less important. The contribution of bryophytes must be accounted for as well.