Carbon Dioxide Flux from Three Arctic Tundra Types in North-Central Alaska, U.S.A.

Vegetation history of northcentral Alaska: A mapped summary of late-quaternary pollen data

Aim Describe the spatial and temporal properties of transitions in the Arctic and develop a conceptual understanding of the nature of these spatial transitions in the face of directional environmental change.Location Arctic tundra ecosystems of the North Slope of Alaska and the tundra‐forest region of the Seward Peninsula, AlaskaMethods We synthesize information from numerous studies on tundra and treeline ecosystems in an effort to document the spatial changes that occur across four arctic transitions. These transitions are: (i) the transition between High‐Arctic and Low‐Arctic systems, (ii) the transition between moist non‐acidic tundra (MNT) and moist acidic tundra (MAT, also referred to as tussock tundra), (iii) the transition between tussock tundra and shrub tundra, (iv) the transition between tundra and forested systems. By documenting the nature of these spatial transitions, in terms of their environmental controls and vegetation patterns, we develop a conceptual model of temporal dynamics of arctic ecotones in response to environmental change.Results Our observations suggest that each transition is sensitive to a unique combination of controlling factors. The transition between High and Low Arctic is sensitive primarily to climate, whereas the MNT/MAT transition is also controlled by soil parent material, permafrost and hydrology. The tussock/shrub tundra transition appears to be responsive to several factors, including climate, topography and hydrology. Finally, the tundra/forest boundary responds primarily to climate and to climatically associated changes in permafrost. There were also important differences in the demography and distribution of the dominant plant species across the four vegetation transitions. The shrubs that characterize the tussock/shrub transition can achieve dominance potentially within a decade, whereas spruce trees often require several decades to centuries to achieve dominance within tundra, and Sphagnum moss colonization of non‐acidic sites at the MNT/MAT boundary may require centuries to millennia of soil development.Main conclusions We suggest that vegetation will respond most rapidly to climatic change when (i) the vegetation transition correlates more strongly with climate than with other environmental variables, (ii) dominant species exhibit gradual changes in abundance across spatial transitions, and/or (iii) the dominant species have demographic properties that allow rapid increases in abundance following climatic shifts. All three of these properties characterize the transition between tussock tundra and low shrub tundra. It is therefore not surprising that of the four transitions studied this is the one that appears to be responding most rapidly to climatic warming.

Background: Tussock tundra is a main component of the Low Arctic vegetation cover. As it is adapted to underlying permafrost, its marginal sites at its southern distribution limit could be excellent indicators of climate change. There are still some scattered outpost stands of tussock tundra in the subarctic-alpine area of northern Fennoscandia, now showing signs of decline. Aims: The objective was to document changes in community structure of the tussock tundra over a 12-year period in experimentally warmed plots and in non-manipulated controls. In addition, the study included a survey of the present cover of tussock tundra in northern Swedish Lapland and a long-term monitoring of annual flowering intensity in the dominant species, the arctic hare's-tail cottongrass, Eriophorum vaginatum. Methods: The extent of tussock tundra in the region was assessed in a helicopter survey in 2005, followed by ground truthing in 2006. Climate and permafrost have been monitored at Latnjajaure, northern Swedish Lapland, since 1992. An experimental warming study employing open-top chambers (OTCs) was initiated at Latnjajaure for a number of habitats in 1993–1995, and all tussock tundra plots were surveyed in 1995 and 2006. Non-manipulated, permanently marked E. vaginatum tussocks were monitored for flowering frequency annually in 1992–2008. Results: The helicopter survey indicated that tussock tundra covers only few km2 in northernmost Sweden. A stand at the lower end of its altitude range was already in an advanced stage of transition into shrub tundra. In the OTCs at Latnjajaure, evergreen boreal dwarf-shrubs (particularly Vaccinium vitis-idaea) increased in above-ground biomass about eightfold between 1995 and 2006, and almost fourfold in the controls. There has been a significant warming trend in the study area of 0.12 °C per year, likely explaining why the control plots have also changed. The annual monitoring of E. vaginatum flowering indicated a tendency for relaxed synchrony of masting episodes in recent years, probably caused by longer growing seasons. Conclusions: Tussock tundra stands at the southern margin of the ecosystem's range are undergoing rapid changes at present. Increased air temperature and permafrost degradation are likely to be the main drivers of the observed change.

A custom oxygen analyzer in conjunction with an infrared carbon dioxide analyzer and humidity sensors permitted simultaneous measurements of oxygen, carbon dioxide, and water vapor fluxes from the shoots of intact barley plants (Hordeum vulgare L. cv Steptoe). The oxygen analyzer is based on a calciazirconium sensor and can resolve concentration differences to within 2 microliters per liter against the normal background of 210,000 microliters per liter. In wild-type plants receiving ammonium as their sole nitrogen source or in nitrate reductase-deficient mutants, photosynthetic and respiratory fluxes of oxygen equaled those of carbon dioxide. By contrast, wild-type plants exposed to nitrate had unequal oxygen and carbon dioxide fluxes: oxygen evolution at high light exceeded carbon dioxide consumption by 26% and carbon dioxide evolution in the dark exceeded oxygen consumption by 25%. These results indicate that a substantial portion of photosynthetic electron transport or respiration generates reductant for nitrate assimilation rather than for carbon fixation or mitochondrial electron transport.

Climate effects on the carbon balance of tussock tundra in the Philip Smith Mountains, Alaska

Shifting patterns of microbial N-metabolism across seasons in upland Alaskan tundra soils

Carbon dioxide output in laparoscopic cholecystectomy

Arctic tundra ecosystems are warming disproportionately in the winter, including a delayed autumn soil freeze‐up. Because microbial processes are extremely sensitive to change in temperature below freezing, overwinter warming strongly stimulates decomposition and nutrient mineralization and ultimately promotes the conversion of sedge‐dominated tussock tundra into shrub tundra. We characterized the responses of decomposition in shrub tundra and tussock tundra soils to changes in the rate and extent of freezing using laboratory simulations to study a range of active layer freeze‐up scenarios that included variation in time held above 0°C (an acclimation period) and final temperature (−2°C to −20°C). We hypothesized that shrub soil decomposers would be more sensitive to the rate of the freezing process than tussock soil decomposers, but less sensitive to the extent of freezing. Although freezing strongly influenced microbial processes, the effects of freezing tended to not significantly vary across the range freezing regimes tested. Unexpectedly, the tussock‐derived soil decomposer community was more sensitive than shrub‐derived soil to the freezing treatments. Freezing conditions stimulated tussock soil to release more water‐extractable organic carbon (WEOC) that was composed of a greater proportion of microbially derived materials than shrub soil under the same conditions. Freezing‐driven changes in the tussock WEOC pool coincided with reduced microbial decomposer biomass, while the shrub decomposer biomass was relatively insensitive to the freezing treatments. These findings suggest that the microbial community of shrub‐dominated soils is more resistant to the soil freezing process than the tussock‐dominated decomposer community and increases available soil N while reducing labile C release as the soils freeze. Because autumn nutrient dynamics set the stage for overwinter tundra biogeochemical conditions, increasing shrub dominance in tussock tundra may therefore promote both plant N availability and decomposer C limitation during thaw.

The Arctic Tundra Simulator (ARTUS) is a computer—based simulation model of Eriophorum vaginatum tussock tundra ecosystems found in north central Alaska. ARTUS simulates the annual patterns of heat and water balance, carbon fixation, plant growth, and nitrogen and phosphorus cycling. ARTUS runs in 1—d time steps for a growing season from 1 May to 17 September and is intended to run for several years. The abiotic section of ARTUS encodes the seasonal input of the environmental driving variables and calculates the resultant thermal and water regimes to define the heat and water environments for the tussock tundra system. The primary driving variables are daily total solar radiation, air temperature, precipitation, surface albedo, wind, and sky conditions. The soil compartment contains three organic horizons, which are recognized by their state of physical and chemical decomposition, and one mineral horizon. Six vascular plant species and four moss species are simulated. The model has seven compartments for each vascular plant species: total nonstructural carbohydrates, total nitrogen, total phosphorus, leaves grown in the current season, leaves grown in previous years, conducting and storage stems plus roots, and absorbing roots. In ARTUS the functional unit of the plant is the shoot system or ramet. Each shoot system consists of leaves, stems, fine roots (which do not have secondary growth and have a limited life—span), and larger roots, which have secondary growth and an extended life—span. Although plant processes are based on individual shoots, the ARTUS model as a whole is based on a square metre of ground. Values per square metre are calculated from the values per shoot by multiplying by the shoot density of each species. The model was validated by comparing calculated and measured peak season biomasses and nutrient contents, and the seasonal progression of environmental processes, biomass, carbohydrate contents, and nutrient contents. ARTUS successfully simulated the seasonality of the physical environment, but simulated thaw depths were deeper than those measured at all sites. The simulated value for total vascular plant production was 77% of the measured value. The simulated values for ecosystem respiration for Eagle Creek were within the range of measured values. Simulations with ARTUS indicated different patterns of growth and different storage—carbohydrate levels in deciduous shrubs, evergreen shrubs, and graminoids. The simulated seasonal course of net primary production of vascular plants and mosses was similar to the pattern measured at Eagle Creek. Sensitivity analysis using ARTUS indicated that the tussock tundra is more sensitive to external environmental factors, such as increased temperature, than to internal ecosystem variables. The development of ARTUS was limited by the unavailability of data on whole—plant carbon balance including root and stem respiration. More data are also needed on decomposition processes and nitrogen and phosphorus cycling. Adequate climatological data for northern Alaska are needed for extensive validations of the model. While caution should be used in basing managerial decisions on model simulations, ARTUS can be used to identify and quantify the magnitude and direction of plant responses to changes in state variables in the model.

In this study, the variations of the carbon dioxide fluxes were investigated with soil temperatures in the grassplot and seasonal variations of carbon dioxide concentrations and fluxes were analysed. Soil temperatures, carbon dioxide concentrations and fluxes were measured on the grassplot in Pukyong National University. Field measurements were carried out 25 times from March in 2010 to March in 2011 with nine points on the grassplot. Seasonal variations of carbon dioxide concentrations and fluxes showed an inverse relation. In summer, carbon dioxide concentrations are lower and carbon dioxide fluxes are higher. In winter, carbon dioxide concentrations are higher and carbon dioxide fluxes are lower. On the grassplot, carbon dioxide emission rate increase when the soil temperature is more than <TEX>$20^{\circ}C$</TEX> and the emission rate decrease when the soil temperatures are less than <TEX>$10^{\circ}C$</TEX>. When the accumulated rainfall for five days before measurement day is 20~100 mm, it is showed that the more rainfall, the more carbon dioxide emissions. Carbon dioxide emission rate from the grassplot to the upper atmosphere was increased or decreased by the factors such as soil temperature, growth and wither of grass and rainfall. The results of this study showed that the emission of carbon dioxide in the grassplot is dominantly controlled by seasonal factors (especially soil temperature and rainfall).

The alpine and polar climatic limit for growth of woody plants is very much dependent on the mean temperatures of the warmest three or four summer months. Tundra plants with perennating buds close to the ground are sheltered by insulating snow cover. Many tundra plants can grow at temperatures 5–10°C below 0°C and also have low optimum temperatures. Total net production of tundra plants may be as high as 900 g/m2/yr as dry weight in moist and eutrophic low alpine shrub tundra and in antarctic moss mats. The variation in tundra plant production is often observed to be greater between different stands (communities) within one locality than between localities, because of very important variation in soil moisture and nutrients between the stands. On a global scale the biomass of vascular plants increases by an order of magnitude from the climatic severe polar desert to semidesert and again from there to moist shrub tundra. The cryptogam biomass increases only 2–10 fold from polar desert to low arctic shrub tundra. To a certain limit unfavourable climatic conditions are worse to above- than to belowground plant parts. Highest root biomass compared to top (up to 20 times higher) is observed in wet monocotyledonous polar and alpine communities. In polar desert root biomass is small again, as compared to tops and also in lower latitudes and altitudes of temperate regions.

Studies of lightning-caused tundra fires were carried out between 1977 and 1983 in three areas of northwestern Alaska (Seward Peninsula [65?35'N], Noatak River [68?00'N], and Kokolik River [69?30'N]) representing a latitudinal gradient of 460 km. Postfire vegetation and permafrost recovery rates were documented in both tussock and low-shrub tundra ecosystems burned up to 10 yr prior to sampling. Within 5 to 6 yr following 1977 tundra fires, total vascular plant cover reached 50 to 1000%o of the unburned control at all sites with the slowest recovery at the northernmost Kokolik River site. This difference may be accounted for by the greater severity of burning at the Kokolik River sites where the fire occurred late in the season (1 August) and where there is a longer interval between fires. Postfire increases in soil thaw in tussock tundra appeared to stabilize or return to prefire levels within the same 5- to 6-yr time span. Many tundra plants appear to be well adapted to fire through one or a combination of strategies described. Eriophorum vaginatum tussocks may be dependent on fire for removal of competing low shrubs and mosses. Seed and seedling fluxes of certain species increase dramatically following fire. A model is presented which relates species fire survival strategies to the severity of burning and the seasonal timing of fire.

Eddy covariance serves as one the most effective techniques for long-term monitoring of ecosystem fluxes, however long-term data integrations rely on complete timeseries, meaning that any gaps due to missing data must be reliably filled. To date, many gap-filling approaches have been proposed and extensively evaluated for mature and/or less actively managed ecosystems. Random forest regression (RFR) has been shown to be stable and perform better in these systems than alternative approaches, particularly when filling longer gaps. However, the performance of RFR gap filling remains less certain in more challenging ecosystems, e.g., actively managed agri-ecosystems and following recent land-use change due to management disturbances, ecosystems with relatively low fluxes due to low signal to noise ratios, or for trace gases other than carbon dioxide (e.g., methane).In an extension to earlier work on gap filling global carbon dioxide, water, and energy fluxes, we assess the RFR approach for gap filling methane fluxes globally. We then investigate a range of gap-filling methodologies for carbon dioxide, water, energy, and methane fluxes in challenging ecosystems, including European managed pastures, Southeast Asian converted peatlands, and North American drylands.Our findings indicate that RFR is a competent alternative to existing research standard gap-filling algorithms. The marginal distribution sampling (MDS) is still suggested for filling short (< 12 days) gaps in carbon dioxide fluxes, but RFR is better for filling longer (> 30 days) gaps in carbon dioxide fluxes and also for gap filling other fluxes (e.g. sensible heat, latent energy and methane). In addition, using RFR with globally available reanalysis environmental drivers is effective when measured drivers are unavailable. Crucially, RFR was able to reliably fill cumulative fluxes for gaps > 3 moths and, unlike other common approaches, key environment-flux responses were preserved in the gap-filled data.

Understanding the carbon dioxide and water fluxes in the Arctic is essential for accurate assessment and prediction of the responses of these ecosystems to climate change. In the Arctic, there have been relatively few studies of net CO2, water, and energy exchange using micrometeorological methods due to the difficulty of performing these measurements in cold, remote regions. When these measurements are performed, they are usually collected only during the short summer growing season. We established eddy covariance flux towers in three representative Alaska tundra ecosystems (heath tundra, tussock tundra, and wet sedge tundra), and have collected CO2, water, and energy flux data continuously for over three years (September 2007–May 2011). In all ecosystems, peak CO2 uptake occurred during July, with accumulations of ∼51–95 g C/m2 during June–August. The timing of the switch from CO2 source to sink in the spring appears to be regulated by the number of growing degree days early in the season, indicating that warmer springs may promote increased net CO2 uptake. However, this increased uptake in the spring may be lost through warmer temperatures in the late growing season that promote respiration, if this respiration is not impeded by large amounts of precipitation or cooler temperatures. Net CO2 accumulation during the growing season was generally lost through respiration during the snow covered months of September–May, turning the ecosystems into net sources of CO2 over measurement period. The water balance from June to August at the three ecosystems was variable, with the most variability observed in the heath tundra, and the least in the tussock tundra. These findings underline the importance of collecting data over the full annual cycle and across multiple types of tundra ecosystems in order to come to a more complete understanding of CO2 and water fluxes in the Arctic.

Silica (SiO2) accumulation by terrestrial vegetation is an important component of the biological silica cycle because it improves overall plant fitness and influences export rates of silica from terrestrial to marine systems. However, most research on silica in plants has focused on agricultural and forested ecosystems, and knowledge of terrestrial silica cycling in the Arctic, as well as the potential impacts of climate change on the silica cycle is severely lacking. We quantified biogenic silica (BSi) accumulation in above and below‐ground portions of three moist acidic tundra (MAT) sites spanning a 300 km latitudinal gradient in central and northern Alaska, USA. We also examined plant silica accumulation across three main tundra types found in the Arctic (MAT, moist non‐acidic tundra and wet sedge tundra (WST)). Biogenic silica concentrations in live Eriophorum vaginatum, a tussock‐forming sedge that is the foundation species of tussock tundra, were not significantly (p &lt; .05) different across the three main sites. Concentrations of BSi in live above‐ground tissue were highest in the graminoid species (0.55 ± 0.07% BSi in sedges from WST, and 0.27 ± 0.01% in E. vaginatum across the three MAT sites). Both inter‐tussock tundra species and shrubs contained substantially lower BSi concentrations than E. vaginatum. Our results have implications for how shifts in vegetation cover associated with climatic warming may alter silica storage in tussock tundra vegetation. Our calculations suggest that shrub expansion via warming will increase BSi storage in Arctic land plants due to the higher biomass associated with shrub tundra, whereas conversion of tussock tundra to WST via permafrost thaw would produce the opposite effect in the terrestrial plant BSi pool. Such changes in the size of the terrestrial vegetation silica reservoir could have direct consequences for the rates and timing of silica delivery to receiving waters in the Arctic. A plain language summary is available for this article.