Niwot Ridge Research Articles

Mercury dynamics in the Rocky Mountain, Colorado, snowpack

Abstract. Gaseous elemental mercury (GEM) was monitored at the Niwot Ridge (NWT) Long-Term Ecological Research (LTER) site (Colorado, USA, 40° N) from interstitial air extracted from the snowpack at depths ranging from the snow surface to 10 cm above the soil. A highly dynamic cycling of mercury (Hg) in this mid-latitude snowpack was observed. Patterns were driven by both GEM production in surface snow and GEM destruction in the deeper snowpack layers. Thorough mixing and vertical transport processes were observed through the snowpack. GEM was photochemically produced near the snow-air interface throughout the entire winter, leading to enhanced GEM levels in interstitial air of surface snow of up to 8 ng m−3. During low-wind periods, GEM in surface snow layers remained significantly above ambient air levels at night as well, which may indicate a potential weak GEM production overnight. Analyses of vertical GEM gradients in the snowpack show that surface GEM enhancements efficiently propagated down the snowpack, with a temporal lag in peak GEM levels observed with increasing depth. Downward diffusion was responsible for much of these patterns, although vertical advection also contributed to vertical redistribution. Destruction of GEM in the lower snowpack layers was attributed to dark oxidation of GEM. Analysis of vertical GEM / CO2 flux ratios indicated that this GEM destruction occurred in the snow and not in the underlying soil. The strong, diurnal patterns of photochemical GEM production at the surface ultimately lead to re-emission losses of deposited Hg back to the atmosphere. The NWT data show that highest GEM surface production and re-emissions occur shortly after fresh snowfall, which possibly resupplies photoreducible Hg to the snowpack, and that photochemical GEM reduction is not radiation-limited as it is strong even on cloudy days.

Temporal variation of atmospheric CH4 and the potential source regions at Waliguan, China

We used a revised dataset of the atmospheric CH4 mixing ratios that were continuously measured at Mount Waliguan Baseline Observatory of western China during 2002–2006 to study the temporal variations and the potential sources regions of ambient CH4. Approximately 58% observed values were filtered as background data by applying a robust local regression mathematic procedure. The median, 10% and 90% percentiles of CH4 background mixing ratios were 1831.8, 1820.7 and 1843.5 ppb (10−9 mol mol−1), respectively; the atmospheric CH4 time series showed large variations with a fluctuation of 200 ppb (maximum minus minimum), but the background CH4 exhibited a much smaller variation (about 38 ppb), indicating strong impacts from local or regional sources and sinks. Ambient CH4 displayed significant diurnal variations in different seasons during 2002–2006, as was the combined result of changes of nearby sources/sinks and local meteorological circulation. The averaged CH4 seasonal cycle showed minimum in spring and winter, maximum in summer (from June to August), as is completely opposite to that observed at Mauna Loa and Niwot Ridge. The incongruous variation should be attributed to enhancing regional/local sources (e.g., herd) around the site as well as the dominant air flow from the polluted north/northeastern region (e.g., Xining and Lanzhou city) in summer. Another possible reason is, compared to Mauna Loa and Niwot Ridge, the photochemical reaction at Waliguan is weaker. An analysis of 5 d back-trajectory (500 hPa) showed that the episode of high CH4 mixing ratios was associated with advection from the heavily populated regions east or southeast (e.g., Xining and Lanzhou) of Waliguan and northwest of Qinghai via Ge’ermu urban area where growing industrial activities as well as crops residue burning provide large sources of CH4, suggesting a large source area; the low values were observed most frequently when air masses originated from the sparsely populated Tibet and south of Qinghai and Xinjiang Uygur Autonomous Region (XUAR). Additionally, atmospheric CH4 would be enhanced when air masses originated from Ningxia and northwest of Gansu areas (mainly growing rice) along the Yellow river region, especially in summer, suggesting agriculture source of CH4. Our study helps to accurately estimate sources and sinks of CH4, and thus is significant for understanding greenhouse effect over a regional scale and predicting global change in future.

Initial Results of an Intercomparison of AMS-Based Atmospheric 14CO2 Measurements

This article presents results from the first 3 rounds of an international intercomparison of measurements of Δ14CO2 in liter-scale samples of whole air by groups using accelerator mass spectrometry (AMS). The ultimate goal of the intercomparison is to allow the merging of Δ14CO2 data from different groups, with the confidence that differences in the data are geophysical gradients and not artifacts of calibration. Eight groups have participated in at least 1 round of the intercomparison, which has so far included 3 rounds of air distribution between 2007 and 2010. The comparison is intended to be ongoing, so that: a) the community obtains a regular assessment of differences between laboratories; and b) individual laboratories can begin to assess the long-term repeatability of their measurements of the same source air. Air used in the intercomparison was compressed into 2 high-pressure cylinders in 2005 and 2006 at Niwot Ridge, Colorado (USA), with one of the tanks “spiked” with fossil CO2, so that the 2 tanks span the range of Δ14CO2 typically encountered when measuring air from both remote background locations and polluted urban ones. Three groups show interlaboratory comparability within l% for ambient level Δ14CO2. For high CO2/low Δ14CO2 air, 4 laboratories showed comparability within 2%. This approaches the goals set out by the World Meteorological Organization (WMO) CO2 Measurements Experts Group in 2005. One important observation is that single-sample precisions typically reported by the AMS community cannot always explain the observed differences within and between laboratories. This emphasizes the need to use long-term repeatability as a metric for measurement precision, especially in the context of long-term atmospheric monitoring.

Initial Results of an Intercomparison of AMS-Based Atmospheric 14CO2 Measurements

This article presents results from the first 3 rounds of an international intercomparison of measurements of Δ 14CO2 in liter-scale samples of whole air by groups using accelerator mass spectrometry (AMS). The ultimate goal of the intercomparison is to allow the merging of Δ 14CO2 data from different groups, with the confidence that differences in the data are geophysical gradients and not artifacts of calibration. Eight groups have participated in at least 1 round of the intercomparison, which has so far included 3 rounds of air distribution between 2007 and 2010. The comparison is intended to be ongoing, so that: a) the community obtains a regular assessment of differences between laboratories; and b) individual laboratories can begin to assess the long-term repeatability of their measurements of the same source air. Air used in the intercomparison was compressed into 2 high-pressure cylinders in 2005 and 2006 at Niwot Ridge, Colorado (USA), with one of the tanks “spiked” with fossil CO2, so that the 2 tanks span the range of Δ 14CO2 typically encountered when measuring air from both remote background locations and polluted urban ones. Three groups show interlaboratory comparability within 1‰ for ambient level Δ 14CO2. For high CO2/low Δ 14CO2 air, 4 laboratories showed comparability within 2‰. This approaches the goals set out by the World Meteorological Organization (WMO) CO2 Measurements Experts Group in 2005. One important observation is that single-sample precisions typically reported by the AMS community cannot always explain the observed differences within and between laboratories. This emphasizes the need to use long-term repeatability as a metric for measurement precision, especially in the context of long-term atmospheric monitoring. DOI: 10.2458/azu_js_rc.55.16382

Allocation of Terrestrial Carbon Sources Using 14CO2: Methods, Measurement, and Modeling

The radiocarbon content of whole air provides a theoretically ideal and now observationally proven tracer for recently added fossil-fuel-derived CO2 in the atmosphere (Cff). Over large industrialized land areas, determination of Cff also constrains the change in CO2 due to uptake and release by the terrestrial biosphere. Here, we review the development of a Δ14CO2 measurement program and its implementation within the US portion of the NOAA Global Monitoring Division's air sampling network. The Δ14CO2 measurement repeatability is evaluated based on surveillance cylinders of whole air and equates to a Cff detection limit of <0.9 ppm from measurement uncertainties alone. We also attempt to quantify additional sources of uncertainty arising from non-fossil terms in the atmospheric 14CO2 budget and from uncertainties in the composition of “background” air against which Cff enhancements occur. As an example of how we apply the measurements, we present estimates of the boundary layer enhancements of Cff and Cbio using observations obtained from vertical airborne sampling profiles off of the northeastern US. We also present an updated time series of measurements from NOAA GMD's Niwot Ridge site at 3475 m asl in Colorado in order to characterize recent Δ14CO2 variability in the well-mixed free troposphere.

Limnology of the Green Lakes Valley: phytoplankton ecology and dissolved organic matter biogeochemistry at a long-term ecological research site

ABSTRACTBackground: Surface waters are the lowest points in the landscape, and therefore serve as excellent integrators and indicators of changes taking place in the surrounding terrestrial and atmospheric environment.Aims: Here we synthesise the findings of limnological studies conducted during the past 15 years in streams and lakes in the Green Lakes Valley, which is part of the Niwot Ridge Long Term Ecological Research (LTER) Site.Methods: The importance of these studies is discussed in the context of aquatic ecosystems as indicators, integrators, and regulators of environmental change. Specifically, investigations into climatic, hydrologic, and nutrient controls on present-day phytoplankton, and historical diatom, community composition in the alpine lake, Green Lake 4, are reviewed. In addition, studies of spatial and temporal patterns in dissolved organic matter (DOM) biogeochemistry and reactive transport modelling that have taken place in the Green Lakes Valley are highlighted.Results and conclusions: The findings of these studies identify specific shifts in algal community composition and DOM biogeochemistry that are indicative of changing environmental conditions and provide a framework for detecting future environmental change in the Green Lakes Valley and in other alpine watersheds. Moreover, the studies summarised here demonstrate the importance of long-term monitoring programmes such as the LTER programme.

Response of soil organic and inorganic nutrients in alpine soils to a 16-year factorial snow and N-fertilization experiment, Colorado Front Range, USA

Alpine ecosystems are thought to be particularly sensitive to small environmental changes in climate and other parameters due to the plants and soil organisms being on the edge of environmental tolerances. Snow distribution is critical to microclimate in the alpine, affecting soil temperature, growing season duration, and nutrient cycling. Moreover anthropogenic nitrogen (N) deposition over the past half century has had a detrimental impact on temperate ecosystems, resulting in soil acidification and a reduction in plant biodiversity. Here we used a snowfence experiment combined with an N-fertilization experiment, at the NSF-funded Niwot Ridge (NWT) Long-Term Ecological Research (LTER) site (3528m ASL), to increase our understanding of how changes in snow properties and N deposition may affect soil processes. The snowfence used in this manipulation resulted in a consistent pattern of snow accumulation, from deep snowpacks near the fence to a shallow snowpack away from the snowfence. As measured after 16 years of the experiment, the amount, timing, and duration of snow cover appears to affect soil properties. Under moderate snow cover and without N addition, the total soil organic carbon (TOC) and total nitrogen (TN) were significantly greater than either under deep or shallow snow. Nitrogen amendments in general worked in the opposite direction of snowpack controls on soil processes. The N addition caused a significant increase under the shallow snow treatments for TOC and TN, while there was a significant decrease of these properties under the moderate snow treatment. In the latter case the N addition did not cause any significant effect on the inorganic N forms but was correlated with a decline of soil pH, and a consequent increase of exchangeable Al and a reduction of exchangeable base cations, which may have influenced soil microbial biomass found in this study. Our results demonstrate how long-term changes in snow properties and N deposition may interact in affecting alpine soil characteristics, with an important response of soil nutrients.

No inter-hemispheric δ13CH4 trend observed

Arising from Kai, F. M., Tyler, S. C., Randerson, J. T. & Blake, D. R. , 194–197 (2011)10.1038/nature10259 To understand the reasons for global CH4 changes since the 1990s, Kai et al.1 evaluated a combined record of observed tropospheric CH4 mole fractions as well as δ13C-CH4 and δD-CH4 measurements from mid latitudes of both hemispheres. Their data set shows a strongly decreasing inter-hemispheric difference (IHD) in δ13C-CH4 from −0.24 ± 0.11‰ during 1989–1993 to −0.10 ± 0.04‰ during 2001–2005, which they largely attribute to decreasing rice emissions during the last two decades. Here we show that the experimental data used by Kai et al.1 are probably biased because the authors used only one continental mountain station (Niwot Ridge, Colorado, USA) as representative of the entire Northern Hemisphere, thereby incorrectly determining the IHD of δ13C-CH4. Our more comprehensive data set—representing background air in both hemispheres—shows that the IHD of δ13C-CH4 has changed by less than 0.05‰ since 1990; the conclusion drawn by Kai et al.1 of a significant reduction of NH microbial sources is, thus, not constrained by our data.

Kai, Tyler, Randerson & Blake reply

Replying to Levin, I. et al. 10.1038/nature11175 (2012) Levin et al.1 report new methane data from Antarctica measured at the University of Heidelberg and combine these data with observations from two other networks to estimate long-term trends and the interhemispheric difference (IHD) in methane isotopes. These observations and the author’s analysis suggest that the δ13C IHD has been mostly uniform over the last three decades. This contrasts with the Niwot Ridge, Baring Head and Pacific Ocean time series that we analysed2 and suggests that both fossil fuel and microbial sources remained mostly constant during the late 1980s and 1990s, following a trajectory of emissions that is more similar to the control scenario in our analysis (Fig. 2 and Supplementary Fig. 2 of Kai et al.2). The estimate of δ13C inter-hemispheric difference is sensitive to different methods and selected observed sites. We note that the inter-hemispheric differences between Barrow and Cape Grim/Baring Head and between MLO and Cape Grim/Baring Head in Fig. 1 of Levin et al.1 were largest near the beginning of the record (circa 1990) and are broadly consistent with comparisons we made in the Supplementary Information of our paper (Supplementary Fig. 9)2. Important next steps, in our opinion, are (1) to use all available observations from both the Kai et al.2 and Levin et al.1 studies to assess trends in methane isotopes and their dependence on the inclusion or exclusion of individual time series, and (2) to quantify changes in microbial and fossil sources consistent with the expanded set of observations presented by Levin et al.1. Even with a levelling off of both fossil and microbial sources during 1990–2005, our forward biogeochemical modelling analysis of rice agriculture provides evidence that emissions from this sector of the budget declined as a consequence of changes in fertilizer application and water management.

Energy and surface moisture seasonally limit evaporation and sublimation from snow-free alpine tundra

This study highlights the importance of landscape position and resultant snow accumulation to the hydrologic balance of snow-free alpine tundra, and suggests that modeling studies must account for seasonally dissimilar partitioning of the energy balance in order to accurately predict evaporation and/or sublimation. The eddy covariance method was used to measure the surface energy balance above high-elevation (3502m above sea level) alpine tundra at Niwot Ridge, CO, over 3 years from 2007 to 2009. During the winter the site was characterized by wind scour, with little snow accumulation. Two co-located towers afforded the opportunity to constrain the influence of complex mountain topography on measurement uncertainty, and overall errors were comparable to other FLUXNET sites. Random measurement uncertainty for the turbulent fluxes was approximately 10% of midday summertime values. The 0.5-h mean energy balance closure was 81% over the entire measurement period, and improved to 91% during the summer when the magnitude of the turbulent fluxes was larger. In spite of 955mm mean annual precipitation, the 24-h mean evaporative fraction was 0.39, typical of dry grassland or rangeland ecosystems. These low values were attributed to rapid, efficient removal of snow by prevailing windy conditions throughout the winter. During the summer when rainfall provided moisture, evaporation was principally limited by available energy. Overall, an average of 39% of annual precipitation was evaporated or sublimated back to the atmosphere. We conclude that the annual distribution of precipitation is an essential control on evaporation and sublimation from this ecosystem.

Phenological Changes in Alpine Plants in Response to Increased Snowpack, Temperature, and Nitrogen

Modified environmental conditions are driving phenological changes in ecosystems around the world. Many plants have already responded to warmer temperatures by flowering earlier and sustaining longer periods of growth. Changes in other environmental factors, like precipitation and atmospheric nitrogen (N) deposition, may also influence phenology but have been less studied. Alpine plants may be good predictors of phenological response patterns because environmental changes are amplified in mountain ecosystems and extreme conditions may make alpine plants particularly sensitive to changes in limiting factors like precipitation, temperature, and N. We tested the effects of increased snowpack, temperature, and N on alpine tundra plant phenology, using snow fence, open-top warming chamber, and N fertilization treatments at the Niwot Ridge Long Term Ecological Research (LTER) site. Flowering phenology of three abundant species was recorded during two growing seasons. Treatment responses varied among species and functional types. Forbs responded to warming by flowering earlier and responded to snowpack and N by flowering later; however, when both snow and N were increased simultaneously, phenology was unchanged. Graminoids flowered earlier in response to N addition. Our results demonstrate that changing environmental conditions influence plant phenology, and specifically highlight that N and multiple factor interactions can yield stronger responses than warming alone.

Comments on 'Possible contribution of triboelectricity to snow - air interactions'

There have been numerous new discoveries of chemical activity at the snow–atmosphere interface over the last 20 years. These observations have stimulated an increasing body of research on snow chemical systems. The findings from this research have led to a general consensus that photochemical processes are the determining control of this snow chemistry. Traditional gasphase chemical reactions have fallen short of fully explaining observed behaviour. Heterogeneous and quasi-liquid layer chemistry has been postulated to play a role in accounting for the discrepancies. The Tkachenko and Kozachkov article is therefore a timely presentation of new ideas for further elucidating snowpack chemical mechanisms. In the following discussion we present recent data relevant to testing hypothesis proposed by Tkachenko and Kozachkov. Some of the hypotheses presented in this paper build upon data from measurements of ozone (O3) in interstitial air of the deep, glacial snowpack at Summit, Greenland, that were presented in our previous work. These experiments reported on positive O3 atmosphere–snowpack gradients, i.e. depleted O3 within the snowpack, and their dynamical behaviour dependent on environmental conditions. We have since taken this research to other locations and further investigated the dependencies of O3 within snow on a variety of parameters. A particular interest of this new research was to further investigate snowpack chemistry in environments with different snowpack conditions (year-round dry, polar snow; seasonal mid-latitude snowpack; snow over permafrost; snow over frozen freshwater lakes). We have also conducted a further 2 years of experiments at Summit, where the measurement depth in the snowpack was increased to 2.5m and experiments were extended year round to include winter observations. Another environment with polar year-round snowpack over glacial ice was investigated at South Pole, Antarctica. From the comparison with these other sites it has become obvious that the Summit snowpack is somewhat unique in that (1) O3 levels within the snowpack are generally higher than observed at other locations, and (2) interstitial air O3 shows a muchmore dynamic dependence on time of day and season than at the other locations that we have investigated. This behaviour might be in agreement with some hypotheses presented by Tkachenko and Kozachkov as they propose that O3 production by the triboelectrical effect would be most pronounced at sites characterised by a thick, glacial snowpack, low humidity, periodic high wind speeds, and low temperatures; all conditions that are typical for Summit. Our series of experiments have shown that different processes dominate snowpack chemistry in non-glacial snowpack. For example, one definite influence on O3 reactions in seasonal, midlatitude snowpack is the role of NO emitted from microbial processes in the subniveal soil. These influences were evident at a high alpine site at Niwot Ridge, CO, in the RockyMountains; snowpack under a canopy at the University of Michigan Biological Station (B. Seok, D. Helmig, M. W. Williams, C. Vogel, P. Curtis, unpubl. data); and snowpack over permafrost at Toolik Field Station, AK (B. Van Dam, D. Helmig, R. Honrath, L. Kramer, C. Toro, unpubl. data). For the Summit snowpack, Helmig et al. noted that O3 atmosphere–snowpack concentration gradients show three

An Energy Balance Snowmelt Model for Application at a Continental Alpine Site

The objectives of this study are to measure and evaluate the energy balance of a continental alpine snowpack during spring snowmelt conditions, to evaluate the performance of a point energy and mass balance model of a snow cover in alpine conditions. The investigation is conducted during the 1997 snowmelt season at Niwot Ridge in the Colorado Front Range. Further comparisons of the modeled results to the previously described measured results are made below the energy budget terms. The fluxes are corroborated using a point energy and mass balance model for a snowpack to determining snowpack energy exchanges, with minor differences found between flux magnitudes. This comparison suggests that the representation of internal snowpack energy and mass exchange processes is generally orrect.

Snow measurement by GPS interferometric reflectometry: an evaluation at Niwot Ridge, Colorado

AbstractSnow is a critical storage component in the hydrologic cycle, but current measurement networks are sparse. In addition, the heterogeneity of snow requires surveying larger areas to measure the areal average. We presented snow measurements using GPS interferometric reflectometry (GPS‐IR). GPS‐IR measures a large area (~100 m2), and existing GPS installations around the world have the potential to expand existing snow measurement networks. GPS‐IR uses a standard, geodetic GPS installation to measure the snow surface via the reflected component of the signal. We reported GPS‐IR snow depth measurements made at Niwot Ridge, Colorado, from October 2009 through June 2010. This site is in a topographic saddle at 3500 m elevation with a peak snow depth of 1.7 m near the GPS antenna. GPS‐IR measurements are compared with biweekly snow surveys, a continuously operating scanning laser system and an airborne light detection and ranging (LIDAR) measurement. The GPS‐IR measurement of peak snowpack (1.36–1.76 m) matches manual measurements (0.95–1.7 m) and the scanning laser (1.16 m). GPS‐IR has RMS error of 13 cm (bias = 10 cm) compared with the laser, although differences between the measurement locations make comparison imprecise. Over the melt season, when the snowpack is more homogenous, the difference between the GPS‐IR and the laser is reduced (RMS = 9 cm, bias = 6 cm). In other locations, the GPS and the LIDAR agree on which areas have more or less snow, but the GPS estimates more snow on the ground on tracks to the west (1.58 m) than the LIDAR (1.14 m). Copyright © 2011 John Wiley & Sons, Ltd.

Atmospheric CO<sub>2</sub> monitoring with single-cell NDIR-based analyzers

Abstract. We describe CO2 concentration measurement systems based on relatively inexpensive single-cell non-dispersive infrared CO2 sensors. The systems utilize signal averaging to obtain precision (1-σ in 100 s) of 0.1 parts per million dry air mole fraction (ppm), frequent calibrations and sample drying in order to achieve state-of-the-art compatibility, and can run autonomously for months at a time. Laboratory tests indicate compatibility among four to six systems to be ±0.1 ppm (1-σ), and field measurements of known reference-gases yield median errors of 0.01 to 0.17 ppm with 1-σ variance of ±0.1 to 0.2 ppm. From May to August 2007, a system co-located with a NOAA-ESRL dual-cell NDIR system at the WLEF tall tower in Wisconsin measured daytime-only daily averages of CO2 that differ by 0.26 ± 0.15 ppm (median ± 1 σ), and from August 2005 to April 2011 a system co-located with weekly NOAA-ESRL network flask collection at Niwot Ridge, Colorado measured coincident CO2 concentrations that differed by −0.06 ± 0.30 ppm (n = 585). Data from these systems are now supporting a wide range of analyses and this approach may be applicable in future studies where accuracy and initial cost of the sensors are priorities.

A statistical proxy for sulphuric acid concentration

Abstract. Gaseous sulphuric acid is a key precursor for new particle formation in the atmosphere. Previous experimental studies have confirmed a strong correlation between the number concentrations of freshly formed particles and the ambient concentrations of sulphuric acid. This study evaluates a body of experimental gas phase sulphuric acid concentrations, as measured by Chemical Ionization Mass Spectrometry (CIMS) during six intensive measurement campaigns and one long-term observational period. The campaign datasets were measured in Hyytiälä, Finland, in 2003 and 2007, in San Pietro Capofiume, Italy, in 2009, in Melpitz, Germany, in 2008, in Atlanta, Georgia, USA, in 2002, and in Niwot Ridge, Colorado, USA, in 2007. The long term data were obtained in Hohenpeissenberg, Germany, during 1998 to 2000. The measured time series were used to construct proximity measures ("proxies") for sulphuric acid concentration by using statistical analysis methods. The objective of this study is to find a proxy for sulfuric acid that is valid in as many different atmospheric environments as possible. Our most accurate and universal formulation of the sulphuric acid concentration proxy uses global solar radiation, SO2 concentration, condensation sink and relative humidity as predictor variables, yielding a correlation measure (R) of 0.87 between observed concentration and the proxy predictions. Interestingly, the role of the condensation sink in the proxy was only minor, since similarly accurate proxies could be constructed with global solar radiation and SO2 concentration alone. This could be attributed to SO2 being an indicator for anthropogenic pollution, including particulate and gaseous emissions which represent sinks for the OH radical that, in turn, is needed for the formation of sulphuric acid.

Observing Climate at High Elevations Using United States Climate Reference Network Approaches

AbstractThe U.S. Climate Reference Network (USCRN) was deployed between 2001 and 2008 for the purpose of yielding high-quality and temporally stable in situ climate observations in pristine environments over the twenty-first century. Given this mission, USCRN stations are engineered to operate largely autonomously with great reliability and accuracy. A triplicate approach is used to provide redundant measurements of temperature and precipitation at each location, allowing for observations at a specific time to be compared for quality control. This approach has proven to be robust in the most extreme environments, from extreme cold (−49°C) to extreme heat (+52°C), in areas of heavy precipitation (4700 mm yr−1), and in locations impacted by strong winds, freezing rain, and other hazards. In addition to a number of stations enduring extreme winter environments in Alaska and the northern United States, seven of the USCRN stations are located at elevations over 2000 m, including stations on Mauna Loa, Hawaii (3407 m) and on Niwot Ridge above Boulder, Colorado (2996 m). The USCRN temperature instruments and radiation shield have also been installed and run successfully at a station on the Quelccaya Ice Cap in Peru (5670 m). This paper reviews the performance of the USCRN station network during its brief lifetime and the potential utility of its triplicate temperature instrument configuration for measuring climate change at elevation.

Multi-Decadal Changes in Tundra Environments and Ecosystems: Synthesis of the International Polar Year-Back to the Future Project (IPY-BTF)

Understanding the responses of tundra systems to global change has global implications. Most tundra regions lack sustained environmental monitoring and one of the only ways to document multi-decadal change is to resample historic research sites. The International Polar Year (IPY) provided a unique opportunity for such research through the Back to the Future (BTF) project (IPY project #512). This article synthesizes the results from 13 papers within this Ambio Special Issue. Abiotic changes include glacial recession in the Altai Mountains, Russia; increased snow depth and hardness, permafrost warming, and increased growing season length in sub-arctic Sweden; drying of ponds in Greenland; increased nutrient availability in Alaskan tundra ponds, and warming at most locations studied. Biotic changes ranged from relatively minor plant community change at two sites in Greenland to moderate change in the Yukon, and to dramatic increases in shrub and tree density on Herschel Island, and in subarctic Sweden. The population of geese tripled at one site in northeast Greenland where biomass in non-grazed plots doubled. A model parameterized using results from a BTF study forecasts substantial declines in all snowbeds and increases in shrub tundra on Niwot Ridge, Colorado over the next century. In general, results support and provide improved capacities for validating experimental manipulation, remote sensing, and modeling studies.

Forecasting Alpine Vegetation Change Using Repeat Sampling and a Novel Modeling Approach

Global change affects alpine ecosystems by, among many effects, by altering plant distributions and community composition. However, forecasting alpine vegetation change is challenged by a scarcity of studies observing change in fixed plots spanning decadal-time scales. We present in this article a probabilistic modeling approach that forecasts vegetation change on Niwot Ridge, CO using plant abundance data collected from marked plots established in 1971 and resampled in 1991 and 2001. Assuming future change can be inferred from past change, we extrapolate change for 100 years from 1971 and correlate trends for each plant community with time series environmental data (1971-2001). Models predict a decreased extent of Snowbed vegetation and an increased extent of Shrub Tundra by 2071. Mean annual maximum temperature and nitrogen deposition were the primary a posteriori correlates of plant community change. This modeling effort is useful for generating hypotheses of future vegetation change that can be tested with future sampling efforts.

Nitrogen deposition decreases acid buffering capacity of alpine soils in the southern Rocky Mountains

Deposition of anthropogenic N can potentially alter biogeochemistry of ecosystems, acidifying soils and surface waters, lowering availability of some nutrient cations, and increasing concentrations of toxic metals. Remote regions in western North America are exhibiting symptoms of ecological change due to N deposition. We used an existing N addition experiment to empirically estimate the thresholds for the loss of acid buffering capacity and nutrient base cations, decreases in pH, as well as increases in toxic metals in response to N deposition in alpine soils of Niwot Ridge in the southern Rocky Mountains. Soil acid buffering capacity was evaluated using laboratory acid titration, and concentrations of extractable cations and soil pH were evaluated in field-collected soils from plots at ambient deposition (8 kg N ha − 1 yr − 1 ), and plots with N added at 20, 40, and 60 kg per hectare per year above the ambient level. Soil acid buffering capacity decreased with increasing N inputs (40% decrease at highest input), and was associated with a decrease in pH, loss of extractable magnesium and increases in aluminum and manganese. The threshold at which acidification occurred was around 28 kg N ha − 1 yr − 1 . Significant variation in soil acid buffering capacity, base cation concentrations, and pH was noted among experimental blocks, independent of treatment effects, possibly reflecting variation in the amount of winter snow cover and its influence on inputs of anthropogenic N, and differences in local dust deposition. The results of this study indicate granitic-derived alpine soils of the southern Rocky Mountains are susceptible to acidification at relatively modest levels of N deposition.