INSTAAR Research Highlights

See also: Science Spotlights - Short examples of INSTAAR research, education, and societal outreach.

Like all archaeological issues, the peopling of the New World must be placed in an environmental context. INSTAAR's Scott Elias has been studying the environmental conditions that may have played the dominant role in shaping the timing and direction of human migration into Alaska from Siberia. Archaeological evidence indicates that the first human migration into Alaska was across the Bering Land Bridge, a broad continental shelf region between Alaska and Siberia that was dry land during the last glaciation when eustatic sea level was low. Human migration occurred as regional climates warmed at the end of the last glacial, about 12,000 years ago. The cold and arid full-glacial climate that the interval 28,000-14,000 years BP appears to have kept Alaska essentially treeless, with no evidence of human inhabitation. Between 12,000 and 10,000 years BP, an interval of accelerated environmental change, bands of hunter-gatherers became established throughout the regions north of the Alaska Range. Flooding of the Bering Land Bridge brought warm Pacific waters into the Arctic Ocean, establishing oceanic circulation patterns that had been blocked for about 80,000 years. In much of Eastern Beringia (unglaciated regions of Alaska and the Yukon Territory), continental climates gave way to more moderate maritime climates. On the basis of fossil insect assemblages, Elias estimates that, by 11,000 years BP, average summer temperatures in Arctic Alaska rose to as much as 7ºC warmer than they are today. This warming was followed by an abrupt reversal, synchronous with the Younger Dryas oscillation in the North Atlantic region. Many large Pleistocene mammals became extinct around this time, forcing people to adopt new hunting strategies and seek different quarries. It remains unclear how directly human hunting contributed to the extinction of megafauna in the New World.

The Bering Land Bridge has long been invoked to explain migrations of terrestrial mammals and humans between Asia and North America during the Pleistocene. However, a growing body of data suggests that the earliest human migrations to North America may have occurred by watercraft along the northwest coast of North America, rather than via a postulated ice-free corridor between the Cordilleran and Laurentide ice sheets. INSTAAR fellow James Dixon is explicitly testing the coastal migration hypothesis through detailed investigations of a remote cave in the Tongass National Forest on Prince of Wales Island, southeast Alaska. In collaboration with paleontologist Timothy Heaton (University of South Dakota), ongoing excavations at site 49-PET-408 have contributed significantly to the mode of initial human colonization of North America. For example, the 1996 discovery of the human remains of an adult male, dated to 9200 BP by AMS 14C, represents the oldest reliably dated human remains from anywhere in Alaska or Canada. Isotopic analysis of bone indicates that the human had a diet based primarily on marine foods. The remains are associated with stone tools including microblades, projectile points, and knives. The presence of exotic lithic materials of distant provenance strongly suggests the use of watercraft, implying that early peoples were engaged in trade and prepared to travel long distances to collect obsidian and other rare stone types. Since this initial discovery, evidence of an even older occupation at the cave has been found. A bone tool, possibly an awl or punch, has been dated to 10,300 BP, making 49-PET-408 the oldest archaeological site on the northwest coast of North America. Ongoing excavations include the participation of scientists, native interns from southeast Alaska, high school, undergraduate, and graduate students, as well as volunteers from across the United States. Research at the site has attracted reporters and film makers from around the world.

In contrast to the Americas, Australia was colonized well before the last glacial maximum. Estimates of initial colonization lie beyond the limit of radiocarbon dating, between 55,000 and 60,000 years BP. Gifford Miller and collaborators have been working to evaluate both the chronology of Pleistocene climate change in this region, as well as the impact of early humans on ecosystem structure. Studying the record of the closed Lake Eyre basin which internally drains large sectors of the continent's interior, they have reconstructed changes in the intensity of monsoonal rainfall over the past 150,000 years. The team has found that at the time of human colonization, the climate over most of the Australian interior was wetter than at any time subsequently. When humans first arrived, the continent was also populated by a diverse array of large marsupials and flightless birds, most of which rapidly became extinct, despite equable climates during this time. The strength of the data in support of humans being responsible for Australian megafaunal extinctions has largely settled a debate that had lasted more than a century. This extinction was even more dramatic than the North American counterpart, with the loss of 60 species, including every marsupial larger than human (19 species). Miller’s team has focused on one member of the extinct megafauna, Genyornis newtoni, a large, ostrich-sized bird that inhabited much of the semi-arid zone, nesting in sand hills near inland lakes. The eggshells of this bird are the most ubiquitous and best-preserved Quaternary fossils in the outback. Miller’s group has now analyzed fossil amino acid ratios from more than 1000 Genyornis eggshells from seven different regions of the outback. Eggshell amino acid racemization kinetics have been carefully studied experimentally, and many fossil samples have been independently dated. This lends confidence to the finding that Genyornis disappeared suddenly and synchronously throughout the outback, about 50,000 years ago. Deposits with Genyornis eggshell often contain the bones of other elements of extinct megafauna, whereas deposits that postdate Genyornis extinction are devoid of these remains, implying that the well-dated Genyornis extinction is representative of Australian megafaunal extinction in general. Although hunting pressures remain a distinct possibility, it is equally likely that systematic burning by early humans disrupted the landscape to the extent that animals with highly specialized diets became extinct, while generalists survived.

Both early and late representatives of the genus Homo evolved in tropical and subtropical environments, only subsequently dispersing to latitudes above 45ºN. The earliest high-latitude Homo settlements are from western Europe, where the effects of warm ocean currents ameliorated climate relative to the colder and drier regions of Eastern Europe and Siberia, which were not colonized until well after 250,000 years BP. INSTAAR Associate John Hoffecker has been working with Russian colleagues on the problem of hominid adaptation to these environments during the Middle and Late Pleistocene. The analysis of large mammal remains from Treugol’naya Cave in the northern Caucasus provides new insights into foraging strategy and diet from the northern margin of the hominid range (44ºN) prior to 250,000 years ago. Taphonomic studies of the Treugol’naya fauna show little evidence of hominid hunting or central-place foraging, and a heavier reliance on plant foods. Coupled to the apparent lack of morphological or technological adaptations to cold temperature, this reliance on plant foods probably excluded Homo populations from northern regions outside western Europe until the appearance of Neanderthals and anatomically-modern humans. At the end of the Middle Pleistocene, Neanderthals colonized many parts of Eastern Europe. They exhibit an extreme cold-adapted morphology and evidence for central-place foraging and the efficient hunting of large mammals. Hoffecker is now leading studies at Mezmaiskaya Cave in the northern Caucasus, which is revealing a sharp contrast with the pre-Neanderthal occupation, including evidence for hunting of bison, sheep, and other large mammals, as well as a foraging strategy that probably entailed advanced planning and scheduling of seasonal resource use. After 40,000 years BP, European Neanderthals were replaced by modern humans, who exhibit a tropical morphology reflecting their recent African ancestry, but having successfully colonized northern latitudes during the Last Glacial thought the use of innovative technologies such as tailored fur clothing and insulated shelters.

Hoffecker, J. F., 1999: Neanderthals and modern humans in Eastern Europe. Evolutionary Anthropology, 7 :129-141.

Miller, G. H., Magee, J. W., Johnson, B. J., Fogel, M., Spooner, N. A., McCulloch, M. T., and Ayliffe, L. K., 1999: Pleistocene extinction of Genyornis newtoni: human impact on Australian megafauna. Science, 283: 205-208.

In recent years, humans have dramatically altered several key global biogeochemical cycles. The effects of humans on the carbon cycle, an integral component of life on Earth and an important part of the Earth's climate system, have received by far the most attention. Atmospheric CO2 levels are now higher than at any time in the recent geologic history of earth, and are rising at rates that are an order of magnitude greater than anything seen in the paleorecord. CO2 is a greenhouse gas which alters the radiative balance of the atmosphere, and therefore earth's climate. It has been argued that we have already seen climate change due to rising CO2 levels, and numerous future projections suggest incipient changes that include higher mean temperatures, significant redistributions of precipitation, a far greater incidence of severe, damaging storms, and perhaps most worrisome, strong nonlinear behavior in the global climate system. Moreover, changes in atmospheric CO2 can directly affect the growth and distribution of both plant and animal life, with cascading potential feedbacks to not only the climate system, but also to the dynamics of natural and managed ecosystems on which we rely. However, while combustion of fossil fuels for energy is the major contributor to our rising atmospheric CO2 levels, this energy also fuels the global economic engine and is one of the primary factors in molding foreign policies. Thus, environmental concerns over a changing carbon cycle have created virtually unprecedented discussions and debates at the highest levels of governments throughout the world.

Several INSTAAR scientists have been actively involved for years in key research on the global carbon cycle, and some of these individuals are international leaders in this arena. Our first clear picture that humans were changing the global carbon cycle came decades ago from repeated measurements of CO2 in the atmosphere, and this early effort has now expanded to include a multisite global network for monitoring CO2 and several other gases in the atmosphere. The majority of this network is run by the carbon cycle group at NOAA here in Boulder, and scientists from NOAA and INSTAAR have a long history of collaboration in analyzing data from the network. Flasks are filled with air at sites around the world every two weeks, and shipped to Boulder. Gas concentrations are measured at NOAA, but subsamples from all flasks are also sent to Jim White’s Stable Isotope Laboratory at INSTAAR. For more than a decade now, White’s laboratory has been measuring the 13C content (and more recently the 18O content) of the CO2 in these flasks, and this data has proven to be enormously useful in understanding the complex dynamics of a changing global carbon cycle. For example, we have known for years that approximately half of the carbon emitted to the atmosphere by human activities is being stored in terrestrial and/or oceanic realms. The long-term implications of land vs. ocean sinks for anthropogenic CO2 are vastly different, thus determining how much of the so-called "missing carbon" is going into each major reservoir has been an enduring and critical question. Since land-atmosphere and ocean-atmosphere exchanges of CO2 create have very different effects on the 13C content of the CO2, the data from White’s laboratory has allowed both White’s group and others around the world to estimate land vs. ocean carbon sinks. More recently, John Miller and Dominic Ferretti have been developing state-of-the-art analytical techniques to expand the isotopic analyses from the flask samples to include 13C of methane and deuterium measurements of water vapor.

A recent collaboration between White, Alan Townsend, Greg Asner from Geological Sciences, and Pieter Tans from NOAA has also highlighted the potential importance of tropical forest ecosystems in storing anthropogenic CO2. Past attempts to use 13C data from the flask network to focus on tropical latitudes were confounded by the strong isotopic effects created by widespread conversion of tropical forests, which discriminate strongly against the heavier isotope during photosynthesis, to predominantly C4 photosynthesizing pastures and croplands, which have a much smaller isotopic effect. Townsendand colleagues quantified a probable range for the isotopic effects of such land conversion; they then used the atmospheric 13C data to separate atmosphere-surface exchanges of CO2 in the tropics between land and ocean realms. Their results suggested that intact tropical forests appear to have been a major sink for CO2 throughout the 1990s, one which is on par with those estimated for mid-latitudes of the Northern Hemisphere. Recent work by Cory Cleveland and Townsend in Costa Rica has suggested one potential mechanism for such a sink: Cleveland and colleagues found that in phosphorus poor soils, which are widespread in the tropics, microbial decomposition is strongly limited by phosphorus. Most systems outside of the tropics show that the microbial community is more carbon than nutrient limited, but in these tropical ecosystems, Cleveland and colleagues showed that new inputs of C, such as might be seen with rising CO2 levels, are stored in soils much longer than one might expect.

Elise Pendall has also been actively involved in studying the potential effects of rising CO2 on terrestrial ecosystems, with an emphasis on grassland systems in the Colorado region. Several earlier studies suggested that rising CO2 may cause a sharp increase in fluxes of relatively labile carbon through the plants and into the soil environment, thus stimulating decomposition and reducing net C storage. However, Pendall and colleagues used a combination of traditional measurements of ecosystem C pools and fluxes with isotopic analyses of those components to show that in a short-grass prairie system experiencing doubled CO2, higher C inputs to the soil did not result in higher decomposition rates, and therefore that significant new soil C storage was occurring. They point out the importance of soil moisture controls over decomposition rates for the new carbon inputs, thus suggesting further complex feedbacks between rising CO2 levels, a changing climate, and the overall response of the terrestrial carbon cycle.

Diane McKnight and her group devote some of their research efforts to another important and poorly understood component of the carbon cycle: the dynamics of organic carbon in aquatic systems. Organic carbon loading to freshwater ecosystems, and the dynamics of its transport, is being greatly altered by human activity. Bob Stallard and others have suggested that the transport of such carbon in river systems, and its potential storage in reservoirs and coastal areas, may be an important missing piece of the puzzle in understanding recent carbon sinks. Predicting the dynamics of organic carbon in aquatic systems is hindered by difficulties in understanding both its quite variable chemistry (and therefore relative resistance to decomposition), and its original source. McKnight and colleagues have developed novel, new analytical techniques that help resolve some of these uncertainties, including both ways to fractionate the organic carbon into functionally different components, and new fluorescence measurements that greatly improve the ability to trace the original sources.

Finally, while numerous significant gaps remain in our understanding of how the carbon cycle, climate system, and ecology of earth interact, it is both difficult and perhaps misguided to address these natural science questions in the absence of human factors. Humans are now central to the workings of the earth, and an understanding of how they behave in terms of making foreign and domestic policy, formulating economic strategies, as well as how the media helps shape public perceptions and opinions, must be integrated with our developing understanding of the physical workings of the carbon cycle and climate. Thus, Jim White and Alan Townsend are co-directors of a large new NSF/IGERT-sponsored graduate training program entitled the Carbon, Climate and Society Initiative (CCSI). This program integrates natural science, social science, and journalism perspectives on key issues of global environmental change, with an emphasis on the changing carbon cycle and climate system. Faculty participants in the CCSI represent nine CU departments and two research institutes, as well as the National Center for Atmospheric Research (NCAR), the National Oceanographic and Atmospheric Administration (NOAA), the Max Planck Institute for Biogeochemistry in Jena, Germany, and the Boulder Daily Camera. However, INSTAAR directorate members and students are playing a central role in this program, as in addition to White and Townsend, Diane McKnight, Mark Williams, and Robin Webb, are all part of the CCSI core faculty, and the first cohort of graduate students supported by the program includes eight INSTAAR graduate students: Keri Holland, Dan Liptzin, Trevor Popp, Annalisa Schilla, Andrew Todd, Natalie Mladenov, Adina Racoviteanu, and Laura Belanger.

Asner, G.P., Townsend, A.R., and Braswell, B.H., 2000: Satellite observation of El Nino effects on Amazon forest productivity. Geophysical Research Letters, 27 (7): 981-984.

Battle, M., Bender, M.L., Tans, P.P., White, J.W.C., Ellis, J.T., Conway, T., and Francey, R.J., 2000: Global carbon sinks and their variability inferred from atmospheric O2 and d13C. Science, 287: 2467-2470.

One potential consequence of climatic changes associated with enhanced accumulation of greenhouse gases in the atmosphere is alteration of hydrologic patterns. With warmer temperatures, evapotranspiration from the land surface may increase, possibly leading to an energized water cycle with greater frequency and intensity of extreme events such as floods and droughts. Because water is a strategic resource in many regions of the world, greater hydrologic variability creates new challenges for water resource managers. The assumption of stationarity, which assumes that the future trajectories of surface water systems are predicted by past variability, has been the mainstay for management of river networks. However, this model will be less reliable in a future when past analogs do not exist. This element of unknown variability compounds the challenges of meeting environmental quality objectives that now must be considered in water resource management. INSTAAR scientists are involved in basic hydrologic research that is advancing knowledge of surface water hydrologic processes at a range of spatial scales, from small streams to large river systems and their estuaries. Furthermore, INSTAAR scientists are engaged in field and modeling studies that address the coupling of elemental cycles and contaminant transport to hydrologic processes.

Although it is recognized that management of rivers through impoundment and land-use change has direct effects on the transport of sediment at the catchment scale, greater understanding of global-scale patterns of sediment transport is critically needed. James Syvitski and colleagues have completed a comprehensive study using data from 59 gauging stations on large rivers to determine predictive equations for sediment rating parameters that are related to river basin morphology and climate. Developing these relationships requires thorough analysis of detailed data sets because the majority of annual sediment transport can occur during relatively short intervals of high flow. Interannual and storm event variability in sediment load is now adequately accounted for in these equations, allowing realistic explorations of long-term sediment load characteristics. Application of these models to ungauged river basins will be invaluable in designing water resource infrastuctures in developing countries, as well as in projecting changes in sediment transport patterns associated with climate change.

It has long been recognized that river networks have general patterns that are consistent across regions with different topographic and geologic characteristics. However, the processes that give rise to these patterns have yet to be explained from a geophysical perspective. One limitation in developing a quantitative understanding of the evolution of river networks has been the difficulty in acquiring detailed data on a number of large-scale river networks. Scott Peckham has developed a comprehensive computer package entitled River Tools, which can generate these data from digital elevation maps (DEMs) of river basins. While this software is used as research tool at INSTAAR, it is simultaneously being released and developed for current applications in water resource management. For example, Peckham has developed a detailed DEM for the Snake River Watershed which flows into Dillon Reservoir in Summit County, Colorado, in order to evaluate the contributions of abandoned mine sites to water quality problems in the watershed.

In the Rocky Mountains, the annual hydrologic cycle is dominated by wintertime accumulation of the snowpack and the melting of the snowpack in spring. Mark Williams, Nel Caine, and Mark Losleben have been studying the long-term record for climate, snowpack, and streamflow from the Green Lakes Valley in the Colorado Front Range which has been obtained through the Niwot Ridge Long-Term Ecological Research project (NWTLTER) and NOAA. These records suggest trends of increasing snow accumulation in late winter (March), earlier average snowmelt, and a decreases in lake ice thickness. Measurements of water flow made by Alex Machado, Mark Williams, and Tad Pfeffer, using an array of 36 snowmelt lysimeters at a subalpine site at Niwot Ridge have given a detailed and large-scale (100 m2) view of the heterogeneities of meltwater flow through snow. Previous work of this type concentrated on smaller areas and used a smaller number of lysimeters. Geostatistical analysis of the lysimeter flow data indicates a typical spacing of approximately 2.4 m between vertical flow channels.

Important biogeochemical processes occur in the upper soil horizons under the snowpack, thus the pattern of snowpack distribution on the landscape influences the water quality of streamflow. Hillary Hamann, working with Mark Williams and Nel Caine is investigating the formation of ice lenses in alpine soils in two small sub-basins on Niwot Ridge, and evaluating their influence on the water quality. A topographically based model, TOPMODEL, is being used to analyze these results.

At another LTER site, the McMurdo Dry Valleys in Antarctica, Diane McKnight, Arne Bomblies, and Mike Gooseff are studying the relationship between climate, streamflow, and water chemistry in glacial meltwater streams. Analysis of data from the initial exploration of the Dry Valleys by members of Scott’s party in 1903 and subsequent records of lake-level rise and stream flow indicate that the period of 1970 to 1995 had much greater stream flow than the previous 70 years, accounting for the 13 m rise in lake level in one of the dry valley lakes. Field measurements demonstrate that water storage in the hyporheic zone within porous alluvial sediments acts as an important control on the streamflow into Dry Valley lakes. The movement of water between the hyporheic zone and open stream channel can be modeled effectively using a transient storage model (OTIS), which is being adapted to account for zones of rapid and slow exchange.

From the perspective of water resource management, options for responding to changes in hydrologic regime associated with variable climate are constrained by water quality and aquatic habitat concerns. In the Colorado Rocky Mountains, the success of the ski industry is dictated by the reliability of early-season snow cover (November and December). Later snowfall and competition among ski areas have increased requests for permits to use mountain streamflow for artificial snow making purposes. However, many mountain streams are contaminated by acid mine drainage, so that withdrawal and redistribution of these waters can exacerbate water quality problems. Diane McKnight, Durelle Scott, and Eric August, in collaboration with scientists at the US Geological Survey, are studying the hydrologic and biogeochemical processes controlling trace metal transport in streams and wetlands at several Rocky Mountain field sites. These studies employ a reactive solute transport model which quantifies chemical processes occurring in the open channel, the hyporheic zone, and in wetland sediments. This model has recently been adapted to include a kinematic wave model for the routing non-steady state flow, as well as a ligand exchange model for trace metals sorption onto particulate phases.

Glaciers and ice sheets play large roles in global hydrology, and especially in rates of sea level change. Work by members of INSTAAR's Geophysics Group in glacier dynamics and mass balance relate to a number of climate change issues on decadal to millennial time scales. Glaciers which terminate in the ocean providing potentially intimate and dramatic coupling between land ice and the ocean, and this coupling appears from paleoclimatic records to be an important modulator of global climate as well as a critical process in sea level change. Investigations of Columbia Glacier, Alaska, by Tad Pfeffer, Mark Meier, and Josh Cohn, in collaboration with colleagues at the US Geological Survey, involve photogrammetric determination of ice flow velocities and strain rates in the part of Columbia Glacier grounded below sea level. The Columbia Glacier is flowing fast (up to 30 m d- 1), and simultaneously retreating rapidly (1 km yr- 1) due to rapid calving of icebergs. The photogrammetry and analysis made form these measurements allow the future retreat to be predicted; the remaining 20-25 km of the glacier still grounded below sea level is likely be evacuated by a combination of thinning and iceberg calving within the next 50 years.

Also within the Geophysics Group, Mark Meier, and Mark Dyurgerov are engaged in an ongoing study of the contribution of mountain and subpolar glaciers to global hydrology. Globally, glaciers exclusive of Antarctica and Greenland cover an area of about 680 • 103 km2. During the period 1961-98, glaciers lost about 7 m of ice, or about 5 • 103 km3 of water, most of which ran off to the ocean, increasing sea level by about 13 mm. Wastage of small glaciers has thus been responsible for about 20% of total sea level rise during this time. In several years during this period (1979, 1990, 1995, 1997, and 1998), all of which were characterized by extreme annual air temperature, glacier volume loss was exceptionally large. Mass balance sensitivity, seasonal mass balance components (accumulation of snow and ablation of snow and ice), and equilibrium line altitude also showed large changes in these years. Interannual mass balance variability was large during the second half of the previous century in more than 30 regions in the world, mostly in the Northern Hemisphere. Mass balance changes correlate closely with climate variables, and particularly with annual air temperature. Rates of glacier wastage increased in Central Asia, the Canadian Arctic, and Alaska while glaciers in Scandinavia, the Caucasus, Altai, and possibly in New Zealand gained in mass and advanced. The lack of synchrony and increase in spatial variability of all parameters seems to be a distinguishing feature of current climate change. Uncertainties persist which limit our ability to accurately evaluate the contribution of mountain and subpolar glaciers. These particularly include the unknown mass balance regimes of individual ice caps around Antarctica and Greenland, Patagonian ice fields, and the largest glaciers in Alaska.

McKnight, D. M., Niyogi, D. K., Alger, A. S., Bomblies, A., Conovitz, P.A., and. Tate, C. M., 1999: Dry Valley streams in Antarctica: ecosystems waiting for water. BioScience, 49: 985-995.

O'Grady, D. B., Syvitski, J. P. M., Pratson, L. F., and Sarg, J. F., 2000: Categorizing the morphologic variability of siliciclastic passive continental margins. Geology, 28: 207-210.

Nitrogen Cycling

Human alteration of the nitrogen cycle is among the most important current global environmental problems. The increase in anthropogenic fixation of N2 and subsequent emissions is proportionately greater than that of CO2. There is growing concern over the effects of these increased N inputs on terrestrial and aquatic ecosystems, including eutrophication, acidification, and alteration of native species biodiversity. INSTAAR scientists are involved in research efforts investigating the ecological effects of increasing N deposition at a multitude of spatial scales.

Carbon and N cycles are usually coupled, as sequestration of CO2 is dependent on the photosynthetic enzymes of plants, and primary production in many terrestrial ecosystems is limited by the supply of N. Thus it is reasonable to hypothesize that increased N deposition will result in greater uptake of CO2. Alan Townsend, along with Tim Seastedt and Greg Asner from CIRES (University of Colorado), have evaluated the coupling of C and N cycles at a global scale based on regional perspectives. They suggest that there may only be a limited capacity of terrestrial systems to sequester more C as N deposition increases, due to increases in N saturation of terrestrial ecosystems in temperate latitudes of the northern hemisphere, and conversion of shrublands and forests to herbaceous dominated agriculture, which lowers the potential long-term C storage. In addition, Townsend and colleagues point out that much of the future increase in N deposition will occur at tropical and subtropical latitudes, where N limitation is much less common. In the tropics, excess N will rapidly lead to a variety of deleterious consequences, including the potential for a reduction, rather than stimulation, of carbon storage.

A regional concern is the potential influence of increased N deposition in the Front Range on ecosystems in the central Rocky Mountains. While the rates of N deposition are relatively low compared to areas such as Europe or the northeastern US, the granitic parent material of the soils, coupled with relatively low rates of primary production and N cycling, significantly decrease the threshold for N saturation of terrestrial and aquatic ecosystems. Evidence from stream chemistry monitoring efforts indicate that periodic N saturation is occurring, whereas paleolimnology suggests that attendant biological changes are manifested in lakes. Mark Williams, Nel Caine, and their students have conducted extensive stream chemistry measurements in the Green Lakes Valley over the past 30 years, and they have found periodic elevation of NO3- concentrations in high-elevation streams during the growing season. During the late 1980s and early 1990s there was a positive correlation between catchment yield and N deposition in the Green Lakes Valley. The highest lakes in this catchment have also experienced significant losses of acid-neutralizing capacity, in part because of higher N deposition related to the orographic increase of total precipitation with elevation. Ongoing research in Williams's and Steve Schmidt’s (EPO Biology) labs has focused on the role of microbes in talus soils in the highest parts of the catchment in chemical transformation of N deposition.

Alexander Wolfe, undergraduate student Alison Van Gorp, and Jill Baron of the USGS have documented striking shifts in diatom species composition in the sediments of several Front Range alpine lakes. Mesotrophic indicator species expand in close correspondence to significant changes in sediment d15N signatures, in synchrony with the history of increases anthropogenic N deposition. Similar trends are present but comparatively muted in lakes west of the Continental Divide, confirming that the offending sources lie to the east in the Denver-Fort Collins urban axis.

Bill Bowman is examining the potential response of alpine terrestrial vegetation to increasing N deposition. He has found that most species have a very limited capacity to respond to increased N availability, so that changes in species composition will occur as N deposition increases over alpine tundra. Katie Suding Nash has compared long-term changes in plant abundance in permanent plots with the results of changes in abundance following N fertilization experiments. The correspondence between these approaches confirms that changes in terrestrial communities are occurring in response to N deposition. Since plant species composition can control as much of the spatial variability in N cycling as variation in microclimate, as demonstrated by Heidi Steltzer, changes in plant species composition brought on by increases in N deposition will induce a positive feedback to N cycling, potentially accelerating fluxes of N between alpine terrestrial and aquatic ecosystems.

Schematic representation of the nitrogen cycle in terrestrial alpine ecosystems of the Colorado Front Range. (William Bowman, Ambio, v. 49, 2000.)

The evidence of significant and directional biological changes associated with increased N deposition has prompted Mark Williams and Kathy Tonnessen of the USGS to estimate a critical load for N deposition in the Colorado Front Range, which they have set at 4 kg/ha/year. They estimate that current rates of N deposition are at or slightly above the threshold of biological change and N saturation. Barbara Inyan and Mark Williams have also analyzed anthropogenic N inputs into catchments near Telluride, Colorado, and provided significant input to land managers and lawmakers used in legislation to minimize environmental damage caused by land development.

Key publications:

Bowman, W. D., 2000: Biotic controls over ecosystem response to environmental change in alpine tundra of the Rocky Mountains. Ambio, 49: 396-400.

Williams, M. W. and Tonnessen, K. A., 2000: Critical loads for inorganic nitrogen deposition in the Colorado Front Range, USA. Ecological Applications, 28: 207-210.

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Research Highlights

Examples of Interdisciplinary Research at INSTAAR(1999/2000):