Main BNZ Graduate Student Web Page

Just click on any name to view their complete BNZ profile of research interests, datafiles, and publications.

Garrett Altmann ( send email )

M.S. Candidate

Advisor: D. Verbyla
Co-Advisors: J. Fox, K. Yoshikawa,

Increases in fire frequency and severity have resulted from a warming climate in boreal Alaska. These changes alter the successional trajectories of plant species, facilitate permafrost degradation, and have the potential to cause changes in ecosystem state. Using GIS and Landsat TM/ETM+ satellite imagery, my research examines how fire affects pond dynamics in areas of discontinuous permafrost. Through a multi-temporal examination of pond sizes related to fire occurrence, my research goal is to determine if high severity fires create successional conditions that facilitate permafrost degradation in the form of thermokarsting (lake surface area expansion) or talik drainage (lake surface area reduction). Determining how these changes vary with topography, soils, and disturbance regime can produce rules for predicting where threshold changes in hydrology are most likely.

Mike Anderson ( send email )

Ph.D. Candidate

Advisor: R. Ruess
Co-Advisors: L. Taylor, C. Mulder,

Alders (Alnus spp.) are circum-boreally distributed trees and shrubs that form root nodule-based symbiotic associations with nitrogen (N) fixing bacteria in the genus Frankia. The N fixed in this symbiosis provides the plants with several ecological options in terms of nutrient use, and also underlies the well-described community and ecosystem-level effects of alders. Specificity of associations between particular genotypes of both organisms appears to vary among plant species and bacterial strains, and particular plant-bacterial combinations can differ widely in terms of host (plant) growth and N-fixation. These genotypic effects can be modulated by environmental variation, as well. Such interactive effects of host and symbiont genotype and environment are thought to be important in the evolutionary development of interspecific interactions in general and, in the case of alder and ¬Frankia, have the potential to impact community structure and ecosystem function at local spatial and ecological temporal scales. However, the factors determining population-level genetic structure in this interaction are poorly understood. My dissertation research has sought to: 1) characterize the degree to which genetic structure of Frankia assemblages in plant nodules correlates with host species and environmental variation in two Alaskan alder species, A. incana ssp. tenuifolia and A. viridis ssp. fruticosa, and 2) examine the role of ‘host choice’ (i.e., active selection of specific bacterial genotypes by the host plant from a larger set with which it is physiologically compatible) in structuring Frankia assemblages across habitats in A. tenuifolia nodules.

Becky Baird ( send email )

M.S. Candidate

Advisor: T. Hollingsworth
Co-Advisors: D. Verbyla, J. Kidd,

Climate has warmed substantially in boreal Alaska since the mid-1970s. The direct effects of rising temperatures on sub-Arctic ecosystems are already being seen in the form of drought stress, increased fire frequency and severity, and increased frequency and severity of herbivorous insect outbreaks. These effects of climate change should have a direct impact on the vegetation of the boreal forest and can lead to a decreased remotely sensed vegetation index. The vegetation index, NDVI, is an effective proxy for plant photosynthesis on a landscape scale, and therefore, an appropriate measure to examine landscape-scale changes in vegetation due to climate change effects. The overarching goal of my research is to assess the change in vegetation at Bonanza Creek Experimental Forest, and partition these changes into a) expected changes through successional processes, and b) unexpected changes which could be due to climate change. Once I have assessed these changes using a combination of remote sensing and field sampling, I will examine associated patterns on a hierarchical scale from landscape level changes to differences in vegetation patches and finally by examining trends in individual vegetation stands.

Andres Baron Lopez ( send email )

M.S. Candidate

Advisor: T. Schuur
Co-Advisors: M. Mack, ,

Thermokarst are among the most active geomorphological features in permafrost terrain. After permafrost thaws, they have the capacity of substantially alter the form and function of the landscape. These features open new niches for plant colonization, create sites of intense microbial activity and trace gas emission, are sources of nutrients and sediment to receiving waters, and alter the local topography with impacts on snow accumulation. As a natural disturbance, they provide an environment where the recovery and re-establishment of soil and plant dynamics may be studied. Understanding the thermokarst changes thru time it’s a critical step in order to elaborate possible pathways of plant recovery, soil physical and chemical alterations and ultimate shapes of ecosystem dynamics post-disturbance.

Elizabeth Belshe ( send email )

Ph.D. Candidate

Advisor: T. Schuur
Co-Advisors: , ,

Robert Burgess ( send email )

Ph.D. Candidate

Advisor: M.B. Leigh
Co-Advisors: L. Taylor, ,

Decomposition of soil organic matter (SOM) is an important process that determines rates of CO2 release, and is mediated exclusively by soil microbes. The response of these microbes to climate change is unknown, as is the identity of most active soil microbes that break down lignocellulose, a major component of SOM. A recently developed technique, stable isotope probing (SIP), allows the identification of microbes performing specific functions. The proposed research will use SIP to identify microbes that degrade lignocellulose in boreal forest soils, and will use this information to determine their abundance and distribution in response to the simulated climate change scenario of long-term snow exclusion. In addition, SIP will be combined with metagenomics to find genes responsible for the breakdown of lignocellulose. Information from this research can be used to scale-up the small-scale processes responsible for CO2 release, thus increasing the accuracy of climate change modeling. In addition, the discovery of novel genes may be applicable to the biotechnological production of cellulosic bio-ethanol.

Cameron Carroll ( send email )

M.S. Candidate

Advisor: K. Kielland
Co-Advisors: P. Doak, Terry Chapin, K. Kellie

The importance of moose as a subsistence food to many Alaskans is indisputable. Impacts of a changing climate upon those resources are therefore of great concern for wildlife managers. Increasing global temperatures have been linked to increases in fire frequency and fire severity at northern latitudes where there is a related shifting toward a more deciduous dominated landscape. In contrast to previous models of boreal forest successional dynamics, this modeling project explores the link between fire severity and moose population responses. It examines the intricacies of response in non-predator-limited moose populations and the paradox that their rapid ability to respond to available forage puts these populations at greater risk of decline in years of extreme environmental events. The model simulates population dynamics of a moose population where fire severity and available browse inputs inform a matrix population model, and explores the interactions between browse production and consumption in response to fire severity, where a supposed linear function becomes non-linear due to the multiplicative effects of both a functional and numerical response.

Amber Churchill ( send email )

M.S. Candidate

Advisor: A.D. McGuire
Co-Advisors: M. Turetsky, ,

Remote sensing in interior Alaska has shown that major wetland complexes are drying, likely due to better drainage induced by permafrost thaw and/or increased evapotranspiration with warming (Riordan et al 2006). However, in other areas, permafrost thaw and groundwater upwelling is leading to flooding due to thermokarst and peat subsidence (Osterkamp and Romanovsky 1999). Thus, Alaskan peatlands could become drier or wetter under future climate regimes depending on landscape position, permafrost stability, and climate change. These changes will increase the variability of CO2 uptake across peatland landscapes in central Alaska, and may ultimately change whether the region acts as a source or sink for atmospheric CO2. At the Alaskan Peatland Experiment, located near the Bonanza Creek Experimental forest in interior Alaska, we are examining the effects of changing water availability on ecosystem processes across a range of peatland types. These peatland types include a moderate rich fen, a thermokarst/collapse scar and a forested peatland with intact surface permafrost. At the rich fen, a large-scale manipulation of water table position (control, lowered, raised) has been ongoing since 2005. The permafrost peatland and adjacent collapse scar represent a natural gradient from dry surface peat (permafrost bog) to inundated conditions (collapse scar bog). CO2 flux measurements at the rich fen from 2005-2008 show that the lowered water table treatment had lower rates of gross primary productivity (GPP) than the control plot. Across all three sites, 2008 CO2 fluxes show that the rich fen and collapse scar sites had similar magnitudes of GPP, but exhibited different seasonal patterns of C uptake (Figure 1). Our continuing research is focusing on understanding how water and nutrient availability influences rates of plant C uptake (GPP and NPP) across the APEX sites. Specifically our goals are to evaluate the effects of water table manipulations (at the rich fen) and permafrost thaw on the growth of plant functional groups and their competition for light, water, and nitrogen. This information will help to identify how various plant functional groups may respond to climate change and drive potential changes in ecosystem structure and function in interior Alaska.

Colette de Roo ( send email )

Ph.D. Student

Advisor: G. Kofinas
Co-Advisors: , ,

I am studying the effects of different forces of change on subsistence harvesting systems. My objective is to model these systems and thus better understand the trade-offs associated with subsistence harvesting under rapidly changing conditions (climate change, fuel costs, etc.).

Russell Dennis ( send email )

M.S. Candidate

Advisor: P. Doak
Co-Advisors: , ,

La'ona DeWilde ( send email )

Ph.D. Candidate

Advisor: T. Chapin
Co-Advisors: D. White, M. Leigh, J. Jones

This is a look at trace metal and bacteria pollution that enters the surface waters in the Fairbanks area from paved parking lots and roads with a focus on flushing events.

Dashiell Feierabend ( send email )

M.S. Candidate

Advisor: K. Kielland
Co-Advisors: A. Powell, C. Hunter,

Northern snowshoe hare (Lepus americanus) populations experience cyclical fluctuations approximately 10 years in length. At peak densities during this cycle, hares disperse into less preferred habitats in order to forage. The different structural cover characteristics of these habitats have consequences on the sources and intensity of predation on snowshoe hares. Cover changes seasonally with the presence of leaves in summer and accumulation of snow in winter, and sources of predation change seasonally according to relative prey availability. I am investigating the relationship between seasonal changes in rates and sources of predation on snowshoe hares, and the structural cover provided by different hare habitats in the Bonanza Creek Experimental Forest, located 20 km southwest of Fairbanks, Alaska. Mortalities of radio-collared snowshoe hares are being tracked and identified year-round in black spruce (Picea mariana) and riparian communities. My study is the first to use precise individual location data from radio telemetry to achieve a high accuracy estimate of snowshoe hare mortality rates and sources, and to identify the impacts of primary predators of snowshoe hares at the peak and decline phase of the hare cycle in interior Alaska.

Daniel Glass ( send email )

M.S. Candidate

Advisor: L. Taylor
Co-Advisors: , ,

A fungal clone library spanning partial ribosomal large subunit (LSU) and internal transcriber spacer (ITS) gene-regions constructed by Taylor et al (2007) from boreal forest soils at the Bonanza Creek LTER near Fairbanks, AK yielded five novel fungal DNA sequences that are divergent from known Phyla, and whose relationships to other fungi are uncertain. Here we report phylogenetic analyses of these sequences. Two of these novel sequences appear to be representatives of the recently discovered soil clone group one (SCG1) described by Schadt et al (2003) while the other three appear to fall within the basal fungal lineages. The latter placements have little statistical support and remain questionable. This may suggest that they represent highly divergent lineages. We conducted a PCR survey, which shows that two representatives of SCG1 and one other novel taxon are abundant throughout the floodplain black spruce (Picea mariana) site. SCG1 has a large distribution throughout other ecosystems and is likely widely distributed throughout the boreal forest as well. The other two taxa were far less frequent and occurred predominantly in one corner of the black spruce site with one taxon also represented by a single clone in an upland site. In order to further investigate the distribution and abundance of these taxa throughout the boreal forest and potentially gain insights into their ecological roles, quantitative real-time PCR (qPCR) will be conducted on soil extracts. Longer portions of the LSU as well as the ribosomal small subunit (SSU) will be amplified and sequenced using taxon-specific primers to obtain more accurate phylogenetic inferences. Together these efforts will provide a basic understanding of the phylogenies and ecologies of these novel fungal taxa.

Brian Heitz ( send email )

Ph.D. Candidate

Advisor: B. Sveinbjornsson
Co-Advisors: R. Ruess, ,

Rebecca Hewitt ( send email )

Ph.D. Candidate

Advisor: T. Chapin
Co-Advisors: T. Hollingsworth, ,

Understanding the complex mechanisms controlling treeline advance or retreat in the arctic has important implications for projecting ecosystem response to direct and indirect effects of global environmental change. Changes in landcover due to a treeline biome shift would alter climate feedbacks (carbon storage and energy exchange) and other ecosystem services such as wildlife and berry habitat, impacting subsistence users. In the boreal forest climate-induced changes in the fire regime may be a more critical driver of landscape processes than the direct effects of warming. Currently modeling of a dynamic arctic treeline may overlook critical factors influencing tree seedling establishment in previously unforested sites. Soil microbes are key drivers of ecosystem processes, yet their role in regulating landscape-scale vegetation change is not known. Comprehensive studies of treeline position have noted that ectomycorrhizal fungi (EMF) may be an important factor delineating the boundary between forest and tundra. Yet, these critical plant-fungal symbioses are sensitive to wildfires. My research examines the role of EMF in mediating biome shifts at arctic treeline using a suite of laboratory and field experiments. Through my experiments I will develop rules for previously developed frame based models (ALFRESCO) to relate wildfire-induced changes in tree establishment at treeline with landscape patterns of treeline movement. I will model treeline expansion as a function of fire severity, mycorrhizae availability, and postfire tree and understory cover. This research integrates plant-microbial symbioses with landscape processes to address the linkages between climate change, fire ecology, and ecosystem processes in two innovative ways: (1) exploring the importance of plant-microbial interactions to vegetation dynamics by linking fungal genomics to community-scale processes and (2) investigating the role of seedling establishment to biome shifts by linking community-scale processes to globally significant trends. Boreal and treeline research done to date suggests that fire and mycorrhizal inoculum availability are important to plant community composition. This research is a critical step in placing these observations into a predictive framework.  

Elizabeth Hoy ( send email )

Ph.D. Candidate

Advisor: E. Kasischke
Co-Advisors: S. Goward, S. Prince, T. Loboda, J. Sullivan

High Northern Latitude (HNL) (boreal) forests contain vast reservoirs of carbon in the form of deep organic soils (> 200 Pg C in the ground layer of boreal forests alone). Because boreal forest organic soils burn, changes to the fire regime have important ramifications to the carbon cycle in this region. Recent climate change has resulted in a significant increase in average area burned across the North American boreal forest, which in turn has resulted in increasing fire frequency in many ecosystems. In interior Alaska, black spruce (Picea mariana) forests represent 45% of the landscape, and are the prevailing forest type (66% of all forests); the deep organic soils in these black spruce forests represent the dominant terrestrial carbon reservoir in this region (1140.4 ± 117.3 Tg C). While recent research has focused on understanding the carbon cycle responses to variations in area burned and depth of burning in mature spruce forests, little research has been conducted on the impact of more frequent burning in this forest type. The goal of this dissertation research is to assess the impacts of changes in fire frequency on carbon reservoirs present in surface organic soils in black spruce forests. To achieve this goal, three integrated studies will be conducted. First, the factors associated with the vulnerability of landscapes to more frequent reburning will be explored using remote sensing data and geospatial analysis techniques. Then, field-based research will be carried out to document how fire frequency impacts the amount of residual soil organic matter remaining following fire. Finally, the newly acquired information from the remote sensing analysis and field based research will be incorporated into a biogeochemical cycling model (the Terrestrial Ecosystem Model) to 1) analyze how recent changes in fire frequency have altered carbon storage in boreal regions and 2) forecast carbon storage within Alaskan boreal forests over the 21st century.

Hanna Lee ( send email )

Ph.D. Candidate

Advisor: T. Schuur
Co-Advisors: , ,

One of the biggest potential feedbacks to global climate change from high latitude ecosystems may come from thawing of permafrost, which stores more than 50% of the total global terrestrial soil carbon. Thawing of permafrost may accelerate decomposition of soil organic matter and increase carbon dioxide (CO2) emissions. When permafrost thaws in ice-rich areas, it creates localized topographical surface subsidence called thermokarst, which can induce variations in soil abiotic properties. Also, depending on the location of thermokarst formation it can create anaerobic conditions in soil. By altering multiple resources in soil, thermokarst can change C-cycling in high latitude ecosystems beyond simple increases in temperature alone. My research was conducted at a subarctic tundra site near Healy, Alaska (Latitude: 63.7ºN), where permafrost thaw and thermokarst development have been observed and monitored for two decades. I established soil gas wells to explain how thermokarst affects soil respiration and to determine which depth in the soil profile has the greatest soil CO2 flux. I also established plot- scale studies to determine how thermokarst affects ecosystem C exchange by measuring above ground CO2 fluxes using clear chambers. Then I collected permafrost soils with different substrate qualities and incubated them at 15°C under aerobic and anaerobic conditions to observe C loss and climate forcing in different environments after permafrost thaw. I found that there was an increase in soil CO2 production where thermokarst development progressed; this was mostly driven by surface soil layer CO2 production rather than deeper soil layer CO2 production This was likely due to changes in the environment such as soil temperature, moisture, and vegetation. I was able to estimate the annual ecosystem carbon balance using surface subsidence created by thermokarst development, thaw depth, and plant biomass. Permafrost soil incubation showed that carbon loss was 3 times greater under aerobic conditions, but climate forcing was 1.15 times greater under anaerobic conditions due to methane emissions. Therefore, permafrost thaw and thermokarst development may stimulate soil CO2 production, ecosystem carbon exchange, but will affect climate warming more under anaerobic conditions.

Mary-Cathrine Leewis ( send email )

Ph.D. Candidate

Advisor: M. Leigh
Co-Advisors: L. Taylor, T. O'Hara, R. Boone

Microorganisms constitute some of the most functionally diverse organisms on the planet, with metabolic capabilities ranging from nutrient cycling to xenobiotic pollutant degradation. Some microorganisms have been shown to degrade toxic and persistent organic pollutants such as polychlorinated biphenyls (PCBs) in soil systems. Plants can stimulate the microbial degradation of aromatic pollutants such as PCBs through the release of aromatic plant secondary metabolites in the root zone. Alaskan tree species are particularly promising because of their high quantity and diversity of secondary compounds. However, almost nothing is known about the capacity or mechanisms for Alaskan tree species to biostimulate pollutant degradation. Stable isotope probing (SIP) has been used in this project to identify microbes responsible for degradation of biphenyl (a PCB analogue), and benzoate (a biphenyl degradation product and toxic PCB degradation product analogue). 13C-labeled biphenyl and benzoate were added to microcosms containing PCB contaminated soils from Drury Gulch, Kodiak, Alaska. 13CO2 respiration was assessed at days 1, 4, and 14 using GC-IRMS and confirmed the degradation of both 13C-labeled biphenyl and benzoate in each of the microcosms. Microbial community analyses of 13C-DNA will be performed to identify organisms and consortia responsible for complete biodegradation of biphenyl and its intermediate, benzoate. The GeoChip microarray, a new tool which allows the study of bacterial, fungal and archaeal genes involved in pollutant biodegradation, biogeochemical and ecological processes, was used to characterize the functional genetic potential of four Alaskan tree species for rhizoremediation of contaminated soils. Preliminary results indicate that microbes with the genetic potential to degrade chlorinated solvents, hydrocarbons, herbicides, pesticides, and other aromatic compounds are present in varying amounts in the rhizosphere of each tree species. Functional microbial community analysis and toxicological analyses still need to be conducted and will provide insight into the mechanisms for detoxification of contaminated Alaskan soils.

Collin Macheel ( send email )

M.S. Candidate

Advisor: D. Misra
Co-Advisors: A. D. McGuire, R. Daanen, M. Darrow

Peatlands store an estimated one quarter of the Earth's terrestrial soil carbon. Predominantly found within northern latitudes, peatlands contribute an estimated 17-28% of global methane emissions and therefore play an important role in the global carbon cycle. The application of models attempting to accurately represent the energy and hydrologic mass transfer in peatlands have been limited by crude numerical representations and over generalizations when considering variable saturation. Variably saturated hydrology and energy transfer processes are important for regulating the oxidation and reduction of methane and the production of carbon dioxide. Here, we capitalize on environmental data that has been collected since 2004 at the Alaska Peatland Experiment (APEX), a heavily instrumented fen located in interior Alaska, in which water table position and soil temperatures have been manipulated in situ. The key goal of this research is to develop numerical simulation models of complex energy transfer and multiphase hydrologic processes and to apply them to organic variably saturated soils in peatlands, with the application to other climatic treatments at the APEX manipulation sites. More complex representations of the unsaturated subsurface and energy transfer within organic soils has the potential to provide insight on the dynamics of subterranean microbiological processes associated with carbon transformations, atmospheric emissions of greenhouse gases, and hydrologic transport. Finite element and volume analysis that use contemporary numerical codes account for seasonal variations of mass and energy transport. The application of a modified van Genuchten equation for variably saturated flow has been used to account for all hydrologic and energy transport processes. The results show that the water table has a distinctive non-linear effect on heat transfer and phase change. This study will contribute to the development of coupled biogeochemical-hydrologic models that are capable of simulating the dynamics of methane, carbon dioxide, and dissolved organic carbon in northern peatlands.

Kimberley Maher ( send email )

Ph.D. Candidate

Advisor: G. Juday
Co-Advisors: , ,

Harvesting non-timber forest products (NTFPs) such as blueberries and firewood is an important activity in Interior Alaska. This projects looks at the management and valuation (both qualitative and quantitative) of non-timber forest products (NTFPs) in the Tanana Valley.

Jordan Mayor ( send email )

Ph.D. Candidate

Advisor: T. Schuur
Co-Advisors: M. Mack, ,

Measuring the ratio of heavy to light stable isotopes of nitrogen (15N:14N, expressed as d15N relative to a standard) provides unique information regarding the nitrogen (N) cycle in forested ecosystems. This is important because the productivity and ecosystem dynamics of many terrestrial and aquatic ecosystems are limited by N availability. Because anthropogenic change can increase N availability through deposition, or alter N mineralization rates through climate warming and landscape modification, monitoring ecosystem responses to shifts in N availability is a research priority. However, to be confident in the interpretation of foliar d15N, mechanistic processes must be better understood. Across 46 plots in interior Alaska, black spruce trees were found to exhibit a wide range of foliar d15N values. This is likely due to one of two main processes in the N cycle known to significantly alter the ratio of 15N:14N in plants. These include the degree of N-dependency on root associated symbiotic fungi (mycorrhizae) and changes to the 15N:14N of soil N sources caused by soil fertility influencing the soil N cycle. This project uses a detailed method for measuring the d15N values of soil N to determine if changing sources of N can alone explain black spruce variability in d15N or if integrating fungal activity will better explain observed patterns. A mechanistic understanding of foliar ?15N is required to fully understand N cycling and to refine the use of ?15N as a proxy of ecosystem N cycling.

Nicole McConnell ( send email )

M.S. Candidate

Advisor: A.D. McGuire
Co-Advisors: M. Turetsky, J. Harden,

Global climate change is affecting the boreal forest in Alaska with increased fire frequency, drought, and permafrost degradation. Boreal ecosystems cover 14% of the vegetated surface found on earth and account for 25-30% of the world’s soil carbon. Peatlands are massive sinks of carbon and destabilization by global climate change will turn these sinks into carbon sources. The rate of carbon release from a peatland is determined by a variety of factors: the availability of oxygen in the soil, the depth of the water table, the microbial activity present, the soil temperature, the type of vegetation present, in addition to the chemical characteristics of the peat. Understanding these controls on peatland respiration is important to understanding future results as changes occur to these ecosystems. I am interested in 1) exploring biotic and abiotic controls on peatland ecosystem respiration including soil temperature, soil moisture, water table depth, frost depth, and vegetation characteristics. 2) I am interested in further exploring one facet of peatland soil respiration: root respiration. Fine roots account for a large proportion of soil respiration in the summer months and I am interested in quantifying the controls on root respiration including soil temperature, soil moisture, and frost depth and determining the contribution of root respiration to overall ecosystem respiration. Little is known about root respiration in Alaskan peatlands and this information will better our understanding of ecosystem respiration as a whole in these ecosystems.

Ann Olsson ( send email )

Ph.D. Student

Advisor: J. Jones
Co-Advisors: , ,

Wildfires in the boreal forests of the northern hemisphere are projected to increase with climate warming. Increased fire frequency will have large impacts on carbon storage through the loss of vegetation, combustion of soils, thawing of permafrost, and alteration of watershed hydrology. An important loss of carbon from terrestrial ecosystems is through hydrologic transport in both surface and subsurface flow. The goal of this research is to quantify how wildfire in the boreal forest impacts soil structure and the resulting hydrologic export of dissolved organic matter and nutrients from headwater catchments. Our study site is the Caribou-Poker Creeks Research Watershed (CPCRW) located in interior Alaska, which is underlain by discontinuous permafrost has had a recent history of wildfires. Concerning soils, we plan to examine how fire affects soil structure, soil respiration, and the production of soluble organic matter in soils, comparing burned and non-burned catchments. At larger scales, we plan to use an end-member mixing model to couple the production of soluble organic molecules in soils with stream export. Northern boreal forests store vast amounts of carbon, and are sensitive to disturbances such as wildfires. Small changes in fire frequency can have a significant effect on soil organic matter storage in hydrologic exports, and shift the global carbon balance.

Caitlin Pries-Hicks ( send email )

Ph.D. Candidate

Advisor: T. Schuur
Co-Advisors: , ,

Permafrost soils store almost 1700 Pg carbon (C)—C that is at risk of being respired to the atmosphere as the climate warms and permafrost thaws. Because loss of permafrost soil C is a positive feedback to climate change, we need to understand how thaw affects both autotrophic and heterotrophic components of C cycling in order to improve future climate predictions. The goals of this proposed study are to understand how warming affects which ecosystem components are driving ecosystem C flux in both a natural gradient of permafrost thaw and an experimental tundra warming experiment. While measuring C fluxes indicates whether an ecosystem is a source or a sink, it cannot explain why the ecosystem is a source or a sink. Partitioning can answer why, such as if an ecosystem is a source because respiration of newly-fixed C from plants is increasing or because deep soil respiration is increasing. I will use both ?13C and ?14C to partition ecosystem respiration into four sources: aboveground plant structures, belowground plant structures, surface soil, and deep soil. 13C separates sources based on plant parameters (type, structure, tissue), while 14C separates sources based on organic C age. I will sample the ?13C and ?14C of sources using incubations and the ?13C and ?14C of ecosystem respiration using field measurements. This study will utilize an established gradient of permafrost thaw and a warming experiment near Healy, Alaska where soil parameters and C fluxes are already being measured. I have tested the dual isotope approach at the thaw gradient and shown the four sources are distinctly separated allowing for constrained calculations of source contribution ranges. I will apply this method to calculate which sources drive differences in ecosystem respiration that have been measured in the thaw gradient and are hypothesized to occur with experimental warming. I will determine whether old, previously stored C is being respired in response to warming—a positive feedback to climate change. | The goals of this study are to understand how climate and vegetation have affected C balance, as measured by C accumulation rates, throughout the Holocene and during modern times and to test the potential of plant communities as proxies for C accumulation. Modern vegetation and soil moisture relationships will be determined so that past changes in vegetation can be connected to changes in soil moisture. Stratigraphic macrofossil analyses will show how vegetation communities changed in each core. Radiocarbon dating of the soil profile at the depths of vegetation change will be used to calculate the age of each distinct plant community. Carbon accumulation rates for each plant community will be determined in soils by measuring C contents, using 14C dates, and mathematical models. Holocene climate records from nearby lakes in Interior Alaska (e.g. Anderson et al. 2001) will be used to determine whether past changes in climate are connected to plant community transitions or changes in C accumulation rates.

Jenny Rhors-Richey ( send email )

Ph.D. Candidate

Advisor: C. Mulder and B. Roy
Co-Advisors: R. Ruess, D. Bret-Harte, L. Winton

Interests: physiology of Alnus fruticosa subsp viridis, susceptibility of drought stressed A. viridis to pathogens and herbivores, pathogens affecting water-relations in A. viridis

Amanda Rinehart ( send email )

Ph.D. Candidate

Advisor: J. Jones
Co-Advisors: , ,

The concentration of dissolved organic carbon (DOC) in arctic and subarctic streams and rivers is declining despite permafrost warming and thawing throughout the region, which is expected to unlock vast quantities of stored carbon. DOC cycling occurs primarily in the vertical and horizontal saturated sediments beneath stream channels, known as the hyporheic zone. Hyporheic processes in watersheds underlain by permafrost have been understudied, therefore, are under represented in carbon cycle models. My research addresses the influence of discontinuous permafrost on hyporheic exchange and DOC uptake in streams of interior Alaska. I am focusing on variation in hydrologic residence time within the subsurface zone as well as carbon and nutrient input from the catchment as controls on DOC uptake. I am also interested in how the interactions between hyporheic exchange and permafrost may lead to changes in stream chemistry as permafrost thaws.

Emily Schwing ( send email )

MS Candidate

Advisor: David Valentine
Co-Advisors: Roger Ruess, John Yarie,

Forest soils generally contain more C than vegetation and this pattern increases with latitude. In boreal forests, proportionately small changes in soil C inputs and outputs can profoundly alter the ecosystem C balance. The largest input of C to soils is the photosynthate plants allocate belowground, termed belowground carbon flux (TBCF). The largest output of C from soil is through soil respiration (Rs). Climate change projections for the boreal forest call for an increase in MAT and MAP resulting in a longer and drier growing season. Many studies suggest that temperature is the main driver of soil C cycling, but the effects of a changing moisture regime should not be ignored. I plan to examine how experimentally-altered soil moisture affects carbon balance. I will quantify TBCF and partition Rs using radiocarbon techniques to explore the affects of moisture stress on soil C dynamics. I hypothesize that (1) moisture stress will cause no net change in TBCF, because of the offsetting effects of lower NPP and higher belowground C allocation and that (2) heterotrophic respiration (Rh) is more sensitive to moisture stress than autotrophic respiration (Ra), because plants continue to invest in root production under drought conditions.

Katie Shea ( send email )

M.S. Candidate

Advisor: M. Turetsky
Co-Advisors: H. Maherali, A. Gordon,

High latitude ecosystems are expected to be among the regions most severely affected by global climate change. In particular, melting permafrost may seriously alter the landscape, ecology and climate forcing potential of northern regions. Since northern peatlands are known to contain approximately one third of global soil carbon, they have the potential to act as large sources of both carbon dioxide (CO2) and methane (CH4) which are known greenhouse gases. Therefore, understanding the impacts of climate change on these sensitive ecosystems is of the utmost importance. My overall research goal is to examine the controls and mechanisms of CH4 production and release pathways from Alaskan peatlands along a gradient of permafrost integrity. Specific objectives are 1) quantification of CH4 flux from intact permafrost, melting permafrost and non-permafrost peatlands and 2) identification of physical and biological controls on CH4 production and release including soil properties, local hydrology and microclimate and plant and microbial community activities. To address these objectives, I will use ecosystem and plant chambers to measure CH4 emissions and plant transport, respectively, and gas capture funnels to measure ebullition, or bubbling, in the three permafrost settings. I will relate these data to soil climate and vegetation community data already being collected through the Bonanza Creek Long Term Ecological Research (LTER) sites that use soil warming and water table manipulations to study the effects of climate change on vegetation and nutrient cycling.

Aditi Shenoy ( send email )

Ph.D. Candidate

Advisor: J. Johnstone
Co-Advisors: K. Kielland, ,

Wildfire plays a key role in shaping boreal forest succession and ecosystem function. Increases in the frequency and spatial extent of severe fires in the boreal forests of interior Alaska have been linked to changes in boreal forest structure and function and are of great importance for climate-albedo feedbacks and ecosystem carbon storage. Conditions caused by severe burning have been found to favor deciduous rather than coniferous recruitment which may result in forests that have a net cooling effect on radiative forcing of the climate due to a higher albedo and higher productivity. I propose to examine the biogeochemical controls on post-fire succession in order to improve our understanding of the competitive differences between these two plant functional types in response to fire severity. I propose to achieve this by using a combination of field measurements of post-fire vegetation recovery rates and site nitrogen status, and remotely sensed vegetation indices derived from satellite data.

Michaela Swanson ( send email )

M.S. Candidate

Advisor: R. Ruess
Co-Advisors: , ,

Rates of C cycling and NPP (net primary productivity) in northern ecosystems depend heavily on N availability across the landscape. Since most N enters these systems through biological N-fixation, alder, with its high capacity to fix N, plays a critical role in the ecosystem response to environmental change. However, because of its high P demands, the abundance, distribution, and N-fixing capacity of alder is tightly controlled by P availability and its ability to assimilate P by associating with ectomycorrhizal fungal (EMF) symbionts that aid in P acquisition by secreting enzymes that mobilize P from recalcitrant organic sources. In other ecosystems, patterns in the availability of P forms vary throughout soil formation and within soil horizons; however, little is known about P biogeochemistry in boreal forests of interior Alaska. My master’s thesis will investigate changes in the distribution of P forms and the strategies alder uses to assimilate P over spatial and temporal scales across the Tanana river floodplains. Understanding how functional traits of individual EMF are coupled with P biogeochemistry at the ecosystem scale will further our knowledge of the controls over successional dynamics, ecosystem function and the response of ecosystems to global climate change.

Delia Vargas Kretsinger ( send email )

M.S. Candidate

Advisor: K. Kielland
Co-Advisors: , ,

Mark Winterstein ( send email )

M.S. Candidate

Advisor: T. Hollingsworth
Co-Advisors: S. Walker, D. Verbyla,

Widespread lake drying has been documented across Interior Alaska. In the Yukon Flats positive trends in temperature and evapotranspiration have been attributed to reductions in the number and size of closed basin ponds. Changes to lake area create available substrate for colonization by terrestrial plants and should initiate a successional response of vegetation. I investigate here the succession of vegetation on drying lake margins in the Yukon Flats, Alaska. Vegetation and environmental variables will be described along a moisture gradient from existing lake shore to historic lake shore.

Suzanne Worker ( send email )

M.S. Candidate

Advisor: K. Kielland
Co-Advisors: Perry Barboza, Christine Hunter,

Global climate change is likely to influence plant/herbivore interactions in a variety of ways, including changes in secondary compound concentration and toxicity. Such changes will be consequential for herbivores like snowshoe hares that rely on woody browse for winter forage. Geophagy has been shown to have a pharmacological role in mediating the effects of ingested plant compounds and has been observed in snowshoe hares in localized areas near Wiseman, Alaska. My research seeks to understand the physiological and ecological consequences of geophagy in snowshoe hares through monitoring of geophagic hares and mineral licks, analysis of soil and browse, and analysis of individual and population parameters of geophagic and non-geophagic hares. Snowshoe hares are a keystone species of the boreal forest, influencing predator population dynamics and governing other important ecosystem processes such as plant species interactions, nutrient cycling, and forest succession. This project offers a unique approach to understanding plant/herbivore interactions in response to changing climate variables, and has the potential to provide information that integrates a wide variety of ecological processes and interactions that are valuable for ecosystem management.

The Bonanza Creek LTER, including this website, is supported by the National Science Foundation through awards DEB-1026415, DEB-0620579, DEB-0423442, DEB-0080609, DEB-9810217, DEB-9211769, DEB-8702629 and by the USDA Forest Service, Pacific Northwest Research Station through agreement number RJVA-PNW-01-JV-11261952-231. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the supporting agencies or the program as a whole.

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Last modified 23-Feb-12
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