The University of Arizona

REU 2012 Projects

Project A: Coupling subsurface ecohydrology and biogeochemistry

Mentor: Dr. Jon Chorover, Soil Water and Environmental Sciences

Students working in the Chorover lab would have the opportunity to beinvolved in a collaborative and multi-disciplinary Critical Zone Observatory (CZO).  The "Critical Zone" is defined as that portion of the Earth's terrestrial surface that extends from the outer periphery of the vegetation canopy to the lower limit of ground water penetration (National Research Council, 2001 "Basic Research Opportunities in Earth Sciences"). REU students would have an opportunity to work at one of two sites within the UA CZO, including the Santa Catalina Mountains (AZ) and the Jemez River Basin (NM). Projects in the Chorover lab would focus on coupling field work and laboratory studies to understand subsurface biogeochemical processes at these sites, and the interaction of biogeochemistry with ecohydrology and landform evolution.

Project B: Biogeochemistry of urban green infrastructure

Mentor: Dr. Mitchell Pavao-Zuckerman, Biosphere 2

Cities can have dramatic negative impacts on the local environment, yet at the same time can provide ecosystem services to residents.  Small-scale green infrastructure [such as green roofs & storm water retention basins] and broader scale [urban forestry] can be used to improve local urban environments and increase the provisioning of ecosystem services.  Students would work on projects investigating the success of arid urban ecosystem green infrastructure projects by monitoring soil biogeochemical and plant/ecosystem ecophysiological responses of experimental applications at Biosphere 2 or green infrastructure applications in the city of Tucson.

Project C: Effect of biota on mineral weathering

Mentor: Dr. Katerina Dontsova, Biosphere 2

Plants, bacteria, and fungi live in close relationship with the soil.  Soil serves as a source of essential nutrients for the plants and associated microorganisms, as well as physical environment that provides access to water and air.  In order to ensure supply of lithogenic elements, plants developed multiple mechanisms that allow them to mine nutrients from the soil.  Among others, these mechanisms include release of organic acids and other organic exudates, and respiration that increases concentration of carbonic acid in the soil.  Organic and inorganic acids influence mineral dissolution and re-precipitation, while released organic compounds improve soil structure.  As a result, plants change chemical, mineralogical, and physical properties of the soil, making it a more hospitable environment for life.  Plants, mycorrhizal fungi, and bacteria can influence soil separately but by co-existing in the complex symbiotic associations they enhance their ability to change soil.  This study would explore plant-soil interactions with the focus on chemical and mineralogical changes that happen in soil under influence of root-mycorrhizae-bacterial associations.

Project D: Abiotic effects of woody plant cover

Mentor: Dr. David Breshears, School of Natural Resources and Environment

The overall theme of the research for this project will be related to gradients of woody plant cover, which can span from grassland to forest. More specifically, key questions focus on abiotic effects of woody plants and their responses to changes in climate and land use. Specifically projects could include assessments of changes in near ground microclimate conditions associated with different densities of woody plant cover, spatial variation in dust production as a function of woody plant cover, and or plant water stress preceding tree mortality along vegetation gradients. Approaches would include hemispherical photography and computational assessments of solar radiation regimes, measurements of dust production using a variety of instruments, and/or measurements of plant water stress and related physiological metrics.

Project E: Native versus exotic desert grass mortality

Mentor: Dr. Travis Huxman, Biosphere 2

The susceptibility of native and exotic grasses to drought and temperature, and potential competitive interactions will be evaluated by growing different species in pots across the environmental gradients at B2. We are designing manipulative experiments in B2 to investigate the impact of droughts, rising temperatures on native and invasive grass communities and how these communities partition resources (soil moisture, nutrients) in these changing environmental conditions. Modeling studies will address the impact of rapid vegetation changes and implications on ecohydrological processes and desertification. These studies will provide suitable cases to link across scales and to link data with modeling exercises.

Project F: Event-scale Landscape Change

Mentor: Dr. Steve DeLong, Biosphere 2

In arid alluvial environments with flashy precipitation regimes, even moderate storms can lead to flash flooding, erosion and sediment transport. Through a combination of environmental sensor network deployment, repeat 3D surveys, and direct observation, we are beginning to quantify landscape response to individual weather events.  These may be useful approaches for predicting how landscapes with change in future climate scenarios. Potential projects may include experimentation, field study of flash floods, or analysis of several years of observational data using GIS and modeling to better understand how landscapes change on human timescales.

Project G: Integrating multiple scale measures of carbon and water flux

Mentor: Dr. Greg Barron-Gafford, Biosphere 2

Research within this project will focus on integrating measures of soil, leaf, plot, and ecosystem scale carbon and/or water flux.  Research projects could be carried out in one of two venues.  Potential sites include a space-for-time substitution of sites representing a woody plant encroachment (WPE) gradient or a mixed conifer forest.  WPE refers to the large-scale vegetative change of an ecosystem from a grassland to a shrubland or woodland.  This transition in vegetative cover has the potential to alter local and regional carbon and water flux, as the grasses and trees differ in their rates of CO2 uptake, sensitivity to temperature, responsiveness to precipitation, and amount of water use.  Previous work within the WPE sites has characterized the temperature sensitivities of leaf, soil, and ecosystem fluxes.  However, a further quantification of how temperature varies within in a canopy, how efficiently these two growth forms utilize light, and how they respond to variations in atmospheric CO2 concentrations are needed.  Within the mixed conifer forest, we will estimate the component fluxes within the sub-canopy and canopy of various coniferous species.  Potential projects include quantifying measures of carbon and water flux in transects radiating from the tower to capture variation due to slope, aspect, and degree of canopy cover.  Methods within either setting will include measures of carbon flux using a variety of scale-specific techniques, measures of plant water status, and hemispherical photography for quantification of incoming solar radiation (a driver of sub-canopy biological activity and soil water loss).

Project H: Volatile organic compounds in semi-arid ecosystems

Mentor: Dr. Kolby Jardine, Biosphere 2

During photosynthesis, plants fix atmospheric carbon dioxide into organic material but release a fraction of it back into the atmosphere in the form of volatile organic compounds (VOCs). By participating in photochemical reactions resulting in the production of secondary organic aerosols and toxic compounds like ozone, VOCs have a strong influence on air quality and climate. Initial research indicates that natural biogenic VOC emissions greatly exceed human caused sources by a factor of 10. A preliminary study during April 1999 demonstrated that significant emissions of terpenoids (isoprene and monoterpenes) and oxygenated VOCs are released by plants in the Sonoran Desert (Geron et al., 2006). Due to the water availability, high temperatures and solar insolation, and productivity of this ecosystem during the Monsoon, we suggest that during this season the Sonoran Desert emits large amounts of VOCs into the atmosphere which impact air quality and climate and may represent a significant fraction of the net exchange of CO2. These conditions may not only result in large emission rates of the VOCs typically observed from forests, but may also include the emissions of VOCs not previously detected in the atmosphere. This is in part because of the high diversity of plant species, many of which are unique to the Sonoran Desert, and the sudden availability of water into a dry system. While cooler than the previous summer months, the high temperatures during the monsoon will favor the volatilization of compounds that are not gasses at lower temperatures. We propose to conduct a long term field study where ecosystem scale fluxes of CO2 and VOCs are monitored at a field site near Tucson, AZ before, during, and after the desert monsoon. Our hypothesis is that the high heat and water availability during the monsoon will lead to large emissions of biogenic VOCs that will strongly alter the regional air quality and climate of the southwest. In addition, we suggest that the water limited ecosystem of the Sonoran desert suddenly becomes carbon limited during the monsoon as a large fraction of the assimilated carbon is lost as VOCs.

To test these ideas, we will set up a field site with a meteorology and chemistry tower at a Sonoran Desert field location to be determined. Instruments such a proton transfer reaction mass spectrometer (PTR-MS) and two infrared gas analyzers (for carbon dioxide and water vapor concentrations and for stable isotope analysis of water vapor) will be housed in a small mobile laboratory. Measurements will consist of branch enclosure flux studies, ambient concentration monitoring, and ecosystem flux measurements using eddy covariance (PTR-MS), and relaxed eddy accumulation (REA). VOC samples collected on tubes packed with solid sorbent will be analyzed by gas chromatography-mass spectrometry (GC-MS) at Biosphere 2.

Project I: Solute fluxes in surface water

Mentor: Dr. Jennifer McIntosh, Hydrology and Water Resources

McIntosh is a Co-PI on the University of Arizona’s recently funded NSF Critical Zone Observatory (CZO) project, and lead for the Surface Water Dynamics subtheme. She proposes to involve NSF REU students in CZO research in the Santa Catalina Mountains and Jemez River Basin. Potential student research questions include: How do solute fluxes in surface waters vary as a function of bedrock lithology and age? How do weathering rates vary as a function of hillslope aspect and water transit times? What are the dominant sources of organic carbon to surface streams, and how do these inputs vary across the landscape? Student research would involve field sample collection, laboratory analyses, and interpretation of data.

Project J: Modeling of soil water dynamics

Mentor: Dr. Marcel Schaap, Soil Water and Environmental Sciences

  • students would participating in modeling (computer simulation of ) of soil-water dynamics in selected soils in the CZO project or the B2 hillslope project. In particular, they would study the relation between soil (geo)morphology and drainage and vegetation dynamics. The students would obtain knowledge on how to implement dynamic eco-hydrological systems in computer models and interpret simulation results.
  • students would participate in measurement of water dynamics in B2 hillslope soils. In particular students would use an existing 1-dimensional representation of the B2 hillslope soils to determine soil water dynamics, but also soil chemical weathering rates. The students would obtain experience with complex measurement and control systems.

Project K: Using digital images to link the hydrologic cycle and ecosystem phenology

Mentor: Dr. Shirley Papuga, School of Natural Resources and Environment

For water-limited ecosystems worldwide, global climate models predict (1) changes in the intra-annual variability of precipitation and (2) a decrease in average annual precipitation. These changes in the timing, frequency and magnitude of precipitation will alter the pulses of soil moisture that drive basic phenological activity in water-limited ecosystems, such as flowering and green up.  As a means to begin to understand these changes, we collect daily digital images within the footprint of an eddy covariance tower. These images serve as a robust digital archive of changes within the ecosystem at a daily scale.  Using MATLAB® image-processing tools, we can develop a quantitative measures from these images which can be directly linked to meteorological and flux data collected at the tower.

Project Objectives:
  • understand phenology and the triggers of phenological events;
  • write and test hypotheses;
  • develop computer programming skills through image processing;
  • make projections about how climate change might impact the timing of phenological events;
  • create a scientific presentation of their work in poster, talk, or paper form.

Project L: Interaction of landscapes, pedogenesis and mass fluxes

Mentor: Dr. Craig Rasmussen, Soil Water and Environmental Sciences

The proposed REU project would include quantifying the interaction among landscape position, soil formation and elemental mass flux. Landscape scale variation in chemical and physical weathering has emerged as a key component modulating terrestrial biogeochemistry. In particular, CO2 consumption associated with mineral weathering and the interaction of this process with pedogenesis and erosion appear to be significant factors controlling long-term patterns in atmospheric CO2 concentration. The REU project would specifically focus on the hypothesis that weathering and mass flux vary predictably with landscape position and climate forcing. Testing of this hypothesis will be accomplished by quantifying the mass flux of elements such as Na and Si from soil profiles located at various landscape positions on north facing slopes embedded within various ecosystems along the environmental gradient encompassed by the Santa Catalina Mountains. The project would include a combination of field sampling, physical and chemical laboratory analyses, and data synthesis with the goal of generating a dataset suitable for publication and/or the pursuit of further funding to address broader scale biogeochemical processes.

Project M: Modeling and predicting our environment

Mentor: Dr. Guo-Yue Niu, Biosphere 2

Our living environment is experiencing unprecedented change under global warming. It is challenging to fully assess and project the impacts of climate change on terrestrial hydrological and ecological processes, e.g., water resources, water quality, flood and drought, biological functions and biodiversity because of their complex, nonlinear interactions. Numerical, process-based computer models describing these coupled processes provide us a way to assess historical changes in our environment and project its future changes. We are developing a terrestrial integrated modeling system (TIMS) of the fully coupled processes of atmosphere, hydrology, ecology, geomorphology, and soil and water chemistry. We are validating TIMS against various measurements available through Landscape Evolution Observatory (LEO) and field experiments. We also use individual models of the above disciplines, including Global Climate Models (GCMs), Weather Research and Forecast (WRF) model, land surface models (NCAR CLM and NCEP Noah), 3-D Catchment hydrological (CATHY) model, soil and water chemistry model (CENTURY and PHREEQC 2), and ecological models (CLM-CNDV and ED), to answer interesting scientific questions. Students would apply these models to a specific site, hillslope, catchment, or a region and contribute to model testing and validation.