Current Research Projects

The influence of mycorrhizae on fine root decomposition and soil carbon processing
Andrew W. Mellon Conservation and Environment Program
Collaborators: Adam Langley, Kitty Gehring, and Nancy Johnson

Ecological Restoration and the Water and Carbon Budgets of Ponderosa Pine Forests
Ecological Restoration Institute
Collaborators: Oleg Menyailo, Tom Kolb, George Koch, Mario Montes-Helu

Hydrology of a scrub-oak woodland under carbon dioxide enrichment
National Science Foundation, 1 April 1999 - 31 March 2002
Collaborators: Jiahong Li, William Dugas, Bert Drake

Interactions between the carbon, nitrogen, and water cycles under carbon dioxide enrichment
Smithsonian Institution, 1 March 1999 - 28 February 2004
Collaborators: Dale Johnson, Paul Dijkstra, Graham Hymus, Bert Drake

An Isotope-Ratio Mass Spectrometer for Ecology and Environmental Biology at Northern Arizona University
National Science Foundation
Collaborators: George Koch, Steve Hart, Dean Blinn, Tom Whitham

Soil health within the Flagstaff Wildland-Urban Interface
US Forest Service, 1 August 1999 - 31 December 2001
Collaborators: Stephen C. Hart, Catherine Gehring

CCLI: The C. Hart Merriam Elevational Gradient: Toward a Unified Ecology Curriculum at Northern Arizona University
National Science Foundation
Collaborators: Neil Cobb, George Koch, Tom Whitham

Summary of Research Projects

CAREER: Ecosystem Responses to Rising CO2 and Climate Change: Feedbacks through the nitrogen cycle
National Science Foundation, 1 July 2001 - 30 June 2006
Collaborators: Paul Dijkstra, Yiqi Luo, Chris Field

Project Summary: Rising atmospheric CO2 and global warming could alter the functioning of terrestrial ecosystems, and a number of experiments have already been established to document these potential changes. These experiments have demonstrated some broad similarities among different ecosystems in their above-ground responses to these global changes, but whether biogeochemical responses below-ground exhibit predictable patterns is largely unknown. For example, elevated CO2 and warming can alter nitrogen availability to plants and nitrogen inputs to and losses from ecosystems, but results to date are equivocal, with empirical support for both increases and decreases in nitrogen availability. However, because of the short time scale of empirical studies to date and the different methods used, contrasting results can not be compared with confidence.

The proposed work will examine how elevated CO2 and warming alter nitrogen cycling in a broad array of terrestrial ecosystems, and how these changes will feed back to affect plant and ecosystem productivity. The research component of this CAREER proposal comprises: 1) N cycling measurements in CO2 and climate change experiments, 2) controlled greenhouse experiments, and 3) integration through modeling. The field experiments will document changes in N cycling in response to increased temperature and elevated CO2 using a long-term 15N tracer technique that will reveal time-integrated effects of these global changes on N cycling. The greenhouse study will explicitly determine the relative importance CO2- and warming-induced changes in soil water content for specific processes in the N cycle, providing a mechanistic underpinning to the field studies. The modeling integration will explore the consequences for longer-term ecosystem responses.

This research complements the proposed CAREER teaching and outreach activities. Observations and experiments (including those proposed here) along a 3000-m elevational gradient near Northern Arizona University will serve as the foundation for inquiry-based laboratories for courses in Ecosystem Ecology and Microbial Ecology. By drawing on publicity surrounding global change and by providing a scientific foundation for understanding these topics, these activities are designed to better engage undergraduate students in the process of science. This career plan brings together the PI’s interests in global change research and science education. The research extends past work on elevated CO2 and carbon and nitrogen cycling and helps outline a broader long-term objective: to understand how interactions among element cycles (here, C, N, and H2O) influence ecosystem processes, including responses to global change. The proposed teaching activities are designed to place results from this research – and the research process itself – in a broader context accessible to the public.

Project Summary: Plant and ecosystem productivity are strongly influenced by nutrient availability, largely determined by rates of decomposition of plant litter. Decomposition rates of above-ground plant tissues (leaves and stems) and associated rates of nutrient mineralization are well characterized by indices of 'litter quality', such as nutrient content, carbon:nutrient ratios, or lignin:nutrient ratios. However, for roots, for reasons that we do not yet understand, relationships between decomposition rate and such indices of litter quality are less consistent. Mycorrhizae are ubiquitous and strongly influence root chemistry, in ways that traditional indices of litter quality are likely to miss. For these reasons, including the mycorrhizal status of decomposing roots could substantially increase our ability to predict root decomposition rates in terrestrial ecosystems.

Most plant species are associated with soil fungi, forming root-fungus associations called mycorrhizae. This association facilitates nutrient and water uptake by plants and provides carbon through photosynthesis to fungi. Much is known about how this association affects living roots and whole plants, but the consequences of mycorrhizal infection for root litter decomposition have never been investigated. Differences in the degree of mycorrhizal infection are likely to strongly influence root decomposition rates, by altering both the carbon and nutrient quality of the roots for decomposer microorganisms. While mycorrhizae usually increase nutrient concentrations in all plant tissues by enhancing nutrient uptake, they also affect root nutrient concentrations because of the high-nutrient content fungal structures that are built inside the root as the mycorrhizal association develops. While high in nutrient content, these structures often contain a recalcitrant carbon skeleton (e.g., chitin), so that rates of decomposition are likely to deviate from predictions based on indices of litter quality.

This project would use greenhouse experiments and natural gradients in the mycorrhizal status of plants to generate root litter material from a broad range of species in which the nutrient content and mycorrhizal status of the root litter vary independently. Following the characterization of litter for nutrient and carbon 'quality' (including fractions not typically examined, such as chitin), mycorrhizal status would be assessed, and decomposition experiments would be conducted under laboratory and field conditions. Comparing decomposition rates of litter material from different treatments would allow a quantitative assessment of the influence of mycorrhizae and associated changes in root chemistry on root decomposition rates, and thus has the potential to substantially advance understanding of the controls over litter decomposition and nutrient availability in terrestrial ecosystems.

Ecological Restoration and the Water and Carbon Budgets of Ponderosa Pine Forests
Ecological Restoration Institute
Collaborators: Oleg Menyailo, Tom Kolb, George Koch, Mario Montes-Helu

Project Summary: Assessing the effects of restoration on water and carbon budgets of ponderosa pine forests is critical to predicting the long-term impacts of restoration on forest productivity and water use, ecosystem level processes with clear implications for wildlife, diversity, and links to critical aquatic habitats in the arid Southwest. If forest productivity increases with forest restoration, one might expect water use to increase in concert, as the two are often tightly correlated. Alternatively, because C4 grasses inhabit the understory in restored stands, we might expect that the greater water-use efficiency conferred by this photosynthetic pathway could allow greater forest productivity with less total stand water use. However, understanding the mechanisms altering water-use in restored and control stands requires sophisticated techniques that enable one to partition water use accurately between trees and understory vegetation, and between plants capable of more water-use-efficient C4 photosynthesis versus plants that utilize C3 photosynthesis. In a companion proposal, Kolb, Koch, and Montes-Helú propose to use sap-flow techniques to measure tree water use, and understory removal treatments to assess water use by the understory vegetation. Here, we propose to complement these efforts by: 1) measuring the source of water taken up by trees and understory vegetation; 2) determining the relative contributions of trees and understory vegetation to total stand water flux, and 3) determine the relative contributions of trees and understory vegetation to total forest productivity and respiration. Novel, stable isotope techniques offer a powerful tool for addressing these questions non-intrusively in restored and control forest stands. This research will help managers assess the mechanisms through which forest restoration alters productivity and water use, and will demonstrate a tool for assessing these impacts that could be applied in other restoration efforts.

Hydrology of a scrub-oak woodland under carbon dioxide enrichment
National Science Foundation, (1 April 1999 - 31 March 2002)
Collaborators: Jiahong Li, William Dugas, Bert Drake

Project Summary. This research integrates field experiments and models focusing on the responses of a Florida scrub-oak ecosystem to elevated atmospheric carbon dioxide (CO2). This research adds to an ongoing study invetigating the effects of elevated CO2 on a naturally-occuring stand of scrub-oak vegetation. The main goal of the ongoing project is to determine the effects of elevated CO2 on ecosystem carbon balance. This proposal expands this research by adding an additional goal: to determine the effects of elevated CO2 on the water cycle, including plant transpiration, evapotranspiration, soil moisture, and water table dynamics, as well as how differential responses of the co-dominant oaks to elevated CO2 mediate these changes.
In particular, this research will show how the responses of the two co-dominant oak species in scrub oak mediate reductions in ecosystem water loss through evapotranspiration, and how these water savings are partitioned between surface soil water stores and the water table. Transpiration in oak individuals in elevated and ambient CO2 treatments will be measured in the field, along with the stable isotope composition of stem water in these species, which indicates the depth in the soil from which the water is obtained. Together, these measurements will determine by how much elevated CO2 reduces transpiration, and how that water savings is partitioned in the soil. This information will be combined with measurements of soil moisture, water table depth, and evapotranspiration, and synthesized through modeling.
This research is important because it focuses on ecosystem responses to elevated CO2 that have not been previously addressed, but that will likely represent critical changes in a future, high-CO2 world. This will be one of the first studies in any ecosystem to develop a detailed hydrologic budget and hydrologic model that partitions water savings in elevated CO2 between surficial and deep water stores in the soil, where savings will have very different biogeochemical consequences. Additionally, this research will determine how individualistic species responses to elevated CO2 mediate changes in system hydrology, and thus will be relevant in predicting changes in hydrology in other systems where species show differential responses. By exploring the interactive effects of elevated CO2 through altered hydrology, this research will substantially advance our knowledge of the responses of terrestrial ecosystems to rising CO2.

Interactions between the carbon, nitrogen, and water cycles under carbon dioxide enrichment
Smithsonian Institution (1 March 1999 - 28 February 2004)
Collaborators: Dale Johnson, Paul Dijkstra, Graham Hymus, Bert Drake

Project Summary. Rising atmospheric CO2 could alter soil nitrogen (N) cycling, shaping the responses of terrestrial ecosystems to elevated CO2. Increased carbon input to soil through increased root growth and increased soil water content due to decreased plant water use in elevated CO2 can all affect soil N transformations and thus N availability to plants. This research will determine the effects of elevated CO2 on nitrogen N cycling in a scrub-oak ecosystem by using a long-term 15N tracer, and will relate observed changes in N cycling to changes in carbon input to soil and in soil hydrology.

An Isotope-Ratio Mass Spectrometer for Ecology and Environmental Biology at Northern Arizona University
National Science Foundation
Collaborators: George Koch, Steve Hart, Dean Blinn, Tom Whitham

Project Summary. This proposal requests funds to purchase an isotope-ratio mass spectrometer for research in the fields of Ecology and Environmental Biology at Northern Arizona University (NAU). This instrument will be used by 15 faculty members in 5 different departments at NAU to address questions such as tracing energy flow through food webs, determining the fate of environmental contaminants (e.g., runoff of fertilizaer from agriculture into aquatic ecosystems), measuring the efficiency with which plants use water, identifying the sources of water to riparian vegetation, and measuring turnover of nitrogen and carbon in terrestrial ecosystems, particularly changes in these processes in response to environmental perturbation. Use would also include applications in Archaeology, determining, for example, when human societies have relied on certain plant species such as corn, which has a distinctive carbon isotope "signature" that is reflected in the bones of consumers. Stable isotope techniques allow the investigation of these questions quantitatively and non-intrusively (without the environmental hazards of radioisotopes), and thus offer considerable advantages over other techniques. Indeed, many of these ecological and environmental questions can only be addressed by using stable isotopes. Use of these techniques at NAU is severely hampered by not having ready access to a mass spectrometer facility. The funds requested in this proposal would relieve this constraint, and thus greatly enhance research in Ecology and Environmental Biology at NAU.

Soil health within the Flagstaff Wildland-Urban Interface
US Forest Service, 1 August 1999 - 31 December 2001
Collaborators: Stephen C. Hart, Catherine Gehring

Project Summary: We propose an integrated, multidisciplinary assessment of the impacts of fuel management and restoration activities on the soil ecosystem in ponderosa pine forests of the Flagstaff Wildland-Urban Interface. The primary goal of the research is to determine the consequences of different field management and restoration activities on key aspects of forest floor and mineral soil structure and function.

CCLI: The C. Hart Merriam Elevational Gradient: Toward a Unified Ecology Curriculum at Northern Arizona University
National Science Foundation
Collaborators: Neil Cobb, George Koch, Tom Whitham

Project Summary: This project will develop an integrated, field-based ecology curriculum at Northern Arizona University that will involve undergraduate students (freshmen to seniors) in state-of-the art research in population, community, and ecosystem ecology. The core of this project is a new field-based quantitative laboratory for General Ecology, BIO 326, a course now required by all majors in Biology, in which students will conduct research along a 3000m elevational gradient spanning desert to tundra ecosystems as a natural experiment. Additionally, this project will enhance introductory biology courses (Introductory Biology and Unity of Life) by adding field exercises along the gradient, which will introduce students to the experimental system they will revisit more comprehensively in General Ecology. The project will also add advanced exercises involving the gradient to existing laboratories of a number of upper division courses in ecology (Entomology, Plant Physiology, Mammalogy, Microbial Ecology, Ecosystem Ecology, Stable Isotope Techniques, Field Ecology). Thus, this project will substantially revise and will provide a unifying theme to the ecology curriculum at Northern Arizona University: Students will visit the same sites in different courses and in different years. They will learn how the same systems and gradients can be approached from different perspectives and used to address some of the major ecological, environmental and conservation challenges of our time. This proposal requests funds to purchase the equipment required to carry out these integrated laboratories in ecology.