We are interested in the response of terrestrial ecosystems to environmental change, and their role in the Earth’s climate system. This is important because as atmospheric CO2 concentrations increase, and temperature and precipitation patterns are altered across much of Earth, we lack the ability to predict if terrestrial ecosystems will be a source or sink of C to the atmosphere.  Our research focuses on C cycling in plants and soils, with expertise in the application of isotopes as tracers of processes.

Our overarching research question is: what is the fate of C in terrestrial ecosystems? This topic spans temporal scales such as whether newly assimilated C is quickly returned to the atmosphere by plant metabolism or sequestered as soil organic matter for centuries to millennia. This topic also covers spatial scales from the study of microbial processes to plant C allocation to landscape-scale climatic controls on ecosystem function.

We combine field measurements, laboratory work, and computational analyses of large continuous datasets. Currently, there are three major themes in our research:

Plant carbon allocation    •   Belowground CO2 fluxes    •   Linking water and carbon

A methodological tool that is used in most of our research is the application of stable and radioactive isotopic tracers, especially radiocarbon (14C) measured by accelerator mass spectrometry (AMS). Radiocarbon is a powerful tool to study terrestrial C cycling on timescales of hours to millennia. It can be used to determine the age of C, the mean residence time or turnover of C pools, and it can also be used as a source tracer. We use natural abundance, bomb spike, and tracer levels of 14C in our research.  We measure 14C with a new state-of-the-art AMS in the Arizona Climate and Ecosystems (ACE) Isotope Lab at NAU.

Plant carbon allocation

Carbon enters terrestrial ecosystems through one well-understood pathway, photosynthesis. Once within the plant, C may be allocated to above- and below-ground structures, and to growth, metabolism, protection, or storage. However, very little is known as to how plants allocate C, and consequently model representations of these processes are simplistic.

We investigate the fate of newly assimilated C in different ecosystems to understand how new C is allocated to plant metabolism. We also study the availability, distribution, and ecological role of nonstructural carbon (NSC; sugars, starch, and lipids) pools in mature trees.  Ongoing work is targeted towards quantifying the age of C stored within, and used by, mature trees under different global change stresses. Here is short film about our recent Coast Redwood work.

Linking water and carbon

Understanding interactions between C and water cycling is increasingly important as precipitation and temperature patterns change, and C cycling is limited by water across ~40% of the Earth’s surface. We have had various projects that that range from semiarid, subalpine, to tropical forest ecosystems. These studies address basic questions of how C and water cycles (including cloud shading and water inputs, snow, and rainfall) are fundamentally linked, and the implications for the fate of terrestrial C with climate change.

Ongoing work on this topic focuses on more than a decade of measurements at the Rocky Mountain Biological Laboratory and field sites on Snodgrass Mountain near Crested Butte, Colorado.  This is the headwaters of the Colorado River and a collaboration with an intensive science effort by Lawrence Berkeley National Laboratory and funded by the Department of Energy. 

Belowground CO2 fluxes

The majority of ecosystem respiration comes from belowground (called soil respiration or the soil CO2 flux). Current models do not adequately represent the mechanisms causing variation in the soil CO2 flux primarily because of methodological difficulties associated with measuring the soil CO2 flux at high temporal frequencies, and separately attributing flux variations to different biotic (plants and microbes) and abiotic processes.  Our research addresses these primary experimental challenges by combining high-time resolution flux measurements and 14C source partitioning techniques.

We are seeking a PhD student or postdoc interested in synthesizing arid ecosystem soil respiration fluxes and establishing new measurements in Northern Arizona.