Current projects      
  The role of multiple climate change factors in shaping community and ecosystem response to global change  

Old-field Community, Climate and Atmospheric Manipulation (OCCAM)

In collaboration with Richard Norby (ORNL), we are using an established multi-factor climatic change experiment in an old-field to test the relative importance of direct and interactive effects of climatic drivers on ecosystem function and processes. This experiment manipulates atmospheric CO2 levels (ambient, elevated + 300 ppm), warming (ambient, elevated +3.5 C), and soil moisture (dry, wet) in a constructed old-field community that consists of seven plant functional types: grasses (C3 and C4), forbs, and nitrogen fixers. One of the unique features of this study is that it was constructed to understand the effects of these treatments on both aboveground and belowground processes simultaneously.

In the winter of 2007 we initiated a woody seedling invasion study (see photo on right) to examine whether multiple climate change factors interact to alter woody plant encroachment into old-field ecosystems and if these changes may be explained by seed characteristics. The results to date suggest an important interaction between plant life history and climatic change effects.

The role of nutrient availability in limiting forest response to global change

The response of terrestrial ecosystems to climatic change is likely constrained by extant traits of the ecosystem such as nitrogen availability. Using a large-scale field experiment (Free-Air CO2 Enrichment Experiment - FACE) that manipulates CO2 levels in a sweetgum forest, we are collaborating with Colleen Iversen and Richard Norby to examine if previously observed changes in carbon allocation to fine roots is due to nitrogen limitation and what an increase in soil carbon from root inputs might mean for long-term ecosystem carbon storage.

We are also interested in how climate change might alter terrestrial and aquatic decomposition and nutrient release and the organisms that are responsible for these processes. We are collaborating with Carri LeRoy (Evergreen State) on these projects.
Interactive effects of climate change and small mammal populations on old-field ecosystem function

In collaboration with Chris Habeck, Rick Lindroth, and Richard Norby, we aim to understand how small mammal activity interacts with a climatic change factor to influence the structure, function and trajectory of a transitional ecosystem.

 
Scaling across levels of biological organization to better understand how climatic change may alter ecosystem function

Belowground respiration is an integrator of ecosystem metabolism and a key component of the interaction between the terrestrial biosphere and the atmosphere. It consists of three components - root respiration, bacterial respiration, and fungal respiration. Each of these components is likely to respond to changes in the global climate differently, yet ecologists have no way of adequately teasing apart root, bacterial, and fungal respiration from bulk soil respiration measurements. In collaboration with Christopher Schadt (ORNL), Hector Castro (ORNL), and Richard Norby (ORNL) we are are using quantitative real-time PCR to assay portions of the ecosystem transcriptome that we hypothesize will be indicative of autotrophic (plant root) and heterotrophic (fungal, bacterial) respiratory activity.
The role of root exudation and rhizosphere microbial community structure and function in regulating ecosystem carbon flow
Plants allocate a significant proportion of their carbon belowground. These often labile carbon substrates serve as the energy source for complex microbial communities that inhabit the rhizosphere and stimulate ecosystem nutrient processes. In spite of its ecological importance, root exudation is poorly understood and even less is known about how changes in the amount and type of root exudate might alter the function of the associated soil microbial community. In collaboration with Hector Castro,Christopher Schadt,, Marie-Anne de Graaff, and Charles Garten (all ORNL) we propose to define specific linkages between microbial diversity and function, and root exudates by merging two technologies - 13C stable isotope probing and DNA sequencing and Functional Gene microArrays - to link root exudate quality and quantity with microbial community structure and function in the rhizosphere.  
 
The role of intra-specific variation in leaf and root litter chemistry among hybrid poplar genotypes in soil carbon sequestration
In collaboration with Stan Wullschleger and Charles Garten (both at ORNL) we are using a basic ecosystem model to test the hypothesis that the intra-specific variation in leaf and root litter chemistry and decomposition rates among hybrid poplar genotypes has large and sustained impacts on soil carbon sequestration and ultimately alters ecosystem trajectories.
The role of foundation species and faunal biodiversity in ecosystem structure and function

Knowledge of how ecosystems respond to the loss or gain of foundation species can substantially reduce uncertainty in forecasts that project ecosystem dynamics from ongoing changes in biological diversity. In collaboration with Aaron Ellison (Harvard Forest), Nick Gottelli (UVM), and Nathan Sanders (UT), we are estabiishing an experiment to determine the direct and indirect mechanisms by which the loss of a foundation tree species from eastern North American forests (Hemlock) leads to changes in core ecosystem processes. One question we are addressing is: Can observed changes in ecosystem processes be accounted for directly by changes in faunal biodiversity, or is there a unique interaction between the loss of the foundation species and the compositional shifts in associated faunal assemblages?

Experimental plots have been established at Harvard Forest (MA) and are being established at Coweeta Hydrological Laboratory (NC).

Herbivore susceptibility and resistance effects on ecosystems

We are also interested in understanding how a single species, or the genetic variation within a species, can impact both community structure and ecosystem function. For example, ongoing research in our lab investigates how chronic insect infestation of piñon pine by a needle scale or stem-boring moth alters the nitrogen and carbon budget of a piñon-juniper woodland. Long-term studies in Arizona have identified trees that are resistant and susceptible to each herbivore, and because piñon resistance to herbivory has a genetic component, the potential exists for linking population-level variation in plant genetics to key ecosystem processes. We collaborate on this project with Tom Whitham (NAU), Steve Hart (NAU) , George Koch (NAU), and Samantha Chapman (Villanova).