Research

While much of my research has focused on wetland plant communities, I am willing to study any organism and work in any ecosystem to answer the questions that interest me. I have worked in systems ranging from tropical streams to desert shrublands and am always looking for ways to explore the causes and consequences of patterns of biological diversity across the planet. My research program combines observational and experimental approaches with modeling to develop and test hypotheses and build towards synthetic ecological theory. Currently, I'm working on the following research projects:

1. The links between biodiversity and ecosystem functioning

2. Succession across a Latitudinal Gradient Network (SLaGNet)

3. The Nutrient Network (NutNet)

4. Effects of the invasive grass Microstegium vimineum on N retention in riparian wetlands following disturbance

1. The links between biodiversity and ecosystem functioning

In addition to understanding what controls the distribution of biological diversity, I am interested in understanding how variability in diversity affects ecosystem functioning. In my work with BioMERGE, I have taken a lead role in trying to synthesize much of the conflicting results from this broad field of science as well as push the field into new and productive areas. In particular, I have focused my work on the following areas:

• Understanding the role of functional diversity in controlling ecosystem functioning - how do we measure functional diversity? Do commonly used functional groups work? Do functional classification schemes based on global patterns apply to smaller scale variability in ecosystem functioning?
• How do we scale up from the small-scale experimental results to predict the effects of biological diversity on ecosystem functioning at spatial scales relevant for management?
• How do we incorporate higher trophic levels into our models which are primarily plant based?

Much of my efforts in this area are focused on a large-scale biodiversity experiment that combines a field experiment at a wetland restoration site with an intensive study of the distribution and plasticity of the morphological and physiological traits likely to influence ecosystem functioning among a long list of herbaceous species recommended for use in wetland restoration projects in the state of North Carolina.

back to top

2. SLaGNet collaborator: Jason Fridley, Syracuse University

This experiment explores the patterns and process of ecological succession—the sequence of communities from initial colonization to climax communities. Old fields in the Northeast U.S. can persist for decades in an herbaceous state, while those of the Southeast U.S. typically support closed pine canopies in less than a decade. We seek to identify the significant drivers of this latitudinal pattern of natural old-field woody invasion across the Eastern Deciduous Forest (EDF), and determine whether predicted climate change will significantly influence the rate at which ecological communities transition from herbaceous- to woody-dominated ecosystems. We will specifically investigate the influence of climate, soil fertility, and species pool by isolating each of these variables. We will use our results to predict whether a warmer climate will change the structure of the EDF by altering the persistence of herbaceous-dominated communities in the current forest-field mosaic.

Climate-sensitive thresholds of competitive balance between growth forms suggests the influence of climate on the rate of succession could result in accelerated movement of early successional tree species northward by promoting the competitive advantage of colonizing trees over herbaceous perennials. However, if herbaceous-dominated communities persist in the NE for reasons other than climate, such as soil fertility or the identity of key species, then northward migration of early successional trees could lag substantially. To address these questions, we have selected six old fields study sites across the full latitudinal gradient of the EDF from Syracuse, NY to Tallahassee, FL. We are currently testing methods in a pilot experiment. In the full experiment, at each site woody pioneer species will be grown on ambient and control soils, with and without interference from two types of perennial old-field herbaceous communities.

back to top

3. NutNet

collaborator: Charles Mitchell, University of North Carolina

Human activity has a range of impacts on ecosystems. The Nutrient Network (NutNet) focuses on two impacts. First, fossil fuel combustion and agricultural fertilization have doubled and quintupled, respectively, global pools of nitrogen and phosphorus, relative to pre-industrial levels.  This has altered the global nutrient budget. Nitrogen and phosphorus often stimulate plant growth, potentially impacting the rest of the food web. Second, habitat loss and degradation, and selective hunting and fishing, remove consumers from food webs. Consumers include both carnivores and herbivores — species like deer that consume plants. At the same time, humans add consumers to food webs for conservation, recreation, and agriculture, as well as by accidental introductions of invasive consumer species. These activities have changed the abundance and identity of consumers.  In spite of the global impacts of these human activities, there have been no globally coordinated experiments to quantify their impacts on ecological systems. The NutNet is a grassroots research effort to investigate these ecological impacts within a coordinated research network comprised of more than 40 grassland sites worldwide. Each NutNet site includes thirty square plots, each of which is 5 meters (about 16 feet) on a side. A nutrient addition treatment was randomly assigned to each plot. The fences were built as a second treatment to exclude herbivores from those plots. These treatments are designed to simulate human-caused global changes. Each year, we determine soil nutrient concentrations, plant biomass, and plant species identity to understand how increased nutrient budgets and changes in consumer populations will impact the ecosystem. For more information on the entire Nutrient Network visit their website here.

back to top

Effects of the invasive grass Microstegium vimineum on N retention in riparian wetlands following disturbance

collaborators: Chantal Reid and Dan Richter

Human activity has shifted the distribution of species across the planet's ecosystems. The introduction of nonnative species has the potential to alter an ecosystem's recruitment of native species, geomorphic processes, hydrologic cycles, fire regimes and nutrient cycling. Because climate is a critical regulator of ecosystem processes, predicted climatic changes are likely to interact with the impacts of invasive species in complex ways. Given these challenges, it is critical to understand how exotic species and climate change may interact to change ecosystem processes in invaded ecosystems and, in turn, how these changes in ecosystem processes impact the success of invasive species.

While invasive species can be a significant management concern, they also hold significant promise as a tool for testing basic ecological hypotheses. What role does species identity play in regulating ecosystem functions ? Invasive species that radically alter patterns of species abundance can serve as valuable natural experiments for investigating what factors regulate community composition. The introduction of species that may differ from native species in key traits presents an opportunity to address the question: What role does species identity play in regulating ecosystem functions?

Wetland habitats, particularly riparian wetlands, may be especially vulnerable to invasive species because they are a receiving area for nutrients, water, and plant propagules. Impacts from land-use change can further increase the susceptibility of riparian ecosystems to invasion by exotic species. Physical disturbance around streams has increased cover of invasive species in many wetlands. Furthermore, both urbanization and agricultural intensification has increased N loading to riparian wetlands due to both runoff and N deposition with consequences for both wetland and stream ecosystems. Riparian zones have been viewed as important “hotspots” for N processing because they can remove significant amounts of N from groundwater through denitrification and plant uptake before it can enter aquatic systems. Thus, the restoration of riparian zones and their associated N removal services for the maintenance of healthy ecosystem are critical and have been a main focus of stream restoration activities. Because invasive species are often inadvertently favored in such sites, a better understanding of their impact can improve mitigation strategies.

Predicting effects of Microstegium vimineum (Mivi), Japanese stilt grass, on N retention in riparian wetlands following disturbance requires a mechanistic understanding of what factors affect the success of this invasive species and how this species alters plant-soil feedbacks under both current and future climatic conditions. Given these pressing research needs, we have assembled a team of scientists with expertise in community ecology (Wright), plant ecophysiology (Chantal Reid), and soil biogeochemistry (Dan Richter) with experience working on the impacts of invasive species and wetland ecosystems. We propose a research program with the following objectives:

1. Determine factors controlling establishment and spread of Mivi at disturbed riparian sites.

2. Determine effects of Mivi on woody seedling recruitment and growth to understand how Mivi affects long-term capacity of riparian systems to sequester N.

3. Develop detailed N budgets in areas with and without Mivi , and under three experimental N availability treatments to determine how invasion alters the capacity of ecosystems to reduce N-loading to downstream ecosystems.

4. Use experimental manipulations of temperature and soil moisture to predict how effects of Mivi on tree-seedling recruitment and N-cycling may change under future climate scenarios.

back to top

Webpage updated 6.26.09 / bmm

A spider and its web sparkle in the morning sun, while the spider eats a beetle snack. These were found in the Duke Forest near one of the research sites.