UNDERSTANDING TERRESTRIAL SYSTEMS

My research focuses primarily on how terrestrial plants respond-to and influence the environment in which they exist.  How will plants respond to anthropogenic global change?  Will increased plant growth help mitigate human CO2 emissions, or will plant growth be limited by some other resource? How will changing climate, CO2 concentrations, and disturbance regimes influence plant production, resource acquisition, and community structure? How do we incorporate answers to these questions into global Earth System Models? 

 

While my primary focus is at the ecosystem scale, these questions require us to view plants at the individual physiological and the community scales as well.  My research involves each of these levels of scope in persuit of a better understanding of the role of plants in global processes.

SYMBIOTIC NITROGEN FIXATION

Despite composing 78% of the atmosphere, nitrogen (N) is the most common limiting element to the growth of terrestrial vegetation.  Nitrogen-fixing plants and their symbiotic bacteria have the greatest natural potential to bring new N into the biosphere. My work on nitrogen-fixing plants focuses on the abundance patterns of nitrogen fixers, the environmental regulators of nitrogen fixation, and theecological effects of these nitrogen fixers. 

Abundance Patterns of N Fixers

 

The ability of N fixers to bring new N into an ecosystem is first determined by their presence at a site. My collaborators and I aim to understand the large-scale distribution patterns of N fixers and the drivers of these patterns. Using Forest Inventory Analysis data, we assess the patterns of N-fixer abudances across North America and the ecological drivers of these abundances. We also use global plant-trait databases to study the particularities of N-fixer seeds and their dispersers in an effort to understand how N fixers are dispersed to particular sites.   

Controls on N Fixation


Once an N fixer establishes at a site, the amount of N that it brings into the ecosystem is determined by its individual fixation rates. I use a combination of greenhouse experiments and field sampling to assess the physiological constraints and competetive advantage or disadvantage of N fixation in different environmental conditions.  Specifically, I focus on how light and soil-nitrogen availability regulate N fixation rates, and if these effects differ in the presence of competing plants.

Ecological Effects of N fixers

Do N fixers have the facilitative effects on the growth of the surrounding forest, as we would expect given their ability to provide N to these systems? I work in a long-term tropical secondary forest dynamics study - the NeoSelvas Bosques Project - using 20 years of tree census data in 8 1-ha tropical forest plots to better understand how N fixers influence the growth of neighboring trees. We find that at our site in Costa Rica, N fixers actually have a negative effect on the growth of neighboring trees and they slow the growth of forests at the plot scale.

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Figure 2_Boxplot N fixed per plant_edite
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RISING CO2, ROOTS, AND THEIR SYMBIONTS

Plant growth is expected to increase as atmospheric CO2 concentrations rise due to human activities.  This increased plant growth has the potential to capture some of the excess anthropogenic CO2, thereby mitigating human impacts on atmospheric CO2 levels.  However, this increased plant growth will likely be limited by plants' ability to acquire the various soil resources needed for extra growth.


 


By experimentally manipulating CO2 concentrations, we can monitor plant growth and allocation responses to elevated levels of CO2.  My work focuses on changes in the biomass, architecture, and mycorrhizal colonization of fine root systems, which are the primary means of soil resource uptake for the plant.  At the Duke Free-Air-CO2-Enrichment (FACE) experiment, we've found that trees in elevated CO2 conditions increase their root length and mycorrhizal colonization, and change their architecture, to increase belowground foraging to sustain increased growth.

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STUDYING BELOW-GROUND SYSTEMS

Despite their major contribution to resource acquisition, carbon storage, and nutrient cycling, we know relatively little about root systems and their associated symbionts.  This is largely due to the major logistical hurdles posed by studying subjects underground.  


In light of these methodological difficulties, I strive to improve the efficiency and accuracy of studying roots and mycorrhizae in situ.  These efforts have involved novel statistical approaches to determining sufficient sample sizes as well as improving the ability of researchers to expand local root measurements to the landscape scale.

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SEEDLING DYNAMICS IN DOMINICA

For forest trees, the seedling stage serves as a major environmental filter for individual survival.  Because of this, the influence of biotic and abiotic environmental factors on the survival and competition of seedlings can largely determine the community composition of the forest canopy over time.  Thus, the species composition of an entire forest system (plants, herbivores, and secondary consumers) is influenced by the dynamics of the tree seedling layer.

 


My collaborators and I use a set of permanent seedling plots located on the island of Dominica in the Lesser Antilles to investigate the predominant abiotic and biotic controls of seedling survival in tropical island rainforests.  We are also investigating the role of native and introduced seed dispersers on the movement of seeds and the influence of dispersal on seedling dynamics in these forests.

Photo Credit: Durland Fish