Terrestrial Ecosystems and Global Change
(Figure from Walker, Taylor, et al. 2020 New Phytologist)
Research in the Taylor Lab centers around the responses of terrestrial ecosystems to various global change drivers and how those responses, in turn, feedback to influence the trajectory of global change. How will changing climate, CO2 concentrations, nutrient inputs, and disturbance regimes influence plant production, resource acquisition, and community structure? Will plants grow larger under higher CO2 concentrations, capture more carbon, and help mitigate human CO2 emissions, or will increased plant growth be limited by some other resource? How do anthropogenic changes in disturbance regimes such as human land use, hurricanes, and invasive species influence carbon and nutrient cycling in terrestrial ecosystems? How do we best incorporate answers to these questions into global Earth System Models?
To address these questions, our work operates at molecular, individual, community, and ecosystem scales. We strive to combine inference from controlled manipulative experiments, natural observational studies, and large-data syntheses to better understand the patterns and mechanisms of ecosystem responses to global change. This multi-scale perspective leads us to conduct work from lowland tropical rainforests to the Alaskan tundra.
Symbiotic Nitrogen Fixation
Despite Composing 78% of the atmosphere, nitrogen (N) is the most common limiting element to the growth of terrestrial plants. Nitrogen-fixing plants and their symbiotic bacteria have the greatest natural potential to bring new N into the biosphere. Our work on nitrogen-fixing plants focuses on the abundance patterns of N fixers, the environmental regulators of N fixation, and the ecological effects of these N fixers in terrestrial ecosystems.
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. Along with collaborators across the globe, we are working to understand the large-scale distribution patterns of N fixers and the drivers of these patterns. Using large-scale datasets such as the U.S. Forest Inventory Analysis data, the Nutrient Network, the CoRRE network, and the GEx network, we assess the patterns of N-fixer abundances in forests and grasslands 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. We use a combination of greenhouse experiments and field sampling to assess the physiological constraints and competitive advantage or disadvantage of N fixation in different environmental conditions. Specifically, we 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 forests, as we would expect given their ability to provide N to these systems? Our data indicate that the actually effect of N fixers can range from strongly positive to strongly negative depending on site and species characteristics. Along with our collaborators, we work at site- and continental-scales to better understand whether N fixers promote or inhibit forest growth and what ecological mechanisms drive these effects.
Plant Responses to Rising CO2
Terrestrial plants have a massive capacity to capture carbon, and the potential for increased plant growth under future elevated CO2 may have a critical mitigating influence on human CO2 emissions to the atmosphere. However, the extent of a future “CO2 Fertilization Effect” and what will regulate this effect are not well understood.
Our research aims to understand how temperate and tropical forests will change in a higher CO2 world. This work involves monitoring aboveground tree growth as well as the belowground communities that help trees obtain the resources needed to fuel extra growth under elevated CO2.
Volcanic CO2 Vents in Tropical Forests
Traditional large-scale CO2 Enrichment experiments have proven logistically impossible in the tropical rainforests that we expect to have a disproportionate effect on the global response to rising CO2. To overcome this challenge, we are working with collaborators at NASA and McGill University to study tropical forests surrounding natural CO2 vents on the flanks of volcanoes in Costa Rica. These vents serve as natural CO2 enrichment experiments that have been operating for hundreds of years. By assessing biomass, growth rates, and nutrient dynamics in these forests, we can gain an entirely new perspective on the future of tropical forests under increasing atmospheric CO2.
Fine-Root Dynamics at the Duke FACE Experiment
By experimentally manipulating CO2 concentrations, we can monitor plant growth and allocation responses to elevated levels of CO2. Along with our collaborators at the College of Charleston, our 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.
Studying Belowground 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, we are working 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.
Tropical Forest Responses to Hurricanes
Hurricanes can create large-scale forest disturbance, and data indicate that the strength of these storms is increasing with rising global temperatures. We work with collaborators at Clemson University to better understand the community dynamics of tropical forests on the island of Dominica in the Caribbean Lesser Antilles. We have established a series of long-term forest monitoring plots across the island. Initial censuses from these plots allow us to monitor long-term recovery from Hurricane David in 1979 and intermediate recovery from Hurricane Dean in 2007. Recently, Dominica was the first site of landfall for Hurricane Maria in 2017, which decimated the island’s forests. We are now monitoring recovery from Maria to better predict the community and ecosystem dynamics of forests following major hurricanes that are predicted to become increasingly common disturbances as the world continues to warm.