Projects

We study ecosystem ecology from microbial to global spatial scales. Below is a partial list of some of our projects.

Soil Organic Carbon Storage and Potential CO₂ Fluxes Under Climate Change Scenarios in Tropical Forest Soils

Because soils contain the largest terrestrial carbon pool, there is much concern that increasing global temperatures may speed up microbial respiration of soil stocks, thereby acting as a positive feedback to global warming. The goal of this project is to understand how tropical forest soil C pools may respond to simulated climate warming in the laboratory, and the mechanisms that underlie these responses. The mechanisms include: i) heterogeneous temperature responses of carbon pools that vary in recalcitrance, ii) temperature-mediated shifts in microbial community composition and functions, and iii) physico-chemical reactions that transfer carbon between pools with different turnover times. To generalize results across the tropics, I am using field-collected soils from a variety of tropical forests. This project is funded by an NSF ADVANCE Fellowship.

Effects of Simulated Climate Warming and Plant Diversity on Belowground Carbon Cycling in a Minnesota Prairie

Prairie grasslands in the United States have great aerial extent, high biodiversity and productivity, and often fertile soils with large carbon stores. Many general circulation models forecast that the climate in this region will be warmer, and long-term meteorological records at the Cedar Creek Natural History Area (CCNHA) in Minnesota confirm that these changes are already occurring. To understand the effects of increasing temperatures and plant diversity on prairie ecosystem structure and function, Dr. David Tilman is establishing a manipulative experiment in an ongoing, long-term field experiment at CCNHA that has numerous plots differing in their plant species compositions and plant species numbers (referred to as the BAC—Biodiversity and Climate Experiment). Infrared heat lamps have been installed to be used to create three experimental treatments: ambient temperatures, low elevated temperatures (1°C over ambient) and high elevated temperatures 3°C over ambient). The experiment is replicated in vegetation plots containing single plant species and multi-species plots (4 or 14 species), thus allowing for generalizations across prairie plant communities that vary in species composition, plant productivity and initial conditions. To understand how these environmental variables impact belowground C cycling, we will be making periodic measurements of soil CO₂ effluxes, soil C storage, microbial biomass and composition, and root productivity. This project is funded by the NSF LTER program.

Interspecific Variation in Leaf Traits and Litter Decomposition Rates in a Dry Tropical Forest

The decomposition of dead leaves and roots is one of the major pathways by which carbon fixed during photosynthesis is returned to the atmosphere as carbon dioxide. One challenge to understanding tropical forest C dynamics is characterizing the functional consequences of the vast biotic diversity contained in those forests. Towards that end, Dr. Manuel Lerdau and I are using a litter bag experiment to measure leaf litter decomposition rates of 26 plant species alone and in combination in the dry forest of northwestern Costa Rica. These species vary in life form, phenology, and relatedness. In addition to chemical measures of litter quality, we are measuring a number of functional leaf traits including toughness, nutrient resorption and specific leaf area. This research is funded by a grant to Dr. Manuel Lerdau from the Mellon Foundation.

Depth-Dependent Variations in Microbial Community Composition, Function and Soil Carbon Storage in a Weathered Tropical Moist Forest Soil

Many tropical forest soils have profiles that extend for meters in depth. Recent studies have found substantial carbon storage in soil organic matter below 1 m depth, and, more intriguingly, substantial microbial activity in deep soil layers. The extent to which deep soil C pools may be mobilized by global environmental changes such as increasing temperatures is unknown. The goal of this project is to determine how soil C concentrations, microbial biomass and composition, and potential respiration vary with depth in Oxisols from Barro Colorado Island, Panama. This project involves Matthew Warner, an undergraduate at UMN and is supported by an NSF REU Supplement.

Effects of Nutrient Amendment on Microbial Community Composition and Function, and Soil Carbon Storage in a Moist Tropical Forest

The goal of this study to understand the effects soil resources have on the structure and function of microbial communities. The collaborators on this project are Joe Yavitt (Cornell), Kyle Harms (LSU), Joe Wright (STRI), Bonnie Keeler (UMN), Adrienne Keller (Macalester College, UMN), and Mark Bradford (UGA). We are investigating how microbial community composition and C cycling functions shift in response to added nutrients as part of an ongoing factorial fertilization experiment in lowland Panamanian moist forests (added nutrient treatments include: N, P, K, NP, NK, NPK, and micronutrients). We predict that increased nutrient availability will shift the community composition of decomposer fungi, which are largely regulated via bottom-up forces, and in turn will have consequences for C and nitrogen storage in soil organic matter. This study is funded by a grant from the Smithsonian Tropical Research Institute to Jennifer Powers and Joe Yavitt.

A Regional-Scale Analysis of Regenerating Tropical Dry Forests in Costa Rica: Measurements and Models of the Linkages among Biodiversity, Ecosystem Function and Carbon Storage

Tropical dry forests (TDF) in Pacific Central America were once extensive, stretching from Mexico to Panama. Currently, TDF have been reduced to less than 1% of this area by land conversion to pastures. Although little is left of the original dry forest, efforts are being made in Costa Rica to regrow TDF through grazing restrictions and fire suppression. The regeneration of TDF has important consequences at a regional scale for both biodiversity and ecosystem functions such as carbon (C) storage. The goals of this project, funded by a NASA New Investigator Award, are to: i) map patterns of regeneration through time in Costa Rican TDF using multitemporal TM imagery, ii) use field measurements to characterize how species differ from one another with respect to key ecophysiological traits, and spectral reflectances, iii) determine the extent to which Hyperion imagery is useful for identifying phenological and taxonomic diversity, and iv) use these data to parameterize simulation models to integrate across spatial scales and make predictions about changes in forest area, diversity and C storage under climate change scenarios. Regional-scale analyses such as this harness the power of remote sensing, i.e. the observation of ecosystem attributes over space and time, while linking these coarse-scale patterns to finer-scale mechanisms.

Variation in plant functional traits and foliar nutrients among individuals and tree species with different leaf habits in a tropical dry forest

The tree species found in tropical dry forests display a wide range of leaf phenological strategies, you can find everything from true evergreen species with long leaf lifespans to drought-deciduous species, and everything in between. We are trying to understand if there is broad correspondence between leaf habit and functional plant traits that affect ecosystem processes. To accomplish this, we have collected replicated samples from 87 species of tropical dry forest trees found in Guanacaste and Palo Verde National Parks in Costa Rica. We are analyzing the following traits for these species: specific leaf area, leaf water content, foliar %N, foliar %P, d13C (as an indicator of water use efficiency), and wood specific gravity.