Natural Methane Emission & Capture
Tree-mediated methane emissions in upland and lowland forests
In the global methane budget, the largest source of uncertainty is from natural emissions, such as those from tree stems. Tree stems have increasingly been recognized as methane sources, due to internal microbial methane production as well as transport from soils, but have also been suggested to serve as global methane sinks due to bark-hosted microbial methane oxidation.
The extent, variability, and drivers are unclear, particularly in upland forests, where stem emissions may partially offset soil methane uptake, and coastal forests, where salinity gradients and lateral flows shape exchanges. I am working in temperate mixed forests (Yale-Myers Forest and Harvard Forest), northern forests (Howland Forest), and subtropical forests (Florida Coastal Everglades) to quantify methane fluxes across the landscape from different ecosystem components across a range of temporal and spatial scales.
Methane emissions from small forested wetlands and terrestrial-aquatic interfaces
Sharp biogeochemical gradients at terrestrial-aquatic interfaces in temperate forests, defined by fluctuating water tables, lead to varying conditions that produce and release methane (CH4) and carbon dioxide (CO2) depending on saturation levels. The dynamic nature of these fluxes, influenced by increasing precipitation and water table fluctuations, makes modeling CH4 emissions in small, transient wetlands challenging.
With the Matthes Lab at Harvard Forest, and along with the Holbrook Lab and Harvey Lab, we are exploring how mechanisms of methane production, oxidation, ebullition, and plant transport vary between perennial and transient wetlands. We are further exploring the potential importance of transient wetlands for net methane flux and lateral transport, the effects of changing precipitation regimes, and scaling and representation of small wetlands methane flux in eddy covariance measurements, reactive transport models, and ecosystem process models.
Methane emissions in blue carbon ecosystems: the NASA Blueflux campaign
Mangroves provide numerous ecosystem services including blue carbon storage, but it is poorly understood how methane emissions may offset carbon uptake in these systems, and how the systems may respond to disturbance from hurricanes and development pressures. In the Blueflux mission, I am leading the field survey team to map methane emissions from mangrove forest sediments, plants, and surface waters, as part of a larger NASA mission mapping vegetation structure and greenhouse gas fluxes through towers, aircraft, and satellites. We are working across a gradient of disturbance classes, from "ghost forest" dead zones to regenerating stands in the wilderness of the Everglades National Park.
You can read more about the project on the NASA project website and from the NASA Earth Observatory.
A conceptual project paper is available here:
Tree species effects on soil greenhouse gas cycling
In addition to directly sequestering and releasing carbon, trees can modulate ecosystem greenhouse gas cycling through the effects of individuals and communities on soil processes and gas exchange. In this project, we asked how soil respiration and methane oxidation varied beneath different tree species and tree species mixtures, finding different flux rates in soils associated with different species and non-linear responses of soil methane uptake to species assemblages.
Paper available here: Tree Species Effects on Soil CO2 and CH4 Fluxes in a Mixed Temperate Forest
