Bald Cypress Knees Contribute to Methane Emissions in a Bottomland Hardwood Wetland
Authors
Skylar Ross1* (sross10@murraystate.edu), Niklas Klauss1, Marissa Miles1, Bassil El Masri1, Jessica Moon1, Kabiraj Khatiwada2, Benjamin Runkle2, Gary Stinchcomb3
Institutions
1Murray State University, Murray, KY; 2University of Arkansas, Fayetteville, AR; 3University of Memphis, TN
Abstract
Relatively little is known about carbon source-sink dynamics in bottomland hardwood wetlands. Identifying carbon pathways and their dominant controlling factors within these systems is critical. Bald cypress (Taxodium distichum) is a dominant tree species of southeastern United States wetlands. These trees can develop exposed woody root structures known as knees, which have been shown to contribute to wetland methane (CH4) emissions. However, there are significant variations in estimates of knee contribution to total ecosystem flux. The team is measuring variation in CH4 and carbon dioxide (CO2) fluxes from individual knees, soils, and 1 m2 plots containing both soils and knees as a function of microtopographic and climatic (i.e., drought and flooding) differences. During moderate-severe drought conditions in the autumn of 2022, knees acted as a source of CH4 even while their surrounding soils acted as a sink. Soil CO2 emissions were significantly related to surrounding knee density during drought conditions (p-value=0.0374), but no relationship was found between soil CH4 uptake and knee density. Similarly, in 1 m2 plots, there was no clear relationship between knee density and CH4 and CO2 flux. During nondrought conditions, in the autumn of 2023, researchers saw an increase in CH4 emissions from individual knees but only from those located at a lower elevation. A historic rainfall event (dropping 17.7 cm of precipitation in 24 hours) in July 2023 also resulted in an approximately 250% increase of CH4 emissions from knees at the lower elevation. The flooding and drought period data suggest fluxes are partially related to hydrologic factors. Sampling is ongoing to better understand the relationships between knee fluxes and surrounding environmental factors (i.e., hydrology, air and soil temperature, soil biogeochemistry). This allows for more accurate wetland ecosystem carbon modeling, which currently does not account for knee fluxes.