Bald Cypress (Taxodium distichum) Knees Are Methane Sources Controlled by Geomorphology, Climate, and Hydrologic Extremes

While woody root structures, such as bald cypress (Taxodium distichum) “knees,” can act as conduits of methane (CH₄), little has been done to explain variation from this flux pathway. We captured spatial (i.e., across knee surface, within sites, between sites) and temporal dynamics of CH₄ from knees, and built empirical models to predict the contribution of knees to net CH₄ fluxes. Knee and soil CH₄ fluxes were measured across seasons within the lower Mississippi Alluvial Valley in a main channel (semi-permanently flooded), side channel (seasonally flooded), and a reservoir edge (artificially flooded). Knees were a net source of CH₄ across all seasons, even during periods of soil CH₄ uptake. During periods of high knee CH₄ efflux, fluxes varied across the knee surface, decreasing with height from the ground. Knee CH₄ fluxes at the main and side channels decreased during a severe drought and increased ∼ ten-fold in summer and two-fold in winter following flooding events. At the reservoir edge, knee fluxes differed between the controlled draw up and draw down at the same water level, likely due to differences in temperature and oxygen availability. Knee CH₄ fluxes were positively correlated with water level (measured from subsurface wells, above ∼−70 cm in the soil profile) and subsurface temperature, but the strength of the relationships differed across geomorphic positions. Cypress knees appear to be an important contributor to wetland CH₄ efflux and accounting for the density of knees is needed to upscale their fluxes and better understand their ecosystem contribution.