Methane fluxes measured by eddy covariance and static chamber techniques at a temperate forest in central Ontario, Canada

Abstract

Methane flux measurements were carried out at a temperate forest (Haliburton Forest and Wildlife Reserve) in central Ontario (45°17´11´´ N, 78°32´19´´ W) from June to October 2011. Continuous measurements were made by an off-axis integrated cavity output spectrometer that measures methane (CH4) at 10 Hz sampling rates. Fluxes were calculated from the gas measurements in conjunction with wind data collected by a 3-D sonic anemometer using the eddy covariance (EC) method. Observed methane fluxes showed net uptake of CH4 over the measurement period with an average uptake flux (±standard deviation of the mean) of −2.7 ± 0.13 nmol m−2 s−1. Methane fluxes showed a seasonal progression with average rates of uptake increasing from June through September and remaining high in October. This pattern was consistent with a decreasing trend in soil moisture content at the monthly timescale. On the diurnal timescale, there was evidence of increased uptake during the day, when the mid-canopy wind speed was at a maximum. These patterns suggest that substrate supply of CH4 to methanotrophs, and in certain cases hypoxic soil conditions supporting methanogenesis in low-slope areas, drives the observed variability in fluxes. A network of soil static chambers used at the tower site showed reasonable agreement with the seasonal trend and overall magnitude of the eddy covariance flux measurements. This suggests that soil-level microbial processes, and not abiological leaf-level CH4 production, drive overall CH4 dynamics in temperate forest ecosystems such as Haliburton Forest.