Methane Sources and Sinks in a High-Elevation Subalpine Forest Inferred from Canopy Tower Profiles

A thorough understanding of the biogeochemical sources and sinks of atmospheric methane is required to assess the current and future influence of greenhouse gases on climate. Recent work has suggested the possibility of aerobic methane emission from plants. There are conflicting conclusions in the literature, however, highlighting the need for a variety of experimental approaches to investigate this phenomenon. We examined the variability of methane in forest air at the Niwot Ridge AmeriFlux site in the Rocky Mountains of Colorado (3050 m elevation) during summer 2007. The site is an upland conifer forest dominated by Pinus contorta (lodgepole pine), Picea engelmannii (Engelmann spruce), and Abies lasiocarpa (subalpine fir). A fast-response optical methane analyzer was used to measure profiles of CH4 at 7 heights within and above the vegetation canopy. Profiles were measured in a 30-min time period and repeated every 2 h over 43 days. Observed CH4 varied over the study from 1780 to 1920 ppb, and the day to day variability was influenced by the urban plume from Denver, 60 km to the southeast. When profiles were examined relative to the top of the tower, there was a regular pattern of excess CH4 in the vegetation canopy at night, and a clear CH4 sink in the soil surrounding the tower. On the basis of wind direction analysis, the excess CH4 in the canopy at night appeared to be related to a nearby wetland source within the nocturnal tower footprint, and not to a source by local aerobic foliar emission. During the daytime, the soil sink persisted. Recent laboratory work by others has highlighted the importance of UV radiation for plant emission of CH4. The high elevation of our research forest is associated with high UV levels, which might be expected to enhance foliar emission rates. While daytime mixing may have obscured possible buildup of CH4 due to foliar emission, there was no evidence for higher emission in the canopy at high compared to low shortwave irradiation. These data will be used to provide bounds for the magnitudes of the soil methanotrophic sink and to assess the possible magnitude of foliar emission in this forest.