Soil, plant, and transport influences on methane in a subalpine forest under high ultraviolet irradiance

Recent studies have demonstrated direct methane emission from plant foliage under aerobic conditions, particularly under high ultraviolet (UV) irradiance. We examined the potential importance of this phenomenon in a high-elevation conifer forest using micrometeorological techniques. Vertical profiles of methane and carbon dioxide in forest air were monitored every 2 h for 6 weeks in summer 2007. Day to day variability in above-canopy CH₄ was high, with observed values in the range 1790 to 1910 nmol mol⁻¹. High CH₄ was correlated with high carbon monoxide and related to wind direction, consistent with pollutant transport from an urban area by a well-studied mountain-plain wind system. Soils were moderately dry during the study. Vertical gradients of CH₄ were small but detectable day and night, both near the ground and within the vegetation canopy. Gradients near the ground were consistent with the forest soil being a net CH₄ sink. Using scalar similarity with CO₂, the magnitude of the summer soil CH₄ sink was estimated at ~1.7 mg CH₄ m⁻ ² h⁻¹, which is similar to other temperate forest upland soils. The high-elevation forest was naturally exposed to high UV irradiance under clear sky conditions, with observed peak UVB irradiance >2 W m⁻ ². Gradients and means of CO₂ within the canopy under daytime conditions showed net uptake of CO₂ due to photosynthetic drawdown as expected. No evidence was found for a significant foliar CH₄ source in the vegetation canopy, even under high UV conditions. While the possibility of a weak foliar source cannot be excluded given the observed soil sink, overall this subalpine forest was a net sink for atmospheric methane during the growing season.