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

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.