A cross-site comparison of ecosystem- and plot-scale methane fluxes from wetlands and uplands

Abstract. Wetland and upland ecosystems play significant but opposing roles in the global methane (CH₄) budget, acting as natural sources and sinks, respectively. Two of the most common approaches for measuring CH₄ fluxes (FCH₄) are chambers, which capture temporally intermittent, fine-scale spatial heterogeneity (ca. 1 m²), and eddy covariance (EC) towers, which cover a larger area (ca. 100–10000 m²) at a longer term. Although chamber and EC observations have been combined in various syntheses and databases to estimate CH₄ budgets, a unified cross-site evaluation of FCH₄ estimates at plot and ecosystem scales is lacking. As a first step toward a systematic spatiotemporal scaling of EC tower and chamber footprints, we quantified the differences between site-level aggregate FCH₄ (EC vs chamber; ΔFCH₄) from ten wetland and upland sites at half-hourly, hourly, daily, weekly, monthly, and annual timescales. We found that ecosystem-scale median FCH₄ was consistently higher than plot-scale FCH₄ at all temporal scales, with the smallest difference at daily timescale (multi-site median ΔFCH₄: 1.36 nmol m^-2 s^-1; ~ 104 % higher ecosystem-scale than plot-scale FCH₄) and largest at annual scales (2.58 nmol m^-2 s^-1; ~ 87 % higher ecosystem-scale than plot-scale FCH₄). In general, the agreement between ecosystem- and plot-scale FCH₄ decreased with finer temporal resolution (from Spearman ⍴ = 0.95 at annual scale to ⍴ = 0.65 at half-hourly scale), while ΔFCH₄ variation was greatest at daily-to-annual scales. Key environmental predictors of ΔFCH₄ included plot-scale spatial heterogeneity, dominant vegetation type, vapor pressure deficit, atmospheric pressure, and friction velocity at the daily and monthly scales. Wind direction was a significant predictor only at the monthly scale, suggesting EC footprint effects. These findings suggest accounting for variation in EC footprint extent, chamber measurement placement and artifacts is key to reconciling multi-scale FCH₄ observations in diverse ecosystems and refining CH₄ budgets.