Hydrological Controls on Ecosystem CO2 and CH4 Exchange in a MIXED Tundra and a FEN within an Arctic Landscape UNDER Current and Future Climates

Abstract:

Variation in CO₂ and CH₄ exchange in years with contrasting weather is strongly affected by hydrology in landscapes underlain by permafrost. Hypotheses for this variation were incorporated into the ecosystem model ecosys which simulated CO₂ and CH₄ fluxes along a topographic gradient within an arctic landscape at Daring Lake, NWT, Canada. Fluxes modelled at mixed tundra and fen sites within the gradient were compared with CO₂ fluxes measured at eddy covariance towers from 2006 to 2009, and with CH₄ fluxes measured with surface chambers in 2008. Slopes and correlation coefficients from regressions of modelled vs. measured CO₂ fluxes were 1.0 ± 0.1 and 0.7 – 0.8 for both sites in all years. At the mixed tundra site, rises in net CO₂ uptake in warmer years with earlier snowmelt were constrained by midafternoon declines in CO₂ influxes when vapor pressure deficits (D) exceeded 1.5 kPa, and by rises in CO₂ effluxes with greater active layer depth (ALD). Consequently annual net CO₂ uptake at this site rose little with warming. At the fen site, CO₂ influxes declined less with D and CO₂ effluxes rose less with warming, so that rises in net CO₂ uptake in warmer years were greater than those at the mixed tundra site. The greater declines in CO₂ influxes with warming at the mixed tundra site were modelled from greater soil-plant-atmosphere water potential gradients that developed in drier soil, and the smaller rises in CO₂ effluxes with warming at the fen site were modelled from O₂ constraints to heterotrophic and below-ground autotrophic respiration that limited their responses to greater ALD. Modelled and measured CH₄ exchange during July and August indicated very small influxes at the mixed tundra site, and larger emissions at the fen site. Emissions of CH₄ modelled during soil freezing in October – November contributed about one-third of the annual total, and so should be included in estimates of annual emissions. These contrasting responses to warming under current climate modelled and measured at the mixed tundra and fen sites were apparent in their contrasting responses modelled under long-term climate change.