Environmental and Vegetation Drivers of Seasonal CO2 Fluxes in a Sub-arctic Forest–Mire Ecotone

Abstract

Unravelling the role of structural and environmental drivers of gross primary productivity (GPP) and ecosystem respiration (R eco) in highly heterogeneous tundra is a major challenge for the upscaling of chamber-based CO2 fluxes in Arctic landscapes. In a mountain birch woodland-mire ecotone, we investigated the role of LAI (and NDVI), environmental factors (microclimate, soil moisture), and microsite type across tundra shrub plots (wet hummocks, dry hummocks, dry hollows) and lichen hummocks, in controlling net ecosystem CO2 exchange (NEE). During a growing season, we measured NEE fluxes continuously, with closed dynamic chambers, and performed multiple fits (one for each 3-day period) of a simple light and temperature response model to hourly NEE data. Tundra shrub plots were largely CO2 sinks, as opposed to lichen plots, although fluxes were highly variable within microsite type. For tundra shrub plots, microsite type did not influence photosynthetic parameters but it affected basal (that is, temperature-normalized) ecosystem respiration (R 0). PAR-normalized photosynthesis (P 600) increased with air temperature and declined with increasing vapor pressure deficit. R 0 declined with soil moisture and showed an apparent increase with temperature, which may underlie a tight link between GPP and R eco. NDVI was a good proxy for LAI, maximum P 600 and maximum R 0 of shrub plots. Cumulative CO2 fluxes were strongly correlated with LAI (NDVI) but we observed a comparatively low GPP/LAI in dry hummocks. Our results broadly agree with the reported functional convergence across tundra vegetation, but here we show that the role of decreased productivity in transition zones and the influence of temperature and water balance on seasonal CO2 fluxes in sub-Arctic forest–mire ecotones cannot be overlooked.