Environmental and vegetation drivers of seasonal CO2 fluxes in a sub-arctic forest–mire ecotone

Unravelling the role of structural and environmental
drivers of gross primary productivity (GPP) and ecosystem
respiration (Reco) in highly heterogeneous tundra
is a major challenge for the upscaling of chamberbased
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,andperformedmultiple fits (onefor
each 3-day period) of a simple light and temperature
response model to hourlyNEE 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 (R0). PAR-normalized photosynthesis (P600)
increased with air temperature and declined with
increasing vapor pressure deficit. R0 declined with soil
moisture and showed an apparent increase with temperature,
which may underlie a tight link between GPP
and Reco. NDVI was a good proxy for LAI, maximum
P600 and maximum R0 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.