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Abstract
AbstractWe present a decadal (1994–2004) record of carbon dioxide flux in a 160‐year‐old black spruce forest/veneer bog complex in central Manitoba, Canada. The ecosystem shifted from a source (+41 g C m−2, 1995) to a sink (−21 g C m−2, 2004) of CO2 over the decade, with an average net carbon balance near zero. Annual mean temperatures increased 1–2° during the period, consistent with the decadal trend across the North American boreal biome. We found that ecosystem carbon exchange responded strongly to air temperature, moisture status, potential evapotranspiration, and summertime solar radiation. The seasonal cycle of ecosystem respiration significantly lagged that of photosynthesis, limited by the rate of soil thaw and the slow drainage of the soil column. Factors acting over long time scales, especially water table depth, strongly influenced the carbon budget on annual time scales. Net uptake was enhanced and respiration inhibited by multiple years of rainfall in excess of evaporative demand. Contrary to expectations, we observed no correlation between longer growing seasons and net uptake, possibly because of offsetting increases in ecosystem respiration. The results indicate that the interactions between soil thaw and water table depth provide critical controls on carbon exchange in boreal forests underlain by peat, on seasonal to decadal time scales, and these factors must be simulated in terrestrial biosphere models to predict response of these regions to future climate.
Highlights
Boreal forests represent 22% of global forest area, and more than half of North American forests (Iremonger et al, 1997; Schlesinger, 1997)
We investigated the role of summertime (DOY 150–250) water table position on ecosystem respiration using measurements at a Sphagnum veneer bog in the northern portion of the eddy-covariance tower footprint, the locus of the largest reservoir of soil carbon at the site (Plate 1)
We investigated the influence of mean annual air temperature, precipitation, photosynthetically active radiation (PAR), and potential evapotranspiration (PET) on annual carbon balance using three different annual bases: calendar year, April 1– March 31, and November 1–October 31
Summary
Boreal forests represent 22% of global forest area, and more than half of North American forests (Iremonger et al, 1997; Schlesinger, 1997). Climate has warmed in the boreal forest in the past century, and precipitation has increased (Myneni et al, 1997; Keyser et al, 2000; Zhang et al, 2000; Stone et al, 2002). The. IPCC Third Assessment Report (2001) predicted that the high northern latitudes will continue to warm more rapidly than the global average in the 21st century, and will experience the largest changes in precipitation. The IPCC (2001) predicted that peatland ecosystems may continue to accumulate carbon in the face of water table drawdown, but this ability would be compromised in the case of extended or more frequent droughts, which may leave deep organic soils susceptible to fire
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