Eddy covariance measurements of the carbon balance of a newly established hybrid poplar plantation

Recent studies have suggested that the potential carbon mitigation benefits of afforestation could be small or even negative in high latitude regions, because reductions in winter albedo due to increased tree cover will contribute greater warming than the net cooling attributable to greater sequestration of atmospheric CO2. However, in Canada's Prairie Provinces, the major interest is in establishing plantations of fast growing deciduous species, notably hybrid poplars and willows. Such species would likely have a smaller impact on winter albedo than evergreen conifers.

An eddy covariance (EC) experiment was set up in early 2005 to investigate the effects of afforestation with hybrid poplar on the energy, water and carbon balances of an agricultural field. Because of known problems with the LICOR open path CO2 sensor during winter, soil respiration rates within the tower footprint were modelled from Vaisala probe measurements of the soil CO2 concentration profile validated against independent repeat measurements made with a LICOR 6400 portable IRGA system. This approach allowed belowground carbon fluxes to be compared with those measured by the EC system, and also to partition the carbon balance into above and belowground components in each of the three growing seasons.

Here we report on observable trends after three growing seasons. Ground cover at the site changed from near-zero following cultivation, to weed cover (with little tree leaf area), and then to significant tree cover. There was a net release of carbon (from decomposing crop residues and dead vegetation) every year, which declined markedly, from about 4 Mt C ha-1 in the first year to close to zero in the third year, while the trees accounted for relatively little uptake.