See more from this Session: Symposium--Mitigation of Greenhouse Gas Emissions In Managed Systems
In Canada, we used the eddy covariance and biometric techniques to assess the long-term carbon source /sink strength of the of hybrid poplar (HP) plantations growing under different soil type and management conditions in Alberta and Ontario. The specific objectives were: (1) to determine the C source-sink relationships in a chronosequence of HP plantations, (2) to study the influence of biophysical variables and soil type on above- and below-ground biomass accumulation, as well as the soil CO2 emissions over the life cycle of HP plantations .
The eddy covariance technique showed how net ecosystem productivity (NEP) and ecosystem water use (E) responded diurnally and seasonally to climate variables. We found that monthly NEP was highest in the month of October when photosynthesis rates were maintained but respiration decreased due to mild temperature. Seasonal E responded more to precipitation and its maxima coincided with maximum monthly precipitation in the month of July. Cumulative E was in good agreement with that calculated using the field water balance method with both following closely cumulative precipitation, indicating little deep drainage of water below 150-cm depth. Comparing C sequestration by HP stands in Alberta showed that Site Class 1 soil (Chernozem) sequestered 10 g C m-2 y-1 while Site Class 3 soil (Luvisol) lost 190 g C m-2 y-1 in the second year of growth.
Using the biometric approach, we found that HP grown on Site Class 3 soil in Ontario sequestered significantly more C per year than either Site Class 1 or 3 soils in Alberta. The differences in sequestration potential may be attributable to the differences in climate (Ontario being warmer and receiving much more precipitation than Alberta) and length of the growing season. However, C loss through soil respiration was also higher for Ontario but greater biomass productivity more than compensated greater soil C loss. In Alberta, soil respiration in the first eight years after plantation averaged 8.23 Mg C ha-1 yr-1 (range 6.04-10.92 Mg C ha-1 yr-1) for site class 1 which was nearly twice of an average of 4.41 Mg C ha-1 yr-1 (range 3.18-7.11 Mg C ha-1 yr-1) for Site Class 3 in the first 12 years after plantation .
Early results using both eddy covariance and biometric techniques suggest that hybrid poplar plantation on former agriculture soils become a sink within 2 to 5 years of plantation. Sites across Canada does show a significant difference in C capture/sequestration potential based on site quality, bio-geo-climatic zone and management regime and intensity.