Six years of CO2 and water vapour exchange measurement in a West Coast Douglas-fir stand

Over the course of six years that include the El Niño events of 1997/1998 and 2002/2003, annual carbon (C) sequestration measured at a Douglas-fir stand on the Canadian west coast varied between 206 and 375 g C m^-2 per year, while during the same period annual evapotranspiration was relatively conservative at 418 to 372 mm per year. The stand was established in 1949 on the east coast of Vancouver Island, BC, Canada and is located on a 5 - 10% slope with a tree density of 1100 stems ha^-1 and a tree height of 35 m. Eddy covariance measurements of CO₂ and water vapour fluxes were initiated in September 1997 and are ongoing. This unique data set has allowed us to analyse how the C and water balances of an ecosystem type typical of the Pacific Northwest change under climate variations induced by El Niño events.

To establish relationships between ecosystem photosynthesis (P_G) and evapotranspiration, P_G was calculated as the sum of net ecosystem productivity (F_NEP) and ecosystem respiration (R_E), i.e. P_G = F_NEP + R_E. Daytime R_E was calculated from the relationship of soil temperature and nighttime F_NEP = - R_E, which was allowed to vary seasonally. This estimate usually agrees to within 10% with R_E estimated as the intercept of the light response of F_NEP. Nighttime measurements were only used for the analysis when friction velocity was above a threshold of 0.3 m/s. The pattern of interannual variability did not vary with either this threshold or the use of an annual instead of a seasonal soil temperature relationship.

Analysis of monthly light response of P_G showed that maximum assimilation rates at high light levels varied from 10 μmol m^-2 s^-1 in December and January to just under 30 μmol m^-2 s^-1 in July and August. Low soil moisture in late summer led to the reduction of maximum assimilation. However, monthly totals of P_G were related linearly to downwelling photosynthetically active radiation and there was little interannual variation in the light use efficiency, which had an overall value of 0.25 g C / mol photons. Similarly, stomatal conductance tended to decrease with decreasing soil moisture, yet this was only important during late summer. Water use efficiency was also roughly constant seasonally as well as for different years. Using monthly values, the stand sequestered 6 g C / kg H₂O with monthly P_G as high as 380 g C m^-2 in summer. The 1997/1998 El Niño event led to an unusually warm spring in 1998, leading to an increase in respiration and consequently low annual F_NEP of 301 g C m^-2. The 2002/2003 El Niño caused an unusually mild early winter in 2002 which again caused an increase in R_E and an annual F_NEP of 206 g C m^-2.