Effects of stand age and weather on carbon dioxide and water vapour fluxes in coastal Douglas-fir forests

Forest harvesting and succession have a major impact on the dynamics of carbon exchange between forests and the atmosphere. This talk presents the differences in stand-level fluxes of carbon dioxide (CO₂) and water vapour and the biophysical factors, which affect respiration, photosynthesis, and evapotranspiration in three different coastal Douglas-fir stands at different stages of development after harvesting. The eddy covariance technique was used to measure stand-level CO₂ and water vapour fluxes in Douglas-fir stands established in 2000, 1988, and 1949 within 50 km of each other on the east coast of Vancouver Island, British Columbia. Between 1999 and 2003, five years of measurements made in the 1949 stand, two years in the 1988 stand, and three years in the 2000 stand were used to contrast annual and seasonal net ecosystem production (NEP) between the three stands. Total annual NEP, ecosystem respiration and gross ecosystem production increased with increasing stand age. For example, in 2002, the 3-year-old stand was a large carbon (C) source (NEP = -610 g C m^-2), the 14-year-old stand was a small C source (NEP= -130 g C m^-2), and the 53-year-old stand was a moderate C sink (NEP = 250 g C m^-2). Annual evapotranspiration was very similar between the two older stands (~ 400 mm) and 150 mm less in the 3-year-old stand. Consequently, water use efficiency also increased with increasing stand age. The timing of maximum and minimum NEP and evapotranspiration differed between stands and was related to differences in stand structural characteristics such as canopy roughness, leaf area, biomass, species composition and phenology. Relative interannual differences in NEP were greatest in the oldest stand and were largely a result of differences in spring temperatures and ecosystem respiration. In contrast, growth of vegetation and drought were most important in controlling C exchange in the youngest stand. Results from this study emphasize the importance of stand age, structural characteristics, and weather variability on C exchange processes in these Douglas-fir stands.