Biological and physical controls over carbon, water and energy exchange along a boreal forest chronosequence

Tower-based eddy covariance measurements were used to quantify and explain patterns of whole ecosystem exchange of CO2, water vapor and energy exchange along a boreal forest age sequence near Thompson, Manitoba. Diurnal patterns of net carbon, water vapor and energy exchange in the four youngest stands in the chronosequence (burns 11, 18, 35 and 70 years in age) were made and compared directly to simultaneous measurements made in the oldest stand (150 year old Northern Old Black Spruce). The daily course of net CO2 exchange in the youngest, primarily deciduous stands showed peaks early in the day, followed by afternoon declines. The older primarily coniferous stands started gaining C later in the morning and showed no significant damping of net CO2 exchange later in the day. Stomatal conductance, and latent heat flux were highest in the youngest stands suggesting patterns may be explained by stand age differences in sensititivity to vapor pressure deficit, with the young deciduous stands being the most sensitive. Below canopy fluxes measured for one month in the growing season in the 35 year old and 150 year old stands were used to determine stand age differences in relative contribution of forest floor to whole canopy carbon, mass and energy exchange