Vertical gradients in photosynthetic gas exchange characterist

change, CO2 concentrations, and refixation of respired CO2 in stands of Populus tremuloides Michx., Pinus banksiana Lamb. and Picea mariana (Mill.) B.S.P. at the northern and southern boundaries of the central Canadian boreal forest. Midsummer gas exchange rates in Populus tremuloides were over twice those of the two conifer species, and Pinus banksiana rates were greater than Picea mariana rates. Gas exchange differences among the species were attributed to variation in leaf nitrogen concentration. Despite these differences, ratios of intercellular CO2 to ambient CO2 (ci/ca) were similar among species, indicating a common balance between photosynthesis and stomatal conductance in boreal trees. At night, CO2 concentrations were high and vertically stratified within the canopy, with maximum concentrations near the soil surface. Daytime CO2 gradients were reduced and concentrations throughout the canopy were similar to the CO2 concentration in the well-mixed atmosphere above the canopy space. Photosynthesis had a diurnal pattern opposite to the CO2 profile, with the highest rates of photosynthesis occurring when CO2 concentrations and gradients were lowest. After accounting for this diurnal interaction, we determined that photosynthesizing leaves in the understory experienced greater daily CO2 concentrations than leaves at the top of the canopy. These elevated CO2 concentrations were the result of plant and soil respiration. We estimated that understory leaves in the Picea mariana and Pinus banksiana stands gained approximately 5 to 6% of their carbon from respired CO2.