VERTICAL GRADIENTS IN PHOTOSYNTHETIC GAS-EXCHANGE CHARACTERISTICS ANDREFIXATION OF RESPIRED CO2 WITHIN BOREAL FOREST CANOPIES

We compared vertical gradients in leaf gas exchange, CO2 concentration s, and refixation of respired CO2 in stands of Populus tremuloides Mic hx., Pinus banksiana Lamb. and Picea mariana (Mill.) B.S.P. at the nor thern and southern boundaries of the central Canadian boreal forest. M idsummer gas exchange rates in Populus tremuloides were over twice tho se of the two conifer species, and Pinus banksiana rates were greater than Picea mariana rates. Gas exchange differences among the species w ere attributed to variation in leaf nitrogen concentration. Despite th ese differences, ratios of intercellular CO2 to ambient CO2 (c(i)/c(a) ) were similar among species, indicating a common balance between phot osynthesis and stomatal conductance in boreal trees. At night, CO2 con centrations were high and vertically stratified within the canopy, wit h 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 sp ace. Photosynthesis had a diurnal pattern opposite to the CO2 profile, with the highest rates of photosynthesis occurring when CO2 concentra tions and gradients were lowest. After accounting for this diurnal int eraction, we determined that photosynthesizing leaves in the understor y experienced greater daily CO2 concentrations than leaves at the top of the canopy. These elevated CO2 concentrations were the result of pl ant and soil respiration. We estimated that understory leaves in the P icea mariana and Pinus banksiana stands gained approximately 5 to 6% o f their carbon from respired CO2.