Brooks, J.R., L.B. Flanagan, G.T. Varney, and J.R. Ehleringer. 1997. Vertical gradients of photosynthetic gas exchange and recycling of respired CO2 within boreal forest canopies. Tree Physiol. 17:1-12.

We compared vertical gradients in leaf gas exchange, CO2 concentrations, and the extent to which understory leaves fix 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 the deciduous species, Populus tremuloides, were over twice that of the two conifer species, and Pinus banksiana rates were greater than rates in Picea mariana. These 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. However, intraspecies gas exchange rates varied geographically. Gas exchange rates in SSA were approximately twice as high as those in NSA, reflecting precipitation differences between the two sites. Intraspecies ci/ca values also varied geographically reflecting the drier conditions in NSA during midsummer. Longer term estimates of ci/ca from carbon isotope data indicated that these geographical differences were short-lived and were related to a midseason drought. The ci/ca values calculated from carbon isotope ratios on leaves collected at the end of the growing season were comparable between the sites, indicating that boreal trees were similar throughout their ranges in balancing photosynthesis and stomatal conductance.

At night CO2 concentrations were high and vertically stratified within the canopy, with maximum concentrations near the soil surface. The gradient in CO2 concentration within the canopy was over 100 ppm at night. In contrast, during the day CO2 gradients were reduced and concentrations throughout the canopy were similar to the well mixed atmosphere above the canopy space. The densest stands, Picea mariana and Populus tremuloides, maintained CO2 gradients of 10 ppm or more during the day, whereas the open stand, Pinus banksiana, maintained only a 2 to 3 ppm gradient.

Photosynthesis and CO2 profiles showed opposite diurnal patterns: the highest rates of photosynthesis occurred when CO2 concentrations and gradients were lowest. After accounting for this diurnal interaction, 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: thus a percentage of CO2 fixed by understory leaves will come from respired CO2. We estimated that understory leaves in the Picea mariana and Pinus banksiana stands gained approximately 5 to 6% of their carbon from respired CO2., maintained only a 2 to 3 ppm gradient.