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Vertical gradients in photosynthetic gas exchange characteristics and refixation of respired CO₂ within boreal forest canopies

J. Renée Brooks (1, 2), Lawrence B. Flanagan (3), Gregory T. Varney (3) and James R. Ehleringer (1)

1. Department of Biology, Stable Isotope Ratio Facility for Environmental Research, University of Utah, Salt Lake City, UT 84112, USA / 2. Department of Biology, University of South Florida, Tampa, FL 33620, USA / 3. Department of Biology, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada / Received December 13, 1995

Summary

We compared vertical gradients in leaf gas exchange, 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 (c_i/c_a) 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.

Keywords: boreal forest, BOREAS, carbon dioxide gradients, carbon isotope discrimination, carbon refixation, photosynthesis.

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ISSN 0829-318X Copyright © 2002–2008 Heron Publishing

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