Vertical gradients in photosynthetic gas exchange characteristics and refixation of respired CO(2) within boreal forest canopies.

We compared vertical gradients in leaf gas exchange, CO(2) concentrations, and refixation of respired CO(2) 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 CO(2) to ambient CO(2) (c(i)/c(a)) were similar among species, indicating a common balance between photosynthesis and stomatal conductance in boreal trees. At night, CO(2) concentrations were high and vertically stratified within the canopy, with maximum concentrations near the soil surface. Daytime CO(2) gradients were reduced and concentrations throughout the canopy were similar to the CO(2) concentration in the well-mixed atmosphere above the canopy space. Photosynthesis had a diurnal pattern opposite to the CO(2) profile, with the highest rates of photosynthesis occurring when CO(2) concentrations and gradients were lowest. After accounting for this diurnal interaction, we determined that photosynthesizing leaves in the understory experienced greater daily CO(2) concentrations than leaves at the top of the canopy. These elevated CO(2) 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 CO(2).

[1]  Pieter P. Tans,et al.  Evidence for interannual variability of the carbon cycle from the National Oceanic and Atmospheric Administration/Climate Monitoring and Diagnostics Laboratory Global Air Sampling Network , 1994 .

[2]  K. G. McNaughton,et al.  Effects of spatial scale on stomatal control of transpiration , 1991 .

[3]  H. Griffiths,et al.  Carbon Isotope Discrimination and the Coupling of CO2 Fluxes within Forest Canopies , 1993 .

[4]  J. D. Stewart,et al.  Photosynthetic acclimation to elevated atmospheric carbon dioxide and UV irradiation in Pinus banksiana , 1993 .

[5]  I. Fung,et al.  Observational Contrains on the Global Atmospheric Co2 Budget , 1990, Science.

[6]  R. Valentini,et al.  Hydrogen and carbon isotope ratios of selected species of a mediterranean macchia ecosystem , 1992 .

[7]  W. Oechel,et al.  Effects of soil temperature on the carbon exchange of taiga seedlings. II. Photosynthesis, respiration, and conductance , 1983 .

[8]  Charles D. Keeling,et al.  The concentration and isotopic abundances of carbon dioxide in rural and marine air , 1961 .

[9]  G. Bonan.  Physiological controls of the carbon balance of boreal forest ecosystems , 1993 .

[10]  J. Ehleringer,et al.  Carbon dioxide concentrations within forest canopies—variation with time, stand structure, and vegetation type , 1996 .

[11]  S. Mulkey,et al.  Ecological Interpretation of Leaf Carbon Isotope Ratios: Influence of Respired Carbon Dioxide , 1989 .

[12]  D. Jacob,et al.  Atmosphere‐biosphere exchange of CO2 and O3 in the central Amazon Forest , 1990 .

[13]  T. Boutton.  10 – Stable Carbon Isotope Ratios of Natural Materials: I. Sample Preparation and Mass Spectrometric Analysis , 1991 .

[14]  W. Oechel,et al.  The photosynthetic capacity, nutrient content, and nutrient use efficiency of different needle age-classes of black spruce (Piceamariana) found in interior Alaska , 1983 .

[15]  K. Jon Ranson,et al.  The Boreal Ecosystem-Atmosphere Study (BOREAS) : an overview and early results from the 1994 field year , 1995 .

[16]  J. Ehleringer,et al.  Carbon Isotope Discrimination and Photosynthesis , 1989 .

[17]  A. McGuire,et al.  Global climate change and terrestrial net primary production , 1993, Nature.

[18]  P. G. Jarvis,et al.  SYSTEMATIC SPATIAL VARIATION OF STOMATAL CONDUCTANCE IN A SITKA SPRUCE PLANTATION , 1982 .

[19]  P. G. Jarvis,et al.  STOMATAL CONDUCTANCE AND PHOTOSYNTHESIS IN A MATURE SCOTS PINE FOREST I. DIURNAL, SEASONAL AND SPATIAL VARIATION IN SHOOTS , 1985 .

[20]  L. Sternberg.  A model to estimate carbon dioxide recycling in forests using 13C12C ratios and concentrations of ambient carbon dioxide , 1989 .

[21]  D. Hollinger,et al.  Vegetation Effects on the Isotopic Composition of Atmospheric CO2 at Local and Regional Scales: Theoretical Aspects and a Comparison Between Rain Forest in Amazonia and a Boreal Forest in Siberia , 1996 .

[22]  J. Ehleringer,et al.  Seasonal variation in the carbon isotopic composition of desert plants , 1992 .

[23]  J. Marshall,et al.  Carbon Isotope Discrimination and Water-Use Efficiency in Native Plants of the North-Central Rockies , 1994 .

[24]  R. Ceulemans,et al.  Variations in photosynthetic, anatomical, and enzymatic leaf traits and correlations with growth in recently selected Populus hybrids. , 1987 .

[25]  F. Meinzer,et al.  Carbon Isotope Discrimination and Photosynthetic Gas Exchange in Coffee Hedgerows During Canopy Development , 1994 .

[26]  W. A. Kaplan,et al.  Carbon dioxide in the atmosphere over the Amazon Basin , 1988 .

[27]  S. T. Gower,et al.  Vertical variation in canopy structure and CO(2) exchange of oak-maple forests: influence of ozone, nitrogen, and other factors on simulated canopy carbon gain. , 1990, Tree physiology.

[28]  F. Bazzaz,et al.  Successional Status, Seed Size, and Responses of Tree Seedlings to CO^2, Light, and Nutrients , 1993 .

[29]  G. Unwin,et al.  Spatial and Temporal Distribution of Photosynthesis and Transpiration by Single Leaves in a Rainforest Tree, Argyrodendron peralatum , 1988 .