Carbon Isotope Effects in Relation to CO2 Assimilation by Tree Canopies
Lucas A. Cernusak, Nerea Ubierna
Abstract
Abstract The carbon atoms deposited in tree rings originate from the CO 2 in the atmosphere to which the tree’s canopy is exposed. Thus, the first control on the stable carbon-isotope composition of tree rings is by δ 13 C of atmospheric CO 2 . There has been an inter-annual trend of decreasing δ 13 C of atmospheric CO 2 over the past two centuries as a result of combustion of fossil fuels and land-use change. Atmospheric CO 2 is, for the most part, well mixed, but the sub-canopy air space can become depleted in 13 C due to inputs from soil and plant respiration when turbulent exchange with the troposphere is hindered, for example by a high leaf area index at night. This is less likely to occur during daytime when turbulence is higher and photosynthesis takes place. Discrimination against 13 C (∆ 13 C) occurs upon assimilation of atmospheric CO 2 by C 3 photosynthesis. Trees using the C 3 photosynthetic pathway comprise the overwhelming majority of all trees. The primary control on the extent of discrimination during C 3 photosynthesis is the drawdown in CO 2 concentration from the air outside the leaf to the site of carboxylation in the chloroplast. Part of this drawdown is captured by c i / c a , that is, the ratio of intercellular to ambient CO 2 concentrations. The c i / c a represents the balance between the CO 2 supply by stomata and its demand by photosynthesis. It can be related to water-use efficiency, the amount of CO 2 taken up by photosynthesis for a given amount of water loss to the atmosphere, assuming a given evaporative demand. To predict time-averaged c i / c a from wood ∆ 13 C, a simplified, linear model can be employed. In this linear model, the slope is determined by $$\overline{b }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mrow> <mml:mi>b</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> </mml:math> , the effective enzymatic discrimination. The value of $$\overline{b }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mrow> <mml:mi>b</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> </mml:math> can be estimated by comparing wood ∆ 13 C to representative measurements of c i / c a . The $$\overline{b }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mrow> <mml:mi>b</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> </mml:math> was originally estimated from observations of leaf tissue to have a value of 27‰. We compiled data for woody stem tissue here, and our analysis suggests that a lower $$\overline{b }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mrow> <mml:mi>b</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> </mml:math> should be used in the simplified model for wood ( $$\overline{b }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mrow> <mml:mi>b</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> </mml:math> = 25.5‰) than for leaves ( $$\overline{b }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mover> <mml:mrow> <mml:mi>b</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>¯</mml:mo> </mml:mrow> </mml:mover> </mml:math> = 27‰). This is also consistent with widespread observations that woody tissues are enriched in 13 C compared to leaves.