Stable carbon isotopes in trees

Fig. 1: Model of the influence of climate variables on the carbon isotopic composition of plant cellulose .

The source of carbon in trees is atmospheric CO₂ The CO₂ enters the tree though the leaf stomata. An isotopic fractionation or discrimination against the heavy isotope ¹³C occurs during the diffusion through the stomata. Generally, the heavier isotopic species diffuse more slowly than the lighter isotopic species. The diffusivity of ¹³CO₂ in air has been calculated to be 4.4% less than that of ¹²CO₂ . This simplistic value has been adopted to describe the fractionation due to diffusion through the leaf stomata. However, in reality, the situation is more complex since isotopes fractionation also occurs in the laminar boundary layer surrounding the leaf, during collision with guard cells, in the intracellular air spaces, and with water vapor .

Another fractionation occurs during carboxylation by the enzyme used to fix carbon, which, for C3 plants, is ribulose-1,5-biphosphate (RuBisCO). The magnitude of the fractionation depends on the photosynthetic pathway (C3, C4, or CAM), the environment, the genotype and many other factors . For C3 plants, this fractionation has been measured and is on the order of -27‰ .

The δ¹³C of tree ring cellulose reflects the δ¹³C of internal CO₂, whose isotopic composition is affected by the rate at which CO₂ enters the leaf and the rate at which CO₂ is taken up for photosynthesis, each step being associated with discrimination of ¹³CO₂. The δ¹³C of cellulose of therefore a measure of the stomatal conductance, which is mainly controlled by moisture stress and humidity, and the photosynthetic rate, which depends on temperature and sunlight hours (Fig. 1) .

Quantitatively, carbon isotope fractionation in response to the atmospheric isotopic signature, diffusion through the stomata, carboxylation, and environmental factors can be described by the following equation :

$\delta^{13}C_{cellulose}=\delta^{13}C_{atm}-a-\left(b-a \right) \left(\frac{c_i}{c_a} \right)$

where:

• $\delta^{13}C_{cellulose}$ is the measured carbon isotopic composition of the cellulose
• $\delta^{13}C_{atm}$ is the carbon isotopic signature of the atmosphere
• $a$ is the diffusion fractionation associated with the transport of CO₂ though the stomata -4.4‰
• $b$ is the fractionation associated with carboxylation, -27‰
• $c_i$ is the internal concentration of CO₂, and
• $c_a$ is the atmospheric carbon dioxide concentration.

The $\frac{c_i}{c_a}$ ratio is mainly controlled by environmental conditions. Over the past 150 years, the concentration of CO₂ in the atmosphere has increased by more than 100 ppm. This carbon dioxide fertilization is thought to affect the tree physiology, and therefore, the δ¹³C of cellulose. Furthermore, this increase had been attributed to the anthropogenic burning of fossil fuels that are depleted in ¹³C relative to that of atmospheric CO₂. Consequently, the carbon isotopic signature of the atmosphere has decreased over the last 150 years by about 1.5‰, a phenomenon known as the Seuss Effect]. This long-term trend is also reflected in plant δ¹³C. Corrections for the increased atmospheric CO₂ and the change in the isotopic signature can be applied in order to retain only the climatic influence on the δ¹³C time series.

References