Stable isotopes

Theory:

• During CO₂-assimilation, plants discriminate against the naturally abundant heavy, stable carbon isotope ¹³C compared to the "normal" ¹²C isotope and the degree of discrimination reflects stomatal opening.
• When stomata are less wide open in dry habitats, plants contain relatively more ¹³C ("they become less choosy").
• This is expressed as a less negative so-called δ¹³C value measured in a mass-spectrometer.
• As can be seen in Fig. 1, there is no δ¹³C gradient from the shores of the lake to the semi-desert habitats on the hillside, indicating no physiologically effective (!) moisture gradient.

A second fingerprint of drought is the abundance of the C4 syndrome, a very efficient mode of photosynthesis adopted by many plants originating in dry areas (e.g. desert vegetation; but also corn, Zea mays). Most other plants perform the C3 mode of photosynthesis (C3 plants). In contrast to C3 plants, C4 plants hardly discriminate against ¹³C, hence can easily be distinguished from C3 plants. Clearly C4 plants become very rare with increasing elevation. Above treeline they do not normally contribute significantly to plant biomass even in semiarid mountains. Fig. 2 illustrates the transition from C4 to C3 plants along the slope of Mt. Kenya in Africa. The climatic treeline is at c. 3800 m.