Carbon isotope discrimination by plants follows latitudinal and altitudinal trends

In an earlier paper we provided evidence that carbon isotope discrimination during photosynthesis of terrestrial C3 plants decreases with altitude, and it was found that this was associated with greater carboxylation efficiency at high altitudes. Changing partial pressures of CO₂ and O₂ and changing temperature are possible explanations, since influences of moisture and light were reduced to a minimum by selective sampling. Here we analyse plants sampled using the same criteria, but from high and low altitudes along latitudinal gradients from the equator to the polar ends of plant distribution. These data should permit separation of the pressure and temperature components (Fig. 1). Only leaves of fully sunlit, non-water-stressed, herbaceous C3 plants are compared. The survey covers pressure differences of 400 mbar (ca. 5000 m) and 78 degrees of latitude (ca 25 K of mean temperature of growth period). When habitats of similar low temperature (i.e. high altitude at low latitude and low altitude at polar latitude) are compared, discrimination increases towards the pole (with decreasing altitude and thus increasing atmospheric pressure). Latitudinally decreasing temperature at almost constant atmospheric pressure (samples from low altitude) is associated with a decrease in discrimination. So, polar low-altitude plants have δ¹³C values half way between humid tropical lowland and tropical alpine plants. It is unlikely that latitudinal changes of the light regime had an effect, since low and high altitude plants show contrasting latitudinal trends in δ¹³C although local altitudinal differences in overall light consumption were small. These results suggest that both temperature and atmospheric pressure are responsible for the altitudinal trends in ¹³C discrimination. Temperature effects may partly be related to increased leaf thickness (within the same leaf type) in cold environments. Theoretical considerations and laboratory experiments suggest that it is the oxygen partial pressure that is responsible for the pressure related change in discrimination. The study also provided results of practical significance for the use of carbon isotope data. Within a community of C3 plants, discrimination in species of similar life form, exposed to similar light, water and ambient CO₂ conditions ranges over 4‰, with standard deviations for 10-30 species of ±0.6 to 1.2‰. This natural variation has to be taken into account by using a sufficient sample size and standardization of sampling in any attempt at ecological site characterization using carbon isotope data. Evidence of a pronounced genotypic component of this variation in ¹³C discrimination in wild C3 plant species is provided. Correlations with dry matter partitioning, mesophyll thickness and nitrogen content are also present.