Distribution and δ 13C Values of Portulacaceae Species of the High Andes in Northern Chile

The distribution of species of Portulacaceae in the northern Andes of Chile was recorded during the late growing season along altitudinal transects from 1500 m up to the vegetation limit. The transects were located towards the east of the towns of Arica (18 °S) and Vallenar (28 °S). Whole plants were analysed for δ ¹³C values to examine the occurrence of different photosynthetic pathways. Three species of Calandrinia (C. thyrsoidea, C. spicata and C. grandiflora) and two species of Philippiamra (P. amaranthoides and P. celosioides) were found to have δ ¹³C values indicative of CAM. Portulaca philippi was the only C4 plant in the sample. The CAM species tend to occupy the lower altitudinal levels, which are also relatively drier. All the C3 species occur in the Andean belt, with more favourable water supply. Their δ ¹³C values averaged ?25.1 ‰, a value significantly lower than the average of lowland C3 Calandrinia species reported elsewhere or the general average for lowland C3 plants of ?28.8 ‰. CAM species are also limited to the western slope of the Andean range, while C3 species occur at both sides, but often have discontinuous distributions north or south of the driest section of the Andes (21–24 °S) indicating that their migration along the western side has probably been prevented by extreme aridity. The C3 species occurring on both sides of the aridity barrier are also annual in habit.