Effects of aridity on plant diversity in the northern Chilean Andes: results of a natural experiment

• Published in: Annals of the Missouri Botanical Garden 1988, Vol.75 (1), p.55-78
• Main Authors: Mary T. Kalin Arroyo, Squeo, Francisco A., Armesto, Juan J., Villagran, Carolina
• Format: Article
• Language: English
• Subjects: adaptacion fisiologica; adaptation; America; Arroyos; Biological and medical sciences; botanical composition; chile; chili; Chorology; Climate models; composicion botanica; composition botanique; Deserts; drought; ecologia vegetal; encuestas; enquete; Flora; Fundamental and applied biological sciences. Psychology; habitat; habitats; Herbs; Paleoclimatology; phytoecologie; Plant cytology, morphology, systematics, chorology and evolution; plant ecology; Plants; precipitacion atmosferica; Precipitation; Rain; secheresse; sequia; soil zonation; Species diversity; surveys; temperatura; temperature; zonalite; zonificacion de suelos
• ISSN: 0026-6493; 2162-4372
• DOI: 10.2307/2399466

Hyperarid climates in western South America from 15⚬S to 29⚬S, extending up to 3,000 m in the northern Chilean Andes, result primarily from the Andes intercepting precipitation from the Intertropical Convergence and the drying effect of the equatorward-running Humboldt Current bathing much of the Pacific coast of South America. Paleobotanical, paleontological, and geological evidence suggests that such harsh climatic conditions developed very recently in Holocene times, following a turbulent Pleistocene history of alternating wet/cold and warm/dry periods. Seasonal climates probably first emerged in the Pliocene after a long, warm/wet Miocene during which precipitation is thought to have increased from east to west, as opposed to west to east, as occurs north of 25⚬S today. For the three remaining intact vegetation belts (desert scrub, Andean, and high Andean) in the northern Chilean Andes above the Atacama Desert, surveys on six transects, each approximately one-fourth of a degree latitude wide, located at 18⚬S, 19⚬S, 21⚬S, 24⚬S, 26⚬S, 28⚬S, revealed only 769 species of vascular plants in 290 genera. Altitudinally, species richness decreases with elevation in the winter rainfall zone, where precipitation is received from the southwest ("invierno chileno"; transects 26⚬S, 28⚬S), to peak at mid elevations in the summer rainfall zone where precipitation is received from the east ("invierno boliviano"; transects 18⚬S, 19⚬S, 21⚬S, 24⚬S). Species richness decreases by 80% and cover by 50% over the very severe rainfall gradient from 18-24⚬S. In spite of its greater reliance on water, the azonal bog flora has been less affected than the zonal flora by aridity, probably because of greater chances of reintroduction of species following their loss through long-distance dispersal. Species richness, when considered on a broad biogeographical scale, was significantly positively correlated with mean annual rainfall, as it was for most 500-m elevational levels when considered separately. The effect of cold temperatures at the higher elevations was completely obliterated by aridity on a biogeographical scale. Although diversity (measured as a synthetic characteristic combining richness and abundance) and species richness showed similar trends as aridity increases, loss of diversity tended to be more gradual in comparison with reductions in species richness. Aridity leads to community features analogous to those seen in the early stages of primary succession, maintained on