• Cactaceae;
  • Growth form;
  • Phytogeographic province;
  • Plant-environment relationship
  • Hunt (1992) and Kiesling (in press).

Abstract. Patterns of species richness are described for 50 columnar, 109 globose and 50 opuntioid cacti species in 318 grid cells (1° x 1°) covering Argentina. Biological richness hypotheses were tested by regressing 15 environmental descriptors against species richness in each group. We also included the collection effort (estimated as the logarithm of the number of herbarium specimens collected in each cell) to estimate the possible error induced by underrepresentation in certain cells. Log-linear regression models accounted for a large portion of the variation in richness of columnar species, opuntioid species, globose species and all forms combined. Opuntioid and globose species richness was associated with percentage summer rainfall. Columnar species richness was associated with the number of frost-free days. The distribution of columnar cacti seems to be limited by freezing temperatures. The results are consistent with the climate favourableness and climatic variation hypotheses, but do not support the hypothesis that energy flow limits regional species richness.

Altitudinal range (a measure of habitat heterogeneity) was related to richness of globose cacti. A floristic affinity analysis between geographic provinces and the distribution of the different growth forms confirmed the high degree of endemism of globose cacti and the cosmopolitanism of the opuntioid ones. Two cells in the Prepuna and northern Monte Provinces showed species richness values that were significantly higher than those predicted by the log-linear model. This is interpreted in terms of Quaternary historical events having affected the main South American biomes.