SPATIAL HETEROGENEITY OF HIGH-MOUNTAIN VEGETATION IN THE ANDEAN DESERT ZONE OF CHILE

In the Andean desert zone of Chile, three vegetational belts can be di stinguished: the subalpine belt (<3,200-3,500 m); the low alpine belt (3,500-4,250 m); and the high alpine belt (4,250-4,450 m). In the Cord illera Dona Ana area (29-degrees-45' S, 60-degrees-59' W) soils show a great variation in chemical and drainage characteristics which may ex plain the patchy distribution of plant species. In the subalpine belt, the shrubby species (> 1 m) are found exclusively on north-facing slo pes in rocky sites with shallow soils, and this may be related to ther mal refuges. In contrast, many subshrubs and perennial herbs grow main ly on south-facing slopes or on hillsides with deep sediments. Most le guminous species grow on soils rich in calcium, and this may be due to their greater ability to fix atmospheric nitrogen. Large areas with h ypothermal anomalies have no vegetation, except when covered by sedime nts of other origins (such as volcanic or intrusive). At the same alti tudinal level, the chemical characteristics, drainage of the substrate , and winter snow accumulation determine which species are present.