Abstract

The pre-andean mountains of the northern Chile (18° 15′S_69° 33′W) corresponds to a narrow strip which is affected by the desert at lower altitud to the west, and the Altiplano, a higher plateau at the east. The vegetation in the area also presents an altitudinal variation, and the pre-andean mountains constitutes the second of four altitudinal floors, commonly called tolar or pre-altiplano shrubland (sensu Gajardo). Ten sampling sites were studied for life forms of plants, absolute and relative vegetal cover, plant diversity and species richness of vascular plants before and after summer rainfall (dry and wet season) in four expeditions during 2002 and 2003 by using transects. The summer rainfall in 2002 was 203 mm, considered a normal precipitation for the region, whereas the following year presented a lower amount of rain of 84.7 mm (less than 50% of normal summer rain for the area). The majority of the species of plants (92,9 %) were native (total n=98 species) and the most abundant life forms were chamaephytes, nanophanerophytes and hemicryptophytes. The results of the transects showed patterns of seasonality; they presented higher vegetal cover, species richness and biodiversity in wet season. In the dry season nanophanerophytes and chamaephytes, mainly tolas (shrubs in Aymara language), are the most abundant life forms. Whereas in the wet season therophytes and hemicryptophytes herbs also are present. The proportional change of vegetal cover between wet and dry season and the presence of terophytes community were lower in the wet season during 2003, which was dryer than 2002. © 2006 Universidad de Concepción.