Víctor Sandoval, Universidad Austral de Chile, Facultad de Ciencias Forestales y Recursos Naturales,Valdivia, Chile.

Structural attributes at different levels of biological organization (species, communities, landscape) and plant biodiversity are described and compared between the basins of Baker and Pascua Rivers in Aisen Region. Four hundred and ninety four vegetation samples were taken in forest ecosystems, bush, pasture, and peat to build tables of plant communities that differentiate and characterize biodiversity. The Shannon index, frequency and phytogeographical origin of species were used. Two hundred and ninety six species distributed in 30 communities, with high diversity of plant landscapes were found. Two hundred and forty three (82 %) of these species are native, and 53 (18 %), exotic, showing high human impact. Bushes are diverse formations, with 12 communities in the Baker basin. Anthropogenic grasslands exist only in the Baker basin. The community with the highest level of species richness was the thicket of Nothofagus antarctica with 109 species, whereas the krummholz of Nothofagus pumilio presented only seven species in the Baker basin. The Shannon index shows that the Magellanic forest (Pascua basin) is the most diverse community (H' = 2.923), whereas the Nothofagus pumilio krummholz is the least diverse one (H' = 0.615). The Baker basin has a higher wealth of native and exotic species and a higher variety of plant, than hose found at the Pascua basin probably due to the intense human intervention in the former.