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The imbalance existing between the demand and degradation of forest resources partially explains the on-going creation of exotic tree plantations. In northwestern Patagonia of Argentina, Pinus ponderosa (Pinaceae) afforestation plays a key role in conserving degraded soil, but may also reduce biodiversity. The ecological sustainability of this activity was assessed in Aguas Frías (38°46´ W, 70°54´ S) and Litrán (38°54´ W, 71°01´ S) forest stations, where species richness and life form diversity of plants were compared in fenced and unfenced landscape units. Eleven units were identified, composed of natural herbaceous-shrubby steppes, xerophilous and hygrophilous meadows, and pure and mixed forests of Nothofagus pumilio (Nothofagaceae) and Araucaria araucana (Araucariaceae), and P. ponderosa planted forests of dissimilar canopy cover. Xerophilous meadows on rocky outcrops held 1/3 of total richness in an extremely restricted area. Hygrophilous meadows exhibited the largest number of families and native and introduced species, and low life form diversity. Dense pine stands showed low light and water availability in the understory, which correlated with low soil cover, hemicryptophyte frequency, richness and life form diversity. As canopy cover decreased, values for these indicators resembled those of the steppe of reference. Within a plantation, greater compatibility between conservation of diversity and wood production requires maintenance of fencing, reduced tree spacing over long rotations and the development of buffer zones and biological corridors. Although such a project may represent a decline in productivity, it will promote enhanced ecosystem and aesthetic values, increasing the likelihood of further economic support from society.