Recent Vegetation Changes along the Forest/Steppe Ecotone of Northern Patagonia

In northern Patagonia over a west-to-east distance of approximately 50 km, the vegetation changes from Nothofagus rainforests through woodlands of Austrocedrus chilensis to steppe dominated by bunchgrasses and shrubs. Analysis of tree population age structures and photographic comparisons of the forest/steppe margin over the past century indicate an expansion of trees into the steppe. Throughout northern Patagonia, Austrocedrus chilensis and other trees began invading the steppe 75 to 100 years ago. During the early decades of the twentieth century, there was also a shift in dominance from bunchgrasses towards spiny shrubs in the steppe near the forest margin. Cessation of frequent burning of the steppe by Indians hunting guanacos at the end of the nineteenth century appears to have permitted the tree invasion. Severe overgrazing by sheep and livestock during the early decades of the present century was the main factor in the shift in dominance from bunchgrasses towards shrubs. European settlement of northern Patagonia also resulted in vast forest fires as reflected by extensive areas of 70 to 100 year-old even-aged Nothofagus and Austrocedrus stands. Present plant community compositions and forest stand structures bear major imprints of impacts associated with both the late nineteenth century period of white settlement and the earlier activities of the aboriginal population.