Post-fire stand development of Austrocedrus-Nothofagus forests in northern Patagonia

In the rainshadow of the Andean Cordillera in northern Patagonia (ca 39° to 43°S) the steep west-to-east decline in precipitation is reflected by a dramatic gradient from rainforest through open woodlands to shruband bunchgrass-dominated steppe. A recent trend towards aridification of Patagonia and consequent westward expansion of xeric communities into the western rainforest district has been a persistent theme in the ecological literature for nearly 50 years. Specifically, it has been suggested that the xeric trees Austrocedrus chilensis and Nothofagus antarctica are extending their ranges westward and replacing the mesic Nothofagus dombeyi which dominates the rainforests. This hypothesis was investigated by analyzing stand age structures and patterns of tree radial growth along a west-to-east gradient of post-fire stands dominated by Nothofagus dombeyi and Austrocedrus chilensis. Age structures of 14 stands in which the ages of 811 trees were determined do not support the hypothesis that xeric tree species are invading stands formerly dominated by mesic species. Both N. dombeyi and A. chilensis form even-aged populations following stand-destroying fires and cease to establish once a dense canopy has formed. There is no evidence of either species successionally replacing the other. Due to its substantially slower growth rate, A. chilensis forms a subcanopy layer beneath taller N. dombeyi despite the similarity of ages for populations of both species.