TECTONIC INFLUENCES ON TREE GROWTH IN NORTHERN PATAGONIA, ARGENTINA -THE ROLES OF SUBSTRATE STABILITY AND CLIMATIC VARIATION

In northern Patagonia, Argentina, we examined the influences of climat ic variation and inter-site variation in substrate stability on the de ndroecological effects of earthquakes. In association with the great e arthquake in 1960 centered off the coast of nearby Valdivia, Chile, ex tensive tree mortality occurred in northern Patagonia in Nothofagus do mbeyi - Austrocedrus chilensis stands on unstable debris fans. To exam ine the effects of the 1960 and earlier earthquakes on tree growth, we developed tree-ring chronologies from samples of the surviving A. chi lensis on unstable debris fan sites and at adjacent nonfan sites of mo re stable substrates. For controlling the effects of regional climatic variation, we also produced a tree-ring chronology from this species in a more distant and undisturbed stand. Strong variations in tree-gro wth patterns on fan sites were associated with the historically docume nted major seismic events of south central Chile that occurred in 1737 , 1751, 1837, and 1960. Tree-ring chronologies from nonfan sites (i.e. , sites of greater substrate stability) showed much less response to t hese earthquakes. On the fan sites, strong growth suppressions were as sociated with the former three earthquakes, whereas strong releases fo llowed the 1960 earthquake. The difference in response is explained by the occurrence of the 1960 earthquake during a period of drought, whi ch in combination with the violent shaking of the ground, resulted in extensive tree mortality followed by growth releases of the survivors. However, severe droughts in the absence of earthquakes also can produ ce tree mortality and subsequent release of the survivors. Consequentl y, the synergistic effects of climatic variation and earthquake events must be carefully considered in developing records of both climatic v ariation and earthquakes.