Influence of droughts on Nothofagus pumilio forest decline across northern Patagonia, Argentina

Abstract

Understanding the influence of climatic variations on forest decline is a major challenge for scientists investigating global changes. Although reductions in tree growth have previously been associated with forest decline, comprehensive efforts to understand these relationships are rare. Based on ring-width variations, we determine the influence of climatic fluctuations on the onset and temporal evolution of Nothofagus pumilio forest decline in the Patagonian Andes. Basal area increment (BAI) data from 294 Nothofagus trees at 11 stands in a 500-km latitudinal transect along the forest-steppe ecotone were used to identify the dominant patterns of regional growth. Three Regional dominant patterns, showing common variations in BAI, were derived. Two BAI patterns show high rates of growth from early to mid-20th century, followed by sustained negative trends over the last 3-6 decades, whereas the third pattern is characterized by a positive trend since the 1960s. Tipping points in growth trends of the first two patterns are associated with two extreme dry-warm climate events in spring-summer of 1942-1943/1943-1944/1944-1945 and 1978-1979. Both severe droughts were preceded by up to 10 yr of wet periods that promoted above-average tree growth. We concluded that severe droughts occurring after wet periods trigger the decline of large, dominant N. pumilio trees with high rates of growth. The coincidence between major changes in regional growth with two of the most severe droughts in the instrumental records shows that climatic variations over northern Patagonia synchronize the beginning of forest decline at a regional scale. As these dry-mesic N. pumilio sites will face more severe droughts in the 21st century, as suggested by future climate scenarios, the areas affected by forest decline would increase substantially.