Contrasting growth trends in Nothofagus pumilio upper-elevation forests induced by climate warming in the Southern Andes

Abstract

The high sensitivity of Nothofagus pumilio growth to climate variations at upper treelines provides a unique opportunity to document changes in tree responses to a warmer climate in the Patagonian Andes. In the context of significant recent temperature and precipitation changes across Patagonia, we conducted a study along the precipitation gradient in the Río de las Vueltas basin, southern Patagonian Andes, to: (1) document differences in N. pumilio growth trends at upper treelines, (2) determine changes in climate-growth relationships along the precipitation gradient, and (3) estimate future growth responses to simulated 21st century warming. For the past 100 years, mean tree-ring width increases progressively from wet to dry treelines in response to less abundant precipitation and less persistent snow cover into the growing season. Mountain aspect regulates snow cover duration and hence growing season length, thereby also influencing tree-ring width. On interannual scale, temperature directly modulates tree-growth variations in wet and mesic treelines, but is inversely related to growth in dry sites. Growth trends show that the approximate 0.56°C temperature increment since the mid-1970’s in the Patagonian Andes dramatically enhanced the recorded long-term increasing growth rates at mesic, but to a lesser extent at humid treelines, suggesting nonlinear interactions between temperature and snow persistence on tree growth. Contrarily, growth rates at dry treelines decreased over the past 100 years. Our predictive statistical models indicate sustained decadal increases in current radial growth rates at wet and mesic sites and decreases at dry treelines towards the end of the 21st century under the simulated future warming scenarios for the southern Andes. However, the nonlinear relationships between warming, snow cover and tree growth, combined with unreliable estimates of precipitation for the region during the 21st century, suggest that these simulated changes in tree growth should be viewed with caution.