Establishment of Nothofagus pumilio at upper treelines across precipitation gradients. Part II: The Southern Patagonian Andes

VILLALBA R.; MARCOTTI, E.; SRUR, ANA; RODRIGUEZ CATON, M.; AMOROSO, M.M.

Berlin

Conferencia; Climate Impacts on Glaciers and Biosphere in Fuego-Patagonia; 2017

Trees at upper treelines are exposed to more extreme environmental conditions than those at lower elevations. Climate changes at the upper treeline facilitate the establishment or intensify the mortality of trees and, consequently, affect species distributions. The structure and density of individuals of Nothofagus pumilio above the upper treeline, together with their temporal patterns of establishment, were determined in three sites located along a west-east precipitation gradient across the southern Patagonian Andes. This work complements previous studies conducted in the northern Patagonian Andes (Part I). Patterns of tree establishment in the southern Andes were compared to regional variations in temperature and precipitation, as well as to the Antarctic Oscillation (AAO or SAM) index, the atmospheric circulation modes related to climate variability in southern Patagonia. Mesic and dry sites along the moisture gradient have a lower density of newly establishedtrees; however, individuals show larger basal diameters, heights, and number of branches than those established in humid sites. In wet areas, the high density of individuals reflects the higher rates of N. pumilio establishment and survival. At drier treelines, the low snow persistence, associated with longer growing seasons, appears to be related to the larger size of individuals. At all sites, patterns of tree establishment are characterized by a gradual  increase in recruitment starting in the mid-1970s followed by abundant establishment during the 1980s. This marked increase in tree establishment above the treeline is concurrent with a period of above-average summer temperatures starting in the year 1977. Contrary to patterns of establishment the documented in northern Patagonia, new recruitment in the Southern Andes has shown a constant rate of tree establishment to present. Whereas establishment above treeline in northern Patagonia is largely modulated by changes in the phase of the Pacific Decadal Oscillation, the stable rate of recruitment in the southern Andes since 1977 to present, appear to be related to the persistent positive trend recorded in AAO during the past decades.