Establishment of Nothofagus pumilio at Upper Treelines Across a Precipitation Gradient in the Northern Patagonian Andes

SRUR, A.; VILLALBA, R.; RODRÍGUEZ-CATÓN, M. ; AMOROSO, M. M.; MARCOTTI, E.

ARCTIC ANTARCTIC AND ALPINE RESEARCH

INST ARCTIC ALPINE RES

Lugar: Boulder, USA; Año: 2016 vol. 48 p. 755 - 766

1523-0430

Trees at upper treelines are exposed to more extreme environmental conditions thanthose at lower elevations. Climate changes at the upper treeline facilitate the establishmentor 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 siteslocated along a west-east precipitation gradient across the Patagonian Andes. Patterns oftree establishment were compared to regional variations in temperature and precipitation,as well as to indexes of atmospheric circulation that modulate northern Patagonianclimate. Mesic and dry sites along the moisture gradient have a lower density of newlyestablished trees; however, individuals show larger basal diameters and greater annualgrowth rates, heights, and number of branches than those established in humid sites. Inwet areas, the high density of individuals reflects the higher rates of N. pumilio establishmentand survival. At drier treelines, low snow persistence, associated with longer growingseasons, appears to be related to the larger size of individuals. At all sites, patternsof tree establishment are characterized by an abrupt increase in recruitment starting inthe mid-1970s and a marked decrease in the late 1990s. The onset of tree establishmentabove the treeline coincides with an increase in regional spring-summer temperaturein the year 1977, concurrent with the negative-to-positive shift in the phase of the PacificDecadal Oscillation (PDO). In contrast, the decrease in N. pumilio establishmentsince the late 1990s coincides with an opposite shift (positive to negative) in the PDO.This recent change in the PDO phase did not significantly modify the mean values butincreased the interannual variability of the spring-summer temperatures in the region.Changes in the PDO, which encompasses complex variations in environmental conditionsat the upper treeline, are more closely related to N. pumilio establishment than arevariations in temperature or precipitation alone. In addition, the distinction between theeffects of changes in mean values versus the effects of climate variability is crucial forproperly predicting forest responses to climate changes.