Climatic Influences on Nothofagus Forest Dynamics in Southern Patagonia.

SRUR, A.; VILLALBA, R.; VILLAGRA, P.E.; MORALES, M.S.; DELGADO, S.

Mendoza

Congreso; CONCORD, Cambio Climático Organizando la Ciencia para la Cordillera Americana; 2006

IANIGLA

Climate-induced changes in Nothofagus pumilio tree growth and stand dynamics were investigated along an altitudinal gradient in El Chalten, Santa Cruz, Argentina (49°S). Sampling plots were located from the lower forest-steppe border (760 m), the mid-elevation mesic forest (950-1010 m) to the krummholz (1100 m) in the transition to alpine vegetation. Dendrochronological analysis shows contrasting growth patterns of Nothofagus pumilio along this altitudinal gradient. Tree ring width at the upper tree-line shows a remarkable positive trend during the 20th century. This positive trend is strongly correlated (r = 0.56, n = 70, p < 0.000) with the increase in mean annual temperature expressed as departures from the means of Rio Gallegos, Punta Arenas and Ushuaia meteorological stations. The growth of trees located at middle elevation shows a slightly negative trend during the last 100 years. The radial growth at this level is related to temperature between 1930 to ca.1976, but temperatures and ring widths show opposite trends thereafter. In contrast to upper-elevation trees, the growth of trees at the lower treeline shows a strong negative trend since the beginning of the 20th century. Temperature variations since 1930 are negatively correlated (r = -0.31, n = 70, p < 0.010) with tree growth at lower elevations. The increase of temperature during the 20th century has favored tree growth at upper treeline. In contrast, increased temperatures and a decrease in precipitation during recent decades are associated with a negative trend in tree growth at lower treeline. During the first relatively cold decades of the 20th century, the growth of the trees located at middle elevations, was regulated by temperature variations. This relationship seems to have change, especially after 1976, when an exacerbated moisture deficit in response to higher temperatures and lower precipitations has been the limiting factor for tree growth in this sector of the forest. Temperature and precipitation changes during the 20th century have also affected Nothofagus stand dynamics at the forest borders. Dendrochronological dating of young individuals above the upper tree-line indicates that the establishment started after 1976, peaking in 1988-89. Tree mortality at the lower forest border is more common during the past decades. Dating of death trees indicates that tree mortality has increased with higher temperatures since mid 1970s. The unusually warm period starting in 1977 has favored the expansion of the forest above the upper treeline and accelerated tree mortality at the low forest-steppe border.