Tree-ring and isotopic variations of Nothofagus pumilio forests along an altitudinal gradient in El Chaltén, Santa Cruz, Argentina

SRUR, A.; VILLALBA, R.; VILLAGRA, P.E.; HERTEL, D.

Malargüe

Congreso; Simposio Internacional - “Reconstrucciones Regionales de las Variaciones Climáticas en América del Sur durante el Holoceno tardío: Una nueva Iniciativa de PAGES”.; 2006

Nothofagus pumilio tree-ring and d13C variations along an altitudinal gradient in El Chalten, Santa Cruz, Argentina were investigated in relation to climate variations during the 20th century. Three sampled plots were established in the upper- middle- and low-sectors of the forest. All trees were measured and cored with increment bores.  Samples from three individuals from each altitudinal stand were isotopically analyzed. The upper-limit forest presents healthy, snow-stunted individuals with diameter <40cm.  The radial growth of theses trees is characterized by a marked positive trend during the 20th century in response to the increase in regional temperature. Due to an increase in photosynthetic rate, individuals in the upper treeline show an increase in water use efficiency, as indicated by the d13C.  In the lower limit, dieback is common among trees resulting in an over-matured stand with low density and coverage. Establishment of new individuals is rare whereas a high rate of mortality is observed during the past 40 years, likely related by the regional increase in temperature and reduced precipitation during the past decades. The radial growth at the lower treeline shows a negative trend during the 20th century. However, trees at this altitudinal level no show a significant increase is water use efficiency, but more likely related to stomatal conductance than photosynthetic rate changes. At middle elevations, the forest presents intermediate characteristics between those recorded for the forests at the upper-lower limits. We speculate that the recorded patterns along the altitudinal gradient are reflecting the dominant climatic trends during the 20th century. The gradual increase in temperature has favored tree growth at the upper treeline. In contrast, higher temperatures, in combination with reduced precipitation, have enhanced water deficit resulting in progressively lower radial growth at the lower treeline.