Influences of climatic and CO(2) concentration changes on radial growth of Nothofagus pumilio in Patagonia

Abstract

Influences of climate and CO(2) concentration variations during the last century on radial growth and water-use efficiency of Nothofagus pumilio were evaluated along an altitudinal gradient in El Chalten (49 degrees 22' S), Santa Cruz, Argentina. We used a combination of dendrochronological and isotopic techniques. Tree growth in the upper treeline shows a positive trend concurrent with increasing regional temperatures. In contrast, due to a gradual increase in water deficits, the rate of radial growth in the xeric forest-steppe ecotone has decreased in the past 100 years. Intermediate-elevation trees from mesic forests show a weak, non-significant negative trend in radial growth during the same interval. delta(13)C reflects soil moisture variations with elevation and its differential influence on stomatal conductance and photosynthetic rate processes. In sites with reduced to severe water deficits, the intrinsic water-use efficiency (IWUE) appears to be influenced by the amount of water in the soil. However, the relationship between IWUE and climate is more obvious in sites with reduced water stress. In drier sites, the photosynthetic rate is severely limited by water deficits so that the reduction in radial growth is not compensated by the increase in IWUE. In contrast with the traditional assumption in dendrochronology indicating that the strong relationships between radial growth and climate are recorded at the forest ecotones, the variations in delta(13)C better reflect water deficits at intermediate inesic forests.