Rainfall-controlled tree growth in high-elevation subtropical treelines

It is generally assumed that tree growth in the upper limit of a forest is mainly controlled by summer temperature. This general statement is mostly based on studies from extra-tropical mountains and has been rarely evaluated in subtropical latitudes frequently characterized by drier climates. In the subtropical mountains from Northwestern Argentina (similar to23degrees S), annual precipitation decreases with elevation from > 1500 mm at 12001500 m. to <200 mm above 4000 m. In consequence, tree growth at high elevations in the region may be seriously limited by water supply. In order to assess the influence of precipitation on tree growth, we evaluated the relationships between climatic variations and radial growth in four species growing at different altitudinal zones: Juglans australis from the montane cloud forest at 1800 m; Alnus acuminata from the montane savanna-like woodland at 2700 m; Prosopis ferox from the subalpine dryland at 3500 m; and Polylepis tarapacana from the high-elevation alpine dryland at 4750 m. Dendrochronological techniques were used to relate variations in annual ring width with instrumental climatic records. Growth rings were correctly dated to the year of formation, and the cross-dated series standardized using autoregressive models to reduce non-climatic signals present in the records. Tree-ring chronologies, ranging from 117 to 341 years, were compared, during the common period, with instrumental climatic records using correlation-function analysis. In spite of the remarkable differences in elevation and environmental conditions among the sampling sites, correlation functions with climate indicated that the radial growth of the four species is largely controlled by precipitation. In most cases, increased precipitation during the previous and current growing seasons favors tree growth, while temperatures are negatively correlated with radial growth, likely due to the negative effect on water availability. These results indicate that the generalized idea of upper-treeline growth limited by summer temperatures should be carefully evaluated in low-latitude environments and does not apply to subtropical areas with severe water deficits or strong moisture seasonalities.