Spatio-temporal variations in Polylepis tarapacana radial growth across the Bolivian Altiplano during the 20th century

We document the dendroclimatological potential of Polylepis tarapacana to estimate past temporal and spatial variations in precipitation across the Bolivian Altiplano (17–23°S). The P. tarapacana chronologies, which presently range between 110 and 705 years in length, represent the highest altitude tree-ring records worldwide. Interannual variations in fourteen tree-ring chronologies located between 3900 and 4850 m elevation were compared using correlation and principal component analyses. There is a strong common signal among the chronologies. For the common interval AD 1890–1999, three dominant patterns of tree growth across the Bolivian Altiplano were identified. The first pattern which explains 52% of the total variance in tree growth is associated with years of above- or below-average tree growth across the whole region. The second and third patterns, which are related to significantly lower common variance (13 and 8%, respectively), are associated with contrasting positive and negative anomalies of growth between the northern and southern sectors, and between the central-western sector and the rest of the region, respectively. In order to determine the climatic significance of the spatial variations of P. tarapacana growth, the interannual patterns in tree growth were compared with spatial anomalies in precipitation across the region. Uniform patterns of above (below) average tree growth are associated with positive (negative) precipitation anomalies in the preceding year that are uniformly distributed across the whole Altiplano. In the westernmost areas of the Altiplano, summer rainfall accounts for more than 80% of total annual precipitation. Therefore, relationships between tree growth and summer or annual precipitation are, in most cases, similar. Contrasting patterns of tree growth are related to opposite anomalies in precipitation during the preceding year in the different sub-regions in the Altiplano (north versus south, central versus north and south). Climatic preconditioning of tree growth was also identified for individual years. For instance, extremely reduced growth in the year 1967 reflects dry conditions in the two preceding years (1965–1966). Long-term intervals (3 or more years) with reduced tree growth resulted from several consecutive years with low precipitation. Our results indicate that the network of tree-ring chronologies of P. tarapacana in the Bolivian Altiplano has the potential to provide annually-resolved precipitation reconstructions in this region for the past 5–7 centuries. This is a valuable source to document decadal-to-century-scale precipitation variability in the Altiplano and its relation to large-scale ocean–atmosphere circulation features.