Validation of Non-destructive Techniques for Estimating the Age and Growth of <i>Polylepis tarapacana</i> in the High Andes of Northern Argentina

Abstract

Accurately determining the age of trees is fundamental for ecological and conservation studies, particularly for slow-growing, threatened species in extreme environments. This study evaluates the effectiveness of a non-destructive technique for estimating the age of apical branches and analyzing the growth of Polylepis tarapacana, a near-threatened tree species that forms the world's highest forests and shrublands in the Argentine Altiplano. The technique relies on counting the external bud marks (scars) left by annual shoots on apical branches. To validate this method, we collected 48 apical branches from across the species' distribution in Argentina. The age estimated from bud mark counts was correlated with the age determined by annual ring counts on cross-sections. Statistical analysis revealed a highly significant positive correlation between the two methods (Spearman's ρ = 0.820, p < 0.001), with a coefficient of determination of R² = 0.672. The overall mean absolute error of the non-destructive technique was 1.11 years. Accuracy varied with age, showing higher relative error in very young branches (1-3 years) but high reliability (relative error of 8.5%) for branches over 13 years old. Furthermore, growth analysis indicated a significant decrease in annual shoot elongation with increasing age, with a reduction observed between the youngest (1-3 years) and oldest (10-12 years) age groups. These results demonstrate that the bud mark counting technique is a statistically valid, accurate, and robust non-destructive method for age estimation in young apical branches (up to ~25 years) of P. tarapacana. Its application provides a valuable tool for demographic monitoring and conservation strategies for this high-altitude species, enabling growth assessment without damaging vulnerable populations.