A Height Threshold Redefines Functional Communities: Structural Transition Between Shrublands and Alpine Forests of Polylepis tarapacana in the Argentine Altiplano

Abstract

The high-Andean forests of Polylepis tarapacana at the global treeline form a structural continuum between shrublands and forests, complicating their functional classification. This study evaluates whether a discrete height threshold – Mean Dominant Height ≥ 2 m – defines a genuine ecological transition between these formations in the Argentine Altiplano, aiming to validate this threshold, identify its predictors, and assess its consequences for the ground-layer vegetation. Ninety-six forest inventory plots were analyzed and classified as “Forest” (mean dominant height ≥ 2 m) or “Shrubland” (mean dominant height < 2 m). Structural variables (basal area, density, diameter distribution), proportions of life forms (arborescent, dwarf tree, shrub, brousse tigrée), allometric parameters, and ground cover composition were compared. Analyses included Principal Component Analysis, non-parametric tests, logistic regression models, and indicator species analysis. The 2 m threshold discriminated two clearly distinct communities. “Forests” (19.4 % of plots) exhibited greater basal area (2.8 times higher), diameter diversity, structural complexity, and a higher proportion of arborescent life forms. They showed a more efficient height- diameter relationship and a distinctive ground cover with greater cover of cushion plants and specialist indicator species (e.g., Senecio nutans). “Shrublands” were dominated by juvenile individuals, shrubby forms, and heliophytic perennial shrubs. The probability of achieving a forest state was positively predicted by the presence of arborescent forms and a favorable substrate (Favorable Substrate Index). The mean dominant height ≥ 2 m threshold constitutes a quantifiable ecological tipping point separating distinct successional and functional states in Polylepis tarapacana. This structural criterion synthesizes profound changes in community architecture, microclimate, and associated biotic assemblages. The findings provide a robust framework for the operational classification, monitoring, and priority conservation of these vulnerable alpine ecosystems, moving beyond purely morphological definitions towards a characterization based on ecological functionality.

Acknowledgments: We thank those researchers who, for years, questioned whether Polylepis tarapacana formations could be called forests, preferring terms like “forestlands” or “shrublands”. Their persistent skepticism challenged us to write this. Thanks to them, we now have clear, height-based criteria aligned with the UN definition. We are grateful for their opposition; it pushed us to do better. We also thank Argentina’s public, free, and co-governed university system. Despite devastating defunding and difficult times, we published this work through collective effort. This paper proves that public university researchers do not give up.